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After all, what use is it to anyone? Someone who already understands this does not need to turn to this article. And those who have no idea but want a basic introductory explanation of what it is are left out. Great, so it is magnetic flux. Ok, now I go to that page and am inundated with another page full of abstruse verbage. Around I go, grasping at circular references, looking for some kind of conceptual foothold to begin with, but left with nothing but abstract mathematical symbols and explanations that may as well be in Greek. I mean, electical engineers aren't the only people interested in what this stuff is.
I couldn't agree more. This is supposed to be an encyclopedia entry, not a college level course. I had exactly the same problem you did. All I wanted was a basic explanation that was a little more complete than my vague grasp of the principles, but instead was bombarded with advanced material that belongs in a technical reference, with every link leading to more of the same. Most of the people who contributed to this ought to go find somewhere else to circle-jerk. 208.42.94.130 ( talk) 08:25, 14 November 2008 (UTC)
Could someone please explain inductance in layman's terms? —Preceding unsigned comment added by 68.121.33.140 ( talk) 03:02, 20 December 2008 (UTC)
It is harder to write a simple explanation of inductance than even its dual, capacitance. Capacitance can easily be explained in terms of slowly varying fields. Also, inductance tends to appear in less than ideal situations more often than, for example, resistance or capacitance. In terms of real circuits operating at less than radio frequencies, it is fairly easy to avoid stray capacitance but somewhat harder to avoid stray inductance. As others have said, a voltage generated (induced) due to a change in current. For self inductance it is a change in current in the same conductor, for mutual inductance in a different conductor. Gah4 ( talk) 23:40, 2 July 2009 (UTC)
Let me amplify this a bit. I have no problem with being presented with the math, eventually, but if I am trying to understand the subject, I need a handhold to grasp to get my feet up off the ground and start climbing. It takes a few more words and metaphors to get this across to someone who doesn't already know the subject intimately, and it only takes a quick reference card to get the subject across to those who already get it. So, if someone who was able to claw their way through the math and jargon to a real conceptual understanding of this material would be willing to spend a little time to help bring the rest of us along, it would be great. The power of Wiki is that the space is not really limited by anything, so this does not need to be an either-or proposition, please keep the technical depth, but provide a layman's framework in which to learn and understand the material as well. Visierl ( talk) 14:42, 26 December 2008 (UTC)
For many things, possibly including inductance, it gets more complicated, and takes more words, as you get deeper into the subject. A favorite example is friction. Friction is commonly taught in first year physics classes, yet rarely taught after that. If you try for a deeper understanding than in the first year course it gets very complicated fast. One complication with friction is that the more ideal the system (two pieces of the same material without grease or other impurities) the less ideal it gets (they tend to stick together greatly increasing the friction constant). Gah4 ( talk) 23:40, 2 July 2009 (UTC)
The equation on the inductance of a solenoid states that it is inversely proportional to "l" (the length of the solenoid). This implies that inductance would be infinite for a zero-length solenoid. Certainly a single-winding solenoid could be made with a length arbitrarily close to 0. I think this equation is an approximation with assumptions (eg. ). Could these assumptions please be stated? Jurgen Hissen 21:23, 18 May 2006 (UTC)
Notice the factor of N^2 in the numerator. Ordinarily when winding a solenoid with real wire, each turn takes up the same amount of length, so the number of turns is proportional to the total length l. As l decreases, N normally decreases in step. Take one of the factors of N from the numerator and l from the denominator to form (N/l). That is ordinarily a constant.
But in the limit of an infinitely thin wire, the inductance of that wire really does go to infinity, even without being wound into a coil. Fortunately that is an abstraction never met in the real world.
I'm actually more worried that the inductance formulae in the table all lack a factor of mu_zero, so something could at least be better explained.
User Phil Ekstrom 20 May 2009 —Preceding unsigned comment added by 207.32.171.65 ( talk) 02:36, 21 May 2009 (UTC)
This page needs a picture of an inductor, and a drawing representation of it (showing turns), and the usual circuit representation for it. Maybe I'll draw these sometime. Fresheneesz 07:47, 22 November 2005 (UTC)
Mutual inductance needs a picture, needs to explain the dot convention, and give a picture for this. I might make these sometime as well. Also, mutual inductance need not occur between circuits - but can occur within circuit. Two nearby inductors can even have mutual inductance while remaining also in series. Fresheneesz 07:47, 22 November 2005 (UTC)
I can't even read the first sentence of this article. Shouldn't it read something like, inductance is the property of a device to produce a voltage proportional to a varying electrical current?
The expression given at the beginning, is only correct for a circuit with a fixed geometry. If the circuit is contorted while current is flowing through it, then the change in geometry creates a back-emf, also, as shown below.
(same poster) is the above assertion true? I have found no occurance in any reference texts.
I suppose one could look at it that way.
But the traditional way of analyzing motor/generators is to assume a fixed inductance for each part (the inductance of the rotating coil, and the inductance of the stationary coil), and calculating a separate "back emf" as a function of the speed of the motor.
I've always thought of a motor as "two fixed-geometry inductors that move relative to each other".
I've honestly never thought about considering a motor as "one big variable-geometry inductor". It's a perfectly self-consistent way to look at it.
But if I had some spinning motor, and someone asked me for "the inductance" of the motor, I'd probably just measure the voltage and current and plug in
, completely ignoring the "variable inductance" of the motor taken as a whole.
-- DavidCary 08:01, 29 Oct 2004 (UTC)
What is R in the mutual inductance equation? It also would help to clarify that ds is the increment along the curve.
-- Colin
I probably agree that describing a motor in terms of d(LI)/dt isn't the best way, but it can be important for solenoids. There was a discussion on comp.dsp (a little off topic) on solenoids driven by SCRs, and the dL/dt term is important. Gah4 ( talk) 01:29, 29 November 2009 (UTC)
Is there a reason lower-case 'i' is used to denote the current here?
I just pluralized "henry" and noted that I typed it "henries". But another appearance of the plural later on in the article was spelled "henrys". Is there an official plural for this unit? (Or is this like the ever-simmering British English versus American English debate?) If you know, could you please edit the article to make the two plurals consistent?
Atlant 11:42, 15 Jun 2005 (UTC)
I'm still missing a feeling for what actually mutual inductance is. Is it something we want to have in a transformer? What would happen if it was very small (i.e. MI is zero), what, if it was a very big number (mutual inductance going to infinity)? Thank you, -- Abdull 08:00, 21 July 2005 (UTC)
I'm not sure why the merge request was made, but I think it's clear from the articles (or any physics textbook) that they're two separate concepts. -- SCZenz 14:21, 31 August 2005 (UTC)
Yes, and also the toasting of toast is due to a toaster. That doesn't mean they're not separate concepts, it just means they're related concepts--so they have separate articles that link to each other. -- SCZenz 22:32, 31 August 2005 (UTC)
I propose that the defn should read;
''Inductance is that property of a conductor (including an inductor) that admits a current through it that is proportional to the time integral of the voltage applied across it.
I know this seems more complicated but I believe it is more relevant to isolated inductances (not mutual inductance or induced voltages) and avoids the confusion of induced 'back emf' . Also the usual causal relationship is that you apply a voltage to the inducatnce in the first place but only then does the current ramp up. A current cannot magically already exist in an isolated indutance with no external changing magnetic field. Any comments on this idea?? Light current 15:49, 1 September 2005 (UTC)
Any comments? Light current 16:18, 1 September 2005 (UTC)
Not to throw a monkey wrench into this general agreement, but consider the case of a device that has reactive properties like a traditional inductor or capacitor, but which does not use magnetic or electrical field mechanisms to do so. Would people say that "That's not an inductance" just because it doesn't use a magnetic field, or with that in mind would the above suggestion have merit? Would a better rephrase of the corresponding capacitance description not be "Inductance is a measure of the amount of electric potential stored for a given electrical current?" (Though in both cases I'd say it should be "stored/required" not just "stored.") A quick followup statement could establish that magnetic fields are the common method of storage, and present the flux-based definition. (Oh, and yes, to Light Current, a current can indeed pre-exist in an isolated inductor with a static magnetic field, for example an SMES.) ( 71.233.165.69 00:41, 26 June 2006 (UTC))
According to fundamental principles, the inductance L is defined as:
Does it follow that:
Well, let's see...
Moral of the story - use the the formula from first principles or if not, state your assumptions, e.g.:
Besides, the opening sentence refers to magnetic flux and current, not their differentials. I recommend beginning with the (always correct) non-differential form and then derive the simplified differential form by explicitly stating the caveat 'for constant L'. Alfred Centauri 22:58, 3 September 2005 (UTC)
I disagree. There are many assumptions that are used in many fields to make the study practical. One is that, for the most part, inductance doesn't change fast compared to the current changes. If one had to go around stating assumptions all the time, there would be no time to actually get anything done. Instead, state when the common assumptions do not apply.
Consider Ohm's law: V=IR. Sounds simple enough, and works well most of the time. Note no indication of the assumption of the low inductance of the resistive material. Many electrical components have less than ideal properties, but are useful within the limitations of those properties. One applicable here, inductors tend to have non-negligible resistance. Wirewound resistors have significant inductance restricting their use at higher frequencies. Sometimes those need to be stated, other times they don't. Gah4 ( talk) 23:10, 2 July 2009 (UTC)
Consider this. If L varies with i and i varies with time, L is an implicit function of time so dL/dt is non-zero. Conversely, if L is a function of time, dL/di is only zero if i is constant. Put another way, if L is constant and i varies, we get v= Ldi/dt. If i is constant and L varies, we get v = idL/dt. If both L and i are functions of time, we get the sum of the terms. Bottom line, L only equals dphi/di if L is constant! Alfred Centauri 01:46, 4 September 2005 (UTC)
Could AC state the physical quantity to which lambda refers. Its new to me. Light current 01:09, 4 September 2005 (UTC)
The "Definition" section is mathematically correct, but it may be misleading to the casual reader. It implies that the inductance of a coil is simply proportional to the number of turns where, in fact, it is proportional to the turns squared. This is because the flux term has a "hidden" ampere-turns term which causes the current to cancel so that the inductance of an air coil becomes a function of turns squared, permeability, and coil geometry. Although this gets cleared up in the formula for an N-turn solenoid in the "Permeability" section, I think this example should be brought up to the "Definitions" section to make it clearer (similar to the way "Capacitance" is defined in the Wiki article in terms of permittivity, area and separation distance). I made a number of changes that, hopefully address this - added an "Inductace of Solenoid" section and removed the "Permeability" section (since there's already a Wiki article on permeability). Feel free to mercilessly change as appropriate... Bert 13:25, 10 April 2006 (UTC)
I would argue that any counter emf is identical to the applied emf in all cases where there is no externally generated magnetic field impinging on the coil. From this stand point I maintain that this counter emf is in fact ficticious. There is only one emf acting across a coil --- that is the applied emf - end of story. The applied emf is the cause, and the current and resulting magnetic field are the effects. The applied emf is countered by the reactance of the coil (giving the effect of inductance) not by some magical 'back emf'. The back emf story would be just like saying that the emf of a battery connected to a resistor is counteracted by a back p.d generated by the resistor somehow. Comments invited. Light current 04:19, 4 September 2005 (UTC)
I suppose this is an old discussion by now, but it seems that the counter emf is important in cases where the magnetic field causes changes that affect the original circuit. In the case of a solenoid actuator, the field pulls a piece of magnetic material into the core, which changes the field seen by the coil. This is the cause of the dL/dt term, from V=d(LI)/dt. Gah4 ( talk) 00:12, 17 February 2010 (UTC)
This can be true because the wire has some inductance and the voltage is applied 'suddenly'. The case 'O' and I were dicussing had zero inductance Light current 18:56, 4 September 2005 (UTC)
I know. But in our thought experiment we were (at least I was) assuming an infinitely short, short circuit?? and a zero loop area. Do you buy that one? ;-) Light current 20:12, 4 September 2005 (UTC)
Agreed Light current 18:56, 4 September 2005 (UTC)
Agreed Light current 18:56, 4 September 2005 (UTC)
Agreed - this is the voltage that the current source generates to drive its current thro the inductance of the loop which we have both agreed exists. Light current 18:56, 4 September 2005 (UTC)
208.61.125.43 20:09, 4 September 2005 (UTC)
Maybe if you look at it the other way around. Does an ideal voltage source generate current? If not, where does the current come from. The way I look at it, the ideal voltage source has to generate the appropriate current to keep the voltage constant. The ideal current source generates the appropriate voltage to keep the current constant. Both have to be able to supply power to the load so I think generate is a fine description. Gah4 ( talk) 00:40, 4 December 2008 (UTC)
I know it generates current and the voltage existing on the resistor is a by product of that current- but the prime mover in this case is still the current source. Why invent an independent source of voltage rthat would not exist if it were not for the current source. If feel this line of argument could take us up a blind alley with no way out! BTW 208.61.125.43 are you a new user or just forgotten to log in?? Light current 20:22, 4 September 2005 (UTC)
Here is where we start to disagree (I think). The voltage across the resistor is identically equal to the voltage of the source (KVL). BUT -- the causal relationship is that the voltage source must first have a voltage at its terminals in order to drive current thro the resistor to create the pd (which of course is the original emf of the source). This is the circular argument to which I refer.!! THe voltage source is Boss, the resistor is completely passive. Light current 18:56, 4 September 2005 (UTC)
No applied voltage/ current from the source. Therefore, voltage spike comes from energy stored in magnetic field. No problem! Light current 20:28, 4 September 2005 (UTC)
Yes, this is the normal action of a resistor. In this case the prime mover is a current source and the resistor has no choice but to develop a pd across itself. equal to IR Light current
Which came first; chicken or egg.?? Light current 00:53, 5 September 2005 (UTC)
Again, perhaps I have not made myself clear in the article and my previous 'rantings'!. I did not say that induced voltage was ficticious (ie voltage induced from an external changing magnetic field) - I said that any concept of 'back emf' in an isolated inductor generating its own changing magnetic field due to an externally applied voltage is a myth. How can you say that the voltage you apply to a lone inductor is induced? It is not - it is applied! After all there can be only one emf/voltage appearing across the ends of an indutor of inductance at any one time. 88.110.246.253 18:56, 4 September 2005 (UTC)
I Agree that we should follow Uncle Alberts maxim (everthing should be as simple as possible -- but no simpler). Here I think the idea of 'back emf' (which I believe has not been taught for over 30 years now at university level), is confusing to everyone (including me). Its a lot simpler to say that for an Isolated inductor connected to a voltage source, V, that V=Ldi/dt. The sign is covered by the sign of di/dt. ie increasing current requires a positive voltage to be applied and to cause the current to decrease requires a negative voltage to be applied. Is this not correct? 18:56, 4 September 2005 (UTC) Light current 18:59, 4 September 2005 (UTC)
The emf is induced in the inductor by its collapsing magnetic field. I have no problem with that. But remember that there is only one source of energy here: in the mag field and cct follows the induction law as if the field were created by another coil somewhere. BTW my spelling is always bad cos I try to type too fast! Light current 20:37, 4 September 2005 (UTC)
In para 3 of Properties of inductance we ahve a completely circular argument. This para seems to be therefore patent nonsense Light current 04:25, 4 September 2005 (UTC)
If, after I have had a chance to explain my thoughts, you still wish to revert the article, I would ask that you copy the controversial material here so that any other interested parties can still see it and comment on it. Thanks Light current 19:07, 4 September 2005 (UTC)
OK I'm going to read that now Light current 19:07, 4 September 2005 (UTC)
I have read that article now, and I think I agree with all of it. But the so called negative sign business is , I assume, the fact that a battery has an opposing electric field across its terminals when open circuited. This is true. However', the opposing electric field would not be there if the battery was not there (causality) . This also reminds me of Newtons Law of motion (cant remember which one) that action and reaction are equal and opposite Light current 19:19, 4 September 2005 (UTC)
Yes I think I agree with this, although I don't really think its relevant to my argument about inductance. Light current 19:24, 4 September 2005 (UTC)
Here is something I wrote for the Electromotive force article:
As there is no external circuit , Kirchoffs voltage does not apply, and hence the sum of the voltage around the circuit (which does not exist) does not need to equal zero. The "emf" is simply an "emf" which has the potential to motive electrons around circuit.
I would hope that the idea of dropping the term "emf" never eventuates. There is no other suitable word(s) to replace it, for the concept that it conveys. IE a "force that motivates electrons!" The term "emf" is not directly interchangeable with "voltage produced by electromagnetic induction" or back emf. There is a big difference.
The volt of course is the unit of emf , potential difference or voltage drop. It is in fact the term "voltage" and likewise the word "amperage" which are badly abused. Extending the logic of doing away with emf is akin to doing away with "mmf"(magneto motive force" What a horrible thought!
If I place a resistor accross the emf, are you als implying that the "voltage" accross the resistor will stop the voltage flow.
The emf , will still be there and there will be a voltage drop accross the resistor. We have a circuit and KVL must be (and will be) satisfied.
Agree --but cant see relevance to my argument about an isolated inductor :-) Light current 19:27, 4 September 2005 (UTC)
Just having looked at the page for the first time since you altered it, I am quite relieved that, (expecting the worst after reading your posts), you have left so much of my material there. Thanks!:-). As you know I'm still confused by the minus sign and the page explanation does nothing to help me. Why must we refer to two voltages equal but of opposite sign when one would do (as the sign is already included in the di/dt or the dØ/dt)? The second one is identically equal to the first in the case of an isolated inductor fed by a switched voltage source -- is it not? The building of the magnetic field does indeed present reactance to the voltage source and this is what we expect from an inductor. However, to say that the 'reactance' is caused by the changing magnetic field the coil is in the process of generating - and this then creates another voltage equal in magnitude but opposite in sense to the applied one, is IMHO an unneccessarily complicated and circular argument for presentation to engineering (or even physics) students Light current 19:53, 4 September 2005 (UTC)
I'm going to agree with that for now. You mean the general integral form of Maxwells equs (from Faradays Law) Presumably you mean magnetic flux density- but lets not argue about that. I have to agree with your above statement.
Im not trying to fuss or complain, I'm just saying that the difference between 'applied emf' and 'induced emf' is extremely confused and needs clarifying. They are (necessarily?) opposite in sign. If we can come up with unambiguous, simple definitions for capacitance, inductance and compare and contrast their similarities/ differences we will have done what most textbooks on electrical engineering have failed to do so far. 88.110.246.253 23:40, 4 September 2005 (UTC)
<!del--- Forgive me butting in here. I agree that the non conservative field is what confuses people. It confuses me. Even the name is intimidating. Is there a simpler name such as changing, not steady or something more friendly that could be used?? Light current 15:02, 5 September 2005 (UTC)--->
Ive just looked at my college notes on Field Theory and guess what? - the topic of non conservative fields is not covered nor even mentioned!. No wonder I've been having trouble all these years understanding electromagnetic induction! Light current 16:49, 7 September 2005 (UTC)
Isnt this just because the field is changing with time? Light current 15:04, 5 September 2005 (UTC)
Before I try to digest the above answer, could you say whether you agree that the sign of any induced voltage is (in some part) determined by the sign of di/dt or dØ/dt? Thanks Light current 22:06, 4 September 2005 (UTC)
I assume your reply means that you agree (not that you could say whether you do or not)Sorry to be pedantic! :-) Light current 23:07, 4 September 2005 (UTC)
Ive just been looking at the page again. The statement I have trouble with is:
where is the Electromotive force (emf) and is the induced voltage. Note that the emf is opposite to the induced voltage.
You see, the way I see it, neither one is defined and the difference between these 2 voltages is not stated. It probably makes complete, unambiguous sense to you, but I'm afraid it doesnt to me (Im not saying its wrong -- just that I dont undersatnd it as it stands). If we were to define what we mean by:
a) the emf
b) the induced voltage
maybe our problems would go away!! What think you? Light current 00:10, 5 September 2005 (UTC)
Is this pd not obviously zero!? Light current 13:30, 5 September 2005 (UTC)
Sorry ,I missed the point about the field changing. You are absolutely correct. Apologies! Light current 14:23, 5 September 2005 (UTC)
OK Im beginning to see that:
a) (we must accept this from previous discussion and Mawells laws etc)
b) the induced emf can exist even with no moveable charges present (ie in an insulator)--
c) If, however, a source of moveable charges exists, these charges will be acted upon by the emf and they will move(necessarily doing work- but lets forget about work at the moment)) to counteract the emf.
d) the induced emf acts like a battery connected to a conductor with the conductor representing the body containing the moveable charges.
e) Therefore, the voltage induced in the conductor must be in opposite sense to the emf in order to obey KVL
Is this a fair summary of your argument so far? Light current 13:52, 5 September 2005 (UTC)
Consider a ring of resistive material with uniform resistivity. Assume that there is a magnetic flux changing at a constant rate through the surface area enclosed by this ring. This changing magnetic flux induces a constant electric field that in turn drives a constant current through the ring of resistive material.
Recall that the line integral of the induced electric field around the path traced by the ring gives the emf (in volts) induced by the changing magnetic flux.
Answer the following questions:
(1) Is it possible to measure the total emf directly with a voltmeter? If so, explain how to do this. If not, propose how to measure the total emf.
It is my opinion that a voltmeter will read zero volts between any two points on the ring as long as the ring is unbroken. This result is the only result consistent with KVL and question 2. Thus, I believe that your 'second try' answer is correct. Alfred Centauri 23:54, 6 September 2005 (UTC)
I disagree. The voltage at all points is the same, but that doesn' mean that is what the voltmeter will read. Put one probe on a spot on the ring. Put the second probe on the same spot, as expected the voltage will be zero. Now slowly move the second probe around the ring, and the voltage will slowly increase until the probe gets back to the beginning. Now you don't need the ring anymore, but note that the flux is going through a loop made up of the voltmeter probe leads. This solution is similar to the problem of contour integration around a singularity. Gah4 ( talk) 21:42, 15 October 2009 (UTC)
(2) KVL says that the sum of the voltage rises and voltage drops around a circuit must equal zero. Does KVL apply here? Explain your answer.
It is true that emf is not a point function precisely because it is defined as the integral of the electric field along a closed path. However, KVL is directly aimed at emf because it is emf that drives charge around a closed circuit. A conservative electric field cannot do this! Nonetheless, your answer is correct otherwise and consistent with the idea that a voltmeter reads zero volts between any two points on the ring for the very reason you gave that the emf is exactly balanced by Idr. Alfred Centauri 00:16, 7 September 2005 (UTC)
(3) Assume the current in the ring is anti-clockwise. If a voltage source were instantly inserted in series with the ring, what is the value and polarity of the voltage required to stop the current in the ring?
Correct. The voltage source should generate a current equal and opposite to the current driven by the emf. Now, doesn't this bother you? After all, you can now measure a voltage across the resistive ring but there is zero current!!!. What happened to Ohm's law? Alfred Centauri 00:19, 7 September 2005 (UTC)
(4) With this voltage source in circuit and with zero current in the ring, magically adjust the resistivity of the material to one-half its former value. Does the current change? Explain. If the current does change, what is the new value of the current?
Correct. Alfred Centauri 00:22, 7 September 2005 (UTC)
(5) Answer (4) again with the resistivity magically adjusted to zero.
Again, correct. But once again, doesn't this bother you? You have a voltage across a short circuit and zero current! How can you explain this? Alfred Centauri 00:22, 7 September 2005 (UTC)
You have to be very careful measuring voltages with changing currents around. The meter leads are unavoidably part of the measuring circuit. Gah4 ( talk) 21:42, 15 October 2009 (UTC)
Doesn't bother me as much as it would have afew days ago! There is an emf generated in this short cct by the changing mag flux. If this emf were not cancelled by my inserted voltage source, then I presume an infinite current would flow. Is this the same thing as a s/c turn in a transformer? The only weird thing to me now is that you can have an emf exist in s/c! Light current 00:49, 7 September 2005 (UTC)
Alfred Centauri 13:49, 5 September 2005 (UTC)
Now that you have become comfortable with the balance of forces, let me thow you a curve ball. [BTW I though all balls were curved!! ;-) Light current 17:26, 7 September 2005 (UTC)
According to Einstein's theory of gravity (General Relativity), the only force on the apple is the push from the deformation. This force causes the apple to be accelerated which is why the apple has weight. So, according to GR, the forces aren't in balance. Enjoy! Alfred Centauri 16:06, 7 September 2005 (UTC)
All the previous arguments (on induction) seem to be leading us down a path of saying that:
Whatever force can exist in the universe, whether mechanical, electromotive (emf), magnetic(changing or not), electric (changing or not), gravitational etc, that force, by its very presence, INDUCES an equal and opposite force on any object to which it is applied. If it is not applied to an object, then the force just exists on its own like an emf in an insulator. But, when applied to an object, that force MUST be resisted by the object (whatever it is) and it must be resisted in a way that leads to the continued existence of the object (ie its not crushed by gravity, it creates an induced voltage to stop current rising to infinity etc,etc)
Would anyone agree with this extreme generalistation of mine?? Light current 15:46, 5 September 2005 (UTC)
In your order. Yes!, Yes, (I knew someone would mention black holes!!) and Yes. OK, SCZenz THanks for your input :-) Light current 01:22, 7 September 2005 (UTC)
Yes I agree this wording needs a lot of work. Unfortunately I am no expert in nuclear physics (or physics in general for that matter - I'm an engineer). I just thought that after my discussions with [User:Alfred Centauri], this business of actions being equal and opposite seems to make sense to me within my limited sphere of knowledge. If it turned out to be a (fairly) general principle, I'm sure it would aid science understanding tremendously. One example I can quote is that of Newtons 3rd? law -to every action there is an equal and opposite reaction. It took me years to see how this could be true in the case (lets say) of an apple resting on the ground. THe apple has the force of gravity acting upon it,and is therfore attracted to the centre of mass of the earth. Since the apple doesnt move, there must be an equal and opposite force acting upwards on the apple. So far so good. THe big question is: how does the earth know what force to exert in response to the apple. The answer came to me one day. When the apple rests on the earth, the earth is slightly deformed by the attractive forces. The more the earth (and apple I suppose)is deformed, the greater its 'upthrust'. Hence the earth is deformed just enough so that the reaction force exerted by the earth is exactly equal to the 'weight' of the apple and a state of mechanical equilibrum then exists. The 'bigger question' is: does this sort of thing happen between other forces and objects or between two different forces like emf and induced voltage. I'm sure people could think of many more examples. Food for thought!!! Light current 02:58, 7 September 2005 (UTC)
Are you implying here that the reaction force is generated by electrostatic repulsion? Light current 05:09, 7 September 2005 (UTC)
OK then, but in an insulating material, is it not true that electrons are tightly bound to their atoms. If so how can there be an electron cloud? Light current 13:40, 7 September 2005 (UTC)
Ok. So regardless of how it happens, in the macroscopic case of the apple on the ground, one could say that the reaction force is induced by the gravitational force. Yes?. The upthrust of a fluid is induced by the dispalcement of that fluid (Archimedes principle). Voltage is induced to counteract emf in a changing magnetic field. Charges are induced on the plate of an electroscope to balance the electric field of a charged object brought close by. Heres a biggie -- electric flux is induced by the presence of charge???. Do you agree with all the above?. If not - say why not Light current 15:24, 7 September 2005 (UTC) Just noticed in my college notes that electric flux used to be called the electric induction. Light current 17:02, 7 September 2005 (UTC)
Possibly Im using the word 'induction' too loosely here , but it will do until someone comes up with a better word!. Light current 00:54, 8 September 2005 (UTC)
If its not always or generally true, then the postulate is erroneous. However, I dont think we can say that yet-- Light current 17:09, 8 September 2005 (UTC)
Apply a force to anything. There is always a reaction (Newton was quite right). Scrub 'induces' if you like and substitute 'causes'. It makes no difference to my postulate. Light current 03:14, 8 September 2005 (UTC)
What it buys us is the notion that all forces in the universe have an equal and opposite reaction. I have yet to think of an application where a new insight might be revealed - but give me time!!. I was hoping that others may latch onto this idea and suggest applications of this postulate. Light current 01:04, 8 September 2005 (UTC)
All forces existing in the universe, cause an (induced)reaction to be generated by any object or force field or medium to which the original force is applied. This reaction force is equal in magnitude and opposite in direction to the original force.
Note on my counterexample above. Of course, it is true that the particle exerts an equal and opposite force on the plates producing the electric field, but that's just an example of Newton's Third Law. (Since your statement doesn't say what the force applies to, it might be read as synonymous with that law--which would mean it was correct, but not new.) -- SCZenz 18:51, 8 September 2005 (UTC)
Ok But were you aware that Newtons 3rd law can be applied to every physical force in the universe?. I was not. I was used to only thinkng of it in the terms of mechanics (as was Newton of course). Light current 22:23, 8 September 2005 (UTC)
I think that comment is a little harsh AC,. After all, it is merely a generalisation of Newtons 3rd law as SCZenz has pointed out and is therfore not nonsense. I'm sure you accept Newtons laws as valid. -- Light current 23:41, 8 September 2005 (UTC)
A) Apple on the ground. Ground reacts to gravity force by induced force pushing up. B) Fluids react to displacement by induced upthrust. C) Electric Flux is induced by a charge. -- Light current 22:38, 8 September 2005 (UTC)
Take a rocket in space. Force on rocket F=ma. this is also equal to the force on the exhausted material. Momentum is conserved in the system. since Force = rate of change of momentum and momentum is not changing, then yes there is no net force in the system but the rocket still accelerates -- Light current 22:51, 8 September 2005 (UTC)
Correct. Would you care to expand on that?-- Light current 22:57, 8 September 2005 (UTC)
No I think I was trying to say that all forces are matched. SCZenz puts it very well in his last post. -- Light current 22:55, 8 September 2005 (UTC)
Newtons third law is applicable to every conceivable force in the universe. -- Light current 23:12, 8 September 2005 (UTC)
OR THe sum of forces in a closed system is zero -- Light current 23:29, 8 September 2005 (UTC)
Thanks for the compliment. Yes I do see what you mean. Correct: sum of all forces in universe is zero. I think I'll quit while I'm ahead! -- Light current 00:09, 9 September 2005 (UTC)
If I'd seen this [2] earlier then we could have made a few short cuts. C'est la vie!
Yes,you are correct. If it aint hurtin', it aint workin'. And no pain - no gain! etc -- Light current 01:22, 9 September 2005 (UTC)
Im sorry to disagree with you but planets in orbit do have a balancing force. Its called centrifugal force. This is what stops then falling into the sun!. Light current 01:08, 8 September 2005 (UTC)
You don't even have to use a rotating frame to see a fictitious force. When you step on the accelerator in your car, the acceleration is forward (hopefully) yet you feel as if a force pushes you backward into your seat. That is acceleration forward 'feels' like a force is pushing backwards. This force you 'feel' doesn't exist - it is fictitious. Now, to make the connection to GR, imagine that you are sitting in a car seat that is facing up. You feel as if you are being pushed back into your seat with a force equal to your weight. But, by the above reasoning, this implies that you are being accelerated up. Yet, you know that you are stationary on the surface of the Earth. According to GR, (this will raise some eyebrows) spacetime itself is accelerating downwards toward the center of the Earth. To be 'stationary' in spacetime would mean that you would also (free) fall towards the center of the Earth. Since the surface of the Earth prevents you from doing this, the surface of the Earth is, in effect, accelerating you. That is, your worldline is not on a geodesic of the spacetime. Wait -what's that I smell? Is it LC's brain short circuiting??? ;<) Alfred Centauri 02:52, 8 September 2005 (UTC)
I would like to avoid the quantum mechanical aspects at the moment if you are agreeable. The reason being that I know very little about quantum mechanics and I feel that the general law of induction should be established or debunked at the macroscopic level first. Light current 01:55, 8 September 2005 (UTC)
That doesn't sound right. When you squeeze something (a solid object, for example) it is the electrons that provide the reaction force. That is, the Exchange interaction that occurs on attempt to force electrons into the same quantum state. Gah4 ( talk) 01:47, 29 November 2009 (UTC)
http://fr.wikipedia.org/wiki/Inductance#Puissance_emmagasin.C3.A9e
pf:
-- HydrogenSu 10:52, 25 December 2005 (UTC)
I just wanted to put up a couple of my sources for putting up some transformer equations in the mutual inductance section. V_secondary = V_primary N_secondary/N_primary. And convsersely the curreent I_secondary = I_primary * N_primary/N_secondary
Fresheneesz 09:49, 14 April 2006 (UTC)
The symbol for current is (I), so why is (i) used here instead? GoldenBoar 18:40, 8 May 2006 (UTC)
I'm no expert, but the old book I have (1937) states that L stands for "Linked Flux", it is not homage to Lenz. I don't want to just go and arbitrarily edit the page, but here is my reference.
p.291 Electrical Engineering Vol 1 Chester L. Dawes, S.B., A.M. McGraw-Hill —The preceding unsigned comment was added by Ninthbit ( talk • contribs) 22:52, 9 February 2007 (UTC).
I have been playing with the formula for self-inductance L of a solenoid, and find that it can usefully be rearranged in terms of the lengths of the wire and the solenoid. Note that the formulae ignore end-effects, so are approximate.
which simplifies to
For an air-cored coil this simplifies further to:
where
Nice, eh? I don't suppose it's original but I haven't seen it elsewhere. I don't think I have made a mistake. GilesW 22:05, 31 May 2007 (UTC)
REAL WORLD PROBLEM! —Preceding unsigned comment added by 167.102.133.216 ( talk) 19:57, 18 January 2008 (UTC)
Yes, it's very nice, and exactly what I was looking for! Now, maybe you can help me with a real-world problem. I am erecting shortwave antennas. I find that my dipole antenna is resonant at too high a frequency. I know that I could add length to my dipole to lower the frequency. But that would mean getting out the soldering iron and doing repairs outside. I rather just add a lumped inductance somewhere in the middle of each arm of the dipole by taking the antenna wire and rolling it around a piece of plastic pipe. My question is will doing this lower the resonant frequency? Because as I roll the inductor, I am reducing the length of the antenna by the same amount. Specifically, the antenna is now resonant at about 5.0 MHz, and I would like it to be resonant at 3.8 MHz. If anyone takes up this challenge, please email me at jcotton@excite.com and let me know that this has been attempted. Thanks for looking —Preceding unsigned comment added by 167.102.133.216 ( talk) 19:55, 18 January 2008 (UTC)
I don't know specifically about that one, but in many cases the wire that goes into the inductor is exactly the right amount that you get the same frequency. Well, in the case of coaxial cables with helical center conductors, the inductance increase results in slower propagation of the EM wave. That comes out the same as if you consider the length of unwound helix. Gah4 ( talk) 01:53, 29 November 2009 (UTC)
Section: Inductance of a solenoid
The equation for inductance displays as L = (μ0 / μr)(N^2 * A) / l, which looks incorrect. When it confused my derivation of relative permeability (μr), I realized there might be a mistake. L = (μ0 * μr * N^2 * A) / l yields the correct results, which I confirmed by deriving μ directly from B/H. I consulted both electronic machinery (transformer) text and electromagnetic text. I am new to this, so I thought someone else should look at it and confirm this before any editing. Thank you for your help. Amanda.Tighe 18:14, 4 June 2007 (UTC)
The page is inconsistent about the use of vs. . I'd suggest explaining that distinction in the intro and getting mutual inductance out of the intro, to be explained later. Ccrrccrr 11:43, 3 August 2007 (UTC)
To be really useful, there should be a table with formulas for wire pair, wire plus wall, coaxial cable (with and without skin effect), rectangle, ring, solenoid ...
BTW, the style of some sections should be checked and corrected by someone with native english, it sounds queer to me. —Preceding unsigned comment added by 84.56.8.130 ( talk) 16:42, 4 November 2007 (UTC)
This is redundant to the section 'Vector field theory derivation', it is an intermediate step to derive the von Neumann formula, and the expression gets singular in the self inductance case. —Preceding unsigned comment added by Rdengler ( talk • contribs) 16:50, 24 November 2007 (UTC)
CONCERNING the formula for Single layer solenoid: I have no idea what the term with the O is? Is it a zero? If so, why include it at all? Is it another term? If so, what? —Preceding unsigned comment added by 167.102.133.216 ( talk) 20:27, 18 January 2008 (UTC)
This is a standard notation in mathematics, see Big O Notation. The complete expression is
This is the inductance of a cylinder with a constant current around its surface.
—Preceding unsigned comment added by Rdengler ( talk • contribs) 17:02, 21 January 2008 (UTC)
The formula for inductance of pair of parallel wires applies only for d>>2a, i.e. no proximity effect. The formula for inductance of parallel wires, high frequency, incorporates the proximity effect. 192.91.147.35 ( talk) 21:16, 30 January 2008 (UTC)
The inductance of a pair of parallel wires exactly is in the low frequency case for all d>2a - there is no proximity or skin effect.
I have removed 'per coil turn' from the definition of inductance.
What is the magnetig flux through a circuit consisting of a thin wire? It is proportional to the number of flux lines that have crossed the wire and now are "captured". This implies that the flux through a circuit with n turns is n times the flux through one turn. Defining the flux geometrically (area times field strenght) leads to inconsistencies and complications. Consider a circular loop with two turns in a constant magnetic field and deform the circuit to a shape looking like an 8. The flux doesn't change.
The magnetic flux through a circuit with a continuous current distribution may be defined by replacing the current distribution with a bunch of thin wires, each carrying a small current. —Preceding unsigned comment added by Rdengler ( talk • contribs) 07:41, 19 August 2008 (UTC)
Ths page does a great job explaining the complex concept of inductance, yet I beleive the most important thing to mention is that inductance is the ratio of magnetic flux created by an inductor to the change in current that induces it.
The formula
L = Φ/i
where L is the inductance in Henries, Φ is magnetic flux in Webers, and i is the current in Amperes, is the simplest formula I know to give a rudimentary understanding of inductance. Is this correct? If it is, I will add it to the page to allow beginners to understand better. -- Skyfinity ( talk) 23:07, 29 November 2008 (UTC)
Here is another comment to the same point. A definition in terms of magnetic flux and current turns the problem on its head. There is a simple definition for flux only in the case of thin wires - and even this case cannot be realized exactly experimentally. A generic definition of inductance is possible in terms of change of current and induced voltage or in terms of the energy of the magnetic field. The connection with the magnetic flux may be useful to illustrate some aspects of electromagnetism. It is not useful as a definition. —Preceding unsigned comment added by Rdengler ( talk • contribs) 16:51, 2 July 2009 (UTC)
In the diagram, the arcs that form the symbols of the inductors are the wrong way round. The convex sides of the arcs should face each other.
Where a single inductor with a core is depicted, the arcs face the core depicted by parallel lines.
I was brought up on BS3939, superseded many years ago by EN60617-4 and the BS version of it.
see: EN60617-4:1996 Graphical symbols for diagrams. Basic passive components ( ISBN 058026745 8). There are numerous engineering publications that use those symbols, mostly correctly(!). GilesW ( talk) 21:35, 15 January 2009 (UTC)
I think 'Coupled inductors' would be more technically correct than 'Mutually inducting inductors' in the text of the diagram (and anywhere else, if applicable). GilesW ( talk) 21:41, 15 January 2009 (UTC)
Could the page include a reference to the units of measurement, and practical help for those who might actually want to use the equations? I assumed MKS units, which seems reasonable in the context of Wikipedia. But when I calculated the inductance of a loop with a radius of 10mm I got 0.049 Henry - obviously a ridiculous result. I had to multiply this by mu-zero to get the much more reasonable answer of 6.2e-8 Henry. Jay.sinnett ( talk) 21:21, 26 June 2009 (UTC)
Can someone add a power series expansion of the solenoid expr terms of w^-1 in the limit w==>inf (i.e. short solenoid limit) Thanks! Woz2 ( talk) 14:30, 9 September 2009 (UTC)
There is a dispute about a superfluous factor -1 in the series expansion of the single layer solenoid inductance. First of all, the -1 (inside the square) is unnatural, useless and very confusing. More to point, the product in the numerator of the term with index m consists of (m-1) factors. For example, for m = 2 it simply is (2m-3) = 1, for m = 3 it is 1*(2m-3) = 1*3. For m = 1 the product thus consists of 0 factors, which is 1 by convention everywhere in mathematics. The only natural thing thus is to remove the factor -1. Of course, it arises if one sets m = 1 in (2m-3). However, there are 0 factors in the numerator of the (m=1)-term. The formalism is clever, and the result looks nice and natural!
Here is a link to a reference with details: http://home.arcor.de/rdengler/Transpo.pdf. Looking for official/original references for the article...
The article still is somewhat incoherent - the capacitance article is superior in this respect.
1) The section "Phasor circuit analysis and impedance" doesn't explain anything about inductance. Should be explained under "phasors" or elsewhere. If there are no objections then I shall remove it.
2) The section "Induced emf" has trivial and clumsy content. If there are no objections then I shall remove it.
3) The sub-sections under "Self-inductance of simple electrical circuits in air" (Inductance of a solenoid, Inductance of a coaxial line) should be moved to a new section "Solenoid and coaxial cable in detail" (or something like that), or, possibly, to other articles (coaxial cable, solenoid), with a link added here. Rdengler ( talk) 16:35, 7 November 2009 (UTC)
I have reverted this edit which has the edit summary changed "wire loop" to coil of wire. The quoted formula contains N respresenting a number of turns of wire. For a wire loop N is 1 and would not be included in the formula. I agree that the use of loop here is problematic and could , perhaps, be improved, but coil of wire is even more so. I think the definition only works if all the turns are essentially co-incident - that is, lying on the same loop. Furthermore, the change was not done consistently, the section continues to talk about wire loop further down. SpinningSpark 16:29, 9 August 2010 (UTC)
The equation for the inductance of a thin solenoid is not correct; one should use Babic and Akyel, Improvement in calculation of the self- and mutual inductance of thin-wall solenoids, eq. (8), IEEE Trans on Magnetics, Vol. 36, No. 4. July 2000 Prof. J.C. Compter —Preceding unsigned comment added by 194.25.102.189 ( talk) 10:04, 2 September 2010 (UTC)
The Lorenz expression for the inductance of a coil is the inductance of a cylinder with a current around its surface (might be indicated in a footnote), and as such is as exact as Maxwell's equations. Improvements (wire or coil thickness, wire spacing) are more complicated and less instructive. B&A use numerical methods. Appears that FEM and numerical methods should be mentioned (with references) under calculation techniques. —Preceding unsigned comment added by Rdengler ( talk • contribs) 08:21, 4 September 2010 (UTC)
In the opening paragraph, I find the following sentence to be confusing: "This is a linear relation between voltage and current akin to Ohm's law, but with an extra time derivate." In fact, the voltage across an inductor is not proportional to the current -- it is 90º out of phase with it if the reactance is perfectly inductive. Jdlawlis ( talk) 01:43, 18 December 2010 (UTC)
-Shouldn't be confusing, taking the time derivative is a linear operation, the statement is mathematically correct. radical_in_all_things ( talk) 08:30, 19 December 2010 (UTC)
The statement is not mathematically correct. It would be correct if it were written: "This is a linear relationship between voltage and the rate of change of current akin to Ohm's law, except that current is replaced by its derivative." The fact that the derivative is a linear operation does not relate to this particular issue. As an example, take an ideal RL circuit with an AC generator. Let be the voltage generated by the AC generator. It follows that the current , where the impedance and the phase constant . The voltage drop across the inductor, . If the voltage across the inductor were indeed proportional to the current, you would be able to multiply the current times a constant to achieve the voltage. Multiplying a cosine function times a constant will only change its amplitude -- it cannot transform it into a sine function. Hence the voltage across the inductor is not proportional to the current through the circuit. These equations can be found in any introductory E&M textbook such as Tipler or Purcell. Jdlawlis ( talk) 23:36, 20 April 2011 (UTC)
The section on "coupled inductors" has a paragraph at the bottom about tuned circuits, starting "When either side of the transformer is a tuned circuit, the amount...". This paragraph is not referenced and may be original research. Does anyone know where this material comes from? —Preceding unsigned comment added by 121.98.140.35 ( talk) 00:01, 20 May 2011 (UTC)
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After all, what use is it to anyone? Someone who already understands this does not need to turn to this article. And those who have no idea but want a basic introductory explanation of what it is are left out. Great, so it is magnetic flux. Ok, now I go to that page and am inundated with another page full of abstruse verbage. Around I go, grasping at circular references, looking for some kind of conceptual foothold to begin with, but left with nothing but abstract mathematical symbols and explanations that may as well be in Greek. I mean, electical engineers aren't the only people interested in what this stuff is.
I couldn't agree more. This is supposed to be an encyclopedia entry, not a college level course. I had exactly the same problem you did. All I wanted was a basic explanation that was a little more complete than my vague grasp of the principles, but instead was bombarded with advanced material that belongs in a technical reference, with every link leading to more of the same. Most of the people who contributed to this ought to go find somewhere else to circle-jerk. 208.42.94.130 ( talk) 08:25, 14 November 2008 (UTC)
Could someone please explain inductance in layman's terms? —Preceding unsigned comment added by 68.121.33.140 ( talk) 03:02, 20 December 2008 (UTC)
It is harder to write a simple explanation of inductance than even its dual, capacitance. Capacitance can easily be explained in terms of slowly varying fields. Also, inductance tends to appear in less than ideal situations more often than, for example, resistance or capacitance. In terms of real circuits operating at less than radio frequencies, it is fairly easy to avoid stray capacitance but somewhat harder to avoid stray inductance. As others have said, a voltage generated (induced) due to a change in current. For self inductance it is a change in current in the same conductor, for mutual inductance in a different conductor. Gah4 ( talk) 23:40, 2 July 2009 (UTC)
Let me amplify this a bit. I have no problem with being presented with the math, eventually, but if I am trying to understand the subject, I need a handhold to grasp to get my feet up off the ground and start climbing. It takes a few more words and metaphors to get this across to someone who doesn't already know the subject intimately, and it only takes a quick reference card to get the subject across to those who already get it. So, if someone who was able to claw their way through the math and jargon to a real conceptual understanding of this material would be willing to spend a little time to help bring the rest of us along, it would be great. The power of Wiki is that the space is not really limited by anything, so this does not need to be an either-or proposition, please keep the technical depth, but provide a layman's framework in which to learn and understand the material as well. Visierl ( talk) 14:42, 26 December 2008 (UTC)
For many things, possibly including inductance, it gets more complicated, and takes more words, as you get deeper into the subject. A favorite example is friction. Friction is commonly taught in first year physics classes, yet rarely taught after that. If you try for a deeper understanding than in the first year course it gets very complicated fast. One complication with friction is that the more ideal the system (two pieces of the same material without grease or other impurities) the less ideal it gets (they tend to stick together greatly increasing the friction constant). Gah4 ( talk) 23:40, 2 July 2009 (UTC)
The equation on the inductance of a solenoid states that it is inversely proportional to "l" (the length of the solenoid). This implies that inductance would be infinite for a zero-length solenoid. Certainly a single-winding solenoid could be made with a length arbitrarily close to 0. I think this equation is an approximation with assumptions (eg. ). Could these assumptions please be stated? Jurgen Hissen 21:23, 18 May 2006 (UTC)
Notice the factor of N^2 in the numerator. Ordinarily when winding a solenoid with real wire, each turn takes up the same amount of length, so the number of turns is proportional to the total length l. As l decreases, N normally decreases in step. Take one of the factors of N from the numerator and l from the denominator to form (N/l). That is ordinarily a constant.
But in the limit of an infinitely thin wire, the inductance of that wire really does go to infinity, even without being wound into a coil. Fortunately that is an abstraction never met in the real world.
I'm actually more worried that the inductance formulae in the table all lack a factor of mu_zero, so something could at least be better explained.
User Phil Ekstrom 20 May 2009 —Preceding unsigned comment added by 207.32.171.65 ( talk) 02:36, 21 May 2009 (UTC)
This page needs a picture of an inductor, and a drawing representation of it (showing turns), and the usual circuit representation for it. Maybe I'll draw these sometime. Fresheneesz 07:47, 22 November 2005 (UTC)
Mutual inductance needs a picture, needs to explain the dot convention, and give a picture for this. I might make these sometime as well. Also, mutual inductance need not occur between circuits - but can occur within circuit. Two nearby inductors can even have mutual inductance while remaining also in series. Fresheneesz 07:47, 22 November 2005 (UTC)
I can't even read the first sentence of this article. Shouldn't it read something like, inductance is the property of a device to produce a voltage proportional to a varying electrical current?
The expression given at the beginning, is only correct for a circuit with a fixed geometry. If the circuit is contorted while current is flowing through it, then the change in geometry creates a back-emf, also, as shown below.
(same poster) is the above assertion true? I have found no occurance in any reference texts.
I suppose one could look at it that way.
But the traditional way of analyzing motor/generators is to assume a fixed inductance for each part (the inductance of the rotating coil, and the inductance of the stationary coil), and calculating a separate "back emf" as a function of the speed of the motor.
I've always thought of a motor as "two fixed-geometry inductors that move relative to each other".
I've honestly never thought about considering a motor as "one big variable-geometry inductor". It's a perfectly self-consistent way to look at it.
But if I had some spinning motor, and someone asked me for "the inductance" of the motor, I'd probably just measure the voltage and current and plug in
, completely ignoring the "variable inductance" of the motor taken as a whole.
-- DavidCary 08:01, 29 Oct 2004 (UTC)
What is R in the mutual inductance equation? It also would help to clarify that ds is the increment along the curve.
-- Colin
I probably agree that describing a motor in terms of d(LI)/dt isn't the best way, but it can be important for solenoids. There was a discussion on comp.dsp (a little off topic) on solenoids driven by SCRs, and the dL/dt term is important. Gah4 ( talk) 01:29, 29 November 2009 (UTC)
Is there a reason lower-case 'i' is used to denote the current here?
I just pluralized "henry" and noted that I typed it "henries". But another appearance of the plural later on in the article was spelled "henrys". Is there an official plural for this unit? (Or is this like the ever-simmering British English versus American English debate?) If you know, could you please edit the article to make the two plurals consistent?
Atlant 11:42, 15 Jun 2005 (UTC)
I'm still missing a feeling for what actually mutual inductance is. Is it something we want to have in a transformer? What would happen if it was very small (i.e. MI is zero), what, if it was a very big number (mutual inductance going to infinity)? Thank you, -- Abdull 08:00, 21 July 2005 (UTC)
I'm not sure why the merge request was made, but I think it's clear from the articles (or any physics textbook) that they're two separate concepts. -- SCZenz 14:21, 31 August 2005 (UTC)
Yes, and also the toasting of toast is due to a toaster. That doesn't mean they're not separate concepts, it just means they're related concepts--so they have separate articles that link to each other. -- SCZenz 22:32, 31 August 2005 (UTC)
I propose that the defn should read;
''Inductance is that property of a conductor (including an inductor) that admits a current through it that is proportional to the time integral of the voltage applied across it.
I know this seems more complicated but I believe it is more relevant to isolated inductances (not mutual inductance or induced voltages) and avoids the confusion of induced 'back emf' . Also the usual causal relationship is that you apply a voltage to the inducatnce in the first place but only then does the current ramp up. A current cannot magically already exist in an isolated indutance with no external changing magnetic field. Any comments on this idea?? Light current 15:49, 1 September 2005 (UTC)
Any comments? Light current 16:18, 1 September 2005 (UTC)
Not to throw a monkey wrench into this general agreement, but consider the case of a device that has reactive properties like a traditional inductor or capacitor, but which does not use magnetic or electrical field mechanisms to do so. Would people say that "That's not an inductance" just because it doesn't use a magnetic field, or with that in mind would the above suggestion have merit? Would a better rephrase of the corresponding capacitance description not be "Inductance is a measure of the amount of electric potential stored for a given electrical current?" (Though in both cases I'd say it should be "stored/required" not just "stored.") A quick followup statement could establish that magnetic fields are the common method of storage, and present the flux-based definition. (Oh, and yes, to Light Current, a current can indeed pre-exist in an isolated inductor with a static magnetic field, for example an SMES.) ( 71.233.165.69 00:41, 26 June 2006 (UTC))
According to fundamental principles, the inductance L is defined as:
Does it follow that:
Well, let's see...
Moral of the story - use the the formula from first principles or if not, state your assumptions, e.g.:
Besides, the opening sentence refers to magnetic flux and current, not their differentials. I recommend beginning with the (always correct) non-differential form and then derive the simplified differential form by explicitly stating the caveat 'for constant L'. Alfred Centauri 22:58, 3 September 2005 (UTC)
I disagree. There are many assumptions that are used in many fields to make the study practical. One is that, for the most part, inductance doesn't change fast compared to the current changes. If one had to go around stating assumptions all the time, there would be no time to actually get anything done. Instead, state when the common assumptions do not apply.
Consider Ohm's law: V=IR. Sounds simple enough, and works well most of the time. Note no indication of the assumption of the low inductance of the resistive material. Many electrical components have less than ideal properties, but are useful within the limitations of those properties. One applicable here, inductors tend to have non-negligible resistance. Wirewound resistors have significant inductance restricting their use at higher frequencies. Sometimes those need to be stated, other times they don't. Gah4 ( talk) 23:10, 2 July 2009 (UTC)
Consider this. If L varies with i and i varies with time, L is an implicit function of time so dL/dt is non-zero. Conversely, if L is a function of time, dL/di is only zero if i is constant. Put another way, if L is constant and i varies, we get v= Ldi/dt. If i is constant and L varies, we get v = idL/dt. If both L and i are functions of time, we get the sum of the terms. Bottom line, L only equals dphi/di if L is constant! Alfred Centauri 01:46, 4 September 2005 (UTC)
Could AC state the physical quantity to which lambda refers. Its new to me. Light current 01:09, 4 September 2005 (UTC)
The "Definition" section is mathematically correct, but it may be misleading to the casual reader. It implies that the inductance of a coil is simply proportional to the number of turns where, in fact, it is proportional to the turns squared. This is because the flux term has a "hidden" ampere-turns term which causes the current to cancel so that the inductance of an air coil becomes a function of turns squared, permeability, and coil geometry. Although this gets cleared up in the formula for an N-turn solenoid in the "Permeability" section, I think this example should be brought up to the "Definitions" section to make it clearer (similar to the way "Capacitance" is defined in the Wiki article in terms of permittivity, area and separation distance). I made a number of changes that, hopefully address this - added an "Inductace of Solenoid" section and removed the "Permeability" section (since there's already a Wiki article on permeability). Feel free to mercilessly change as appropriate... Bert 13:25, 10 April 2006 (UTC)
I would argue that any counter emf is identical to the applied emf in all cases where there is no externally generated magnetic field impinging on the coil. From this stand point I maintain that this counter emf is in fact ficticious. There is only one emf acting across a coil --- that is the applied emf - end of story. The applied emf is the cause, and the current and resulting magnetic field are the effects. The applied emf is countered by the reactance of the coil (giving the effect of inductance) not by some magical 'back emf'. The back emf story would be just like saying that the emf of a battery connected to a resistor is counteracted by a back p.d generated by the resistor somehow. Comments invited. Light current 04:19, 4 September 2005 (UTC)
I suppose this is an old discussion by now, but it seems that the counter emf is important in cases where the magnetic field causes changes that affect the original circuit. In the case of a solenoid actuator, the field pulls a piece of magnetic material into the core, which changes the field seen by the coil. This is the cause of the dL/dt term, from V=d(LI)/dt. Gah4 ( talk) 00:12, 17 February 2010 (UTC)
This can be true because the wire has some inductance and the voltage is applied 'suddenly'. The case 'O' and I were dicussing had zero inductance Light current 18:56, 4 September 2005 (UTC)
I know. But in our thought experiment we were (at least I was) assuming an infinitely short, short circuit?? and a zero loop area. Do you buy that one? ;-) Light current 20:12, 4 September 2005 (UTC)
Agreed Light current 18:56, 4 September 2005 (UTC)
Agreed Light current 18:56, 4 September 2005 (UTC)
Agreed - this is the voltage that the current source generates to drive its current thro the inductance of the loop which we have both agreed exists. Light current 18:56, 4 September 2005 (UTC)
208.61.125.43 20:09, 4 September 2005 (UTC)
Maybe if you look at it the other way around. Does an ideal voltage source generate current? If not, where does the current come from. The way I look at it, the ideal voltage source has to generate the appropriate current to keep the voltage constant. The ideal current source generates the appropriate voltage to keep the current constant. Both have to be able to supply power to the load so I think generate is a fine description. Gah4 ( talk) 00:40, 4 December 2008 (UTC)
I know it generates current and the voltage existing on the resistor is a by product of that current- but the prime mover in this case is still the current source. Why invent an independent source of voltage rthat would not exist if it were not for the current source. If feel this line of argument could take us up a blind alley with no way out! BTW 208.61.125.43 are you a new user or just forgotten to log in?? Light current 20:22, 4 September 2005 (UTC)
Here is where we start to disagree (I think). The voltage across the resistor is identically equal to the voltage of the source (KVL). BUT -- the causal relationship is that the voltage source must first have a voltage at its terminals in order to drive current thro the resistor to create the pd (which of course is the original emf of the source). This is the circular argument to which I refer.!! THe voltage source is Boss, the resistor is completely passive. Light current 18:56, 4 September 2005 (UTC)
No applied voltage/ current from the source. Therefore, voltage spike comes from energy stored in magnetic field. No problem! Light current 20:28, 4 September 2005 (UTC)
Yes, this is the normal action of a resistor. In this case the prime mover is a current source and the resistor has no choice but to develop a pd across itself. equal to IR Light current
Which came first; chicken or egg.?? Light current 00:53, 5 September 2005 (UTC)
Again, perhaps I have not made myself clear in the article and my previous 'rantings'!. I did not say that induced voltage was ficticious (ie voltage induced from an external changing magnetic field) - I said that any concept of 'back emf' in an isolated inductor generating its own changing magnetic field due to an externally applied voltage is a myth. How can you say that the voltage you apply to a lone inductor is induced? It is not - it is applied! After all there can be only one emf/voltage appearing across the ends of an indutor of inductance at any one time. 88.110.246.253 18:56, 4 September 2005 (UTC)
I Agree that we should follow Uncle Alberts maxim (everthing should be as simple as possible -- but no simpler). Here I think the idea of 'back emf' (which I believe has not been taught for over 30 years now at university level), is confusing to everyone (including me). Its a lot simpler to say that for an Isolated inductor connected to a voltage source, V, that V=Ldi/dt. The sign is covered by the sign of di/dt. ie increasing current requires a positive voltage to be applied and to cause the current to decrease requires a negative voltage to be applied. Is this not correct? 18:56, 4 September 2005 (UTC) Light current 18:59, 4 September 2005 (UTC)
The emf is induced in the inductor by its collapsing magnetic field. I have no problem with that. But remember that there is only one source of energy here: in the mag field and cct follows the induction law as if the field were created by another coil somewhere. BTW my spelling is always bad cos I try to type too fast! Light current 20:37, 4 September 2005 (UTC)
In para 3 of Properties of inductance we ahve a completely circular argument. This para seems to be therefore patent nonsense Light current 04:25, 4 September 2005 (UTC)
If, after I have had a chance to explain my thoughts, you still wish to revert the article, I would ask that you copy the controversial material here so that any other interested parties can still see it and comment on it. Thanks Light current 19:07, 4 September 2005 (UTC)
OK I'm going to read that now Light current 19:07, 4 September 2005 (UTC)
I have read that article now, and I think I agree with all of it. But the so called negative sign business is , I assume, the fact that a battery has an opposing electric field across its terminals when open circuited. This is true. However', the opposing electric field would not be there if the battery was not there (causality) . This also reminds me of Newtons Law of motion (cant remember which one) that action and reaction are equal and opposite Light current 19:19, 4 September 2005 (UTC)
Yes I think I agree with this, although I don't really think its relevant to my argument about inductance. Light current 19:24, 4 September 2005 (UTC)
Here is something I wrote for the Electromotive force article:
As there is no external circuit , Kirchoffs voltage does not apply, and hence the sum of the voltage around the circuit (which does not exist) does not need to equal zero. The "emf" is simply an "emf" which has the potential to motive electrons around circuit.
I would hope that the idea of dropping the term "emf" never eventuates. There is no other suitable word(s) to replace it, for the concept that it conveys. IE a "force that motivates electrons!" The term "emf" is not directly interchangeable with "voltage produced by electromagnetic induction" or back emf. There is a big difference.
The volt of course is the unit of emf , potential difference or voltage drop. It is in fact the term "voltage" and likewise the word "amperage" which are badly abused. Extending the logic of doing away with emf is akin to doing away with "mmf"(magneto motive force" What a horrible thought!
If I place a resistor accross the emf, are you als implying that the "voltage" accross the resistor will stop the voltage flow.
The emf , will still be there and there will be a voltage drop accross the resistor. We have a circuit and KVL must be (and will be) satisfied.
Agree --but cant see relevance to my argument about an isolated inductor :-) Light current 19:27, 4 September 2005 (UTC)
Just having looked at the page for the first time since you altered it, I am quite relieved that, (expecting the worst after reading your posts), you have left so much of my material there. Thanks!:-). As you know I'm still confused by the minus sign and the page explanation does nothing to help me. Why must we refer to two voltages equal but of opposite sign when one would do (as the sign is already included in the di/dt or the dØ/dt)? The second one is identically equal to the first in the case of an isolated inductor fed by a switched voltage source -- is it not? The building of the magnetic field does indeed present reactance to the voltage source and this is what we expect from an inductor. However, to say that the 'reactance' is caused by the changing magnetic field the coil is in the process of generating - and this then creates another voltage equal in magnitude but opposite in sense to the applied one, is IMHO an unneccessarily complicated and circular argument for presentation to engineering (or even physics) students Light current 19:53, 4 September 2005 (UTC)
I'm going to agree with that for now. You mean the general integral form of Maxwells equs (from Faradays Law) Presumably you mean magnetic flux density- but lets not argue about that. I have to agree with your above statement.
Im not trying to fuss or complain, I'm just saying that the difference between 'applied emf' and 'induced emf' is extremely confused and needs clarifying. They are (necessarily?) opposite in sign. If we can come up with unambiguous, simple definitions for capacitance, inductance and compare and contrast their similarities/ differences we will have done what most textbooks on electrical engineering have failed to do so far. 88.110.246.253 23:40, 4 September 2005 (UTC)
<!del--- Forgive me butting in here. I agree that the non conservative field is what confuses people. It confuses me. Even the name is intimidating. Is there a simpler name such as changing, not steady or something more friendly that could be used?? Light current 15:02, 5 September 2005 (UTC)--->
Ive just looked at my college notes on Field Theory and guess what? - the topic of non conservative fields is not covered nor even mentioned!. No wonder I've been having trouble all these years understanding electromagnetic induction! Light current 16:49, 7 September 2005 (UTC)
Isnt this just because the field is changing with time? Light current 15:04, 5 September 2005 (UTC)
Before I try to digest the above answer, could you say whether you agree that the sign of any induced voltage is (in some part) determined by the sign of di/dt or dØ/dt? Thanks Light current 22:06, 4 September 2005 (UTC)
I assume your reply means that you agree (not that you could say whether you do or not)Sorry to be pedantic! :-) Light current 23:07, 4 September 2005 (UTC)
Ive just been looking at the page again. The statement I have trouble with is:
where is the Electromotive force (emf) and is the induced voltage. Note that the emf is opposite to the induced voltage.
You see, the way I see it, neither one is defined and the difference between these 2 voltages is not stated. It probably makes complete, unambiguous sense to you, but I'm afraid it doesnt to me (Im not saying its wrong -- just that I dont undersatnd it as it stands). If we were to define what we mean by:
a) the emf
b) the induced voltage
maybe our problems would go away!! What think you? Light current 00:10, 5 September 2005 (UTC)
Is this pd not obviously zero!? Light current 13:30, 5 September 2005 (UTC)
Sorry ,I missed the point about the field changing. You are absolutely correct. Apologies! Light current 14:23, 5 September 2005 (UTC)
OK Im beginning to see that:
a) (we must accept this from previous discussion and Mawells laws etc)
b) the induced emf can exist even with no moveable charges present (ie in an insulator)--
c) If, however, a source of moveable charges exists, these charges will be acted upon by the emf and they will move(necessarily doing work- but lets forget about work at the moment)) to counteract the emf.
d) the induced emf acts like a battery connected to a conductor with the conductor representing the body containing the moveable charges.
e) Therefore, the voltage induced in the conductor must be in opposite sense to the emf in order to obey KVL
Is this a fair summary of your argument so far? Light current 13:52, 5 September 2005 (UTC)
Consider a ring of resistive material with uniform resistivity. Assume that there is a magnetic flux changing at a constant rate through the surface area enclosed by this ring. This changing magnetic flux induces a constant electric field that in turn drives a constant current through the ring of resistive material.
Recall that the line integral of the induced electric field around the path traced by the ring gives the emf (in volts) induced by the changing magnetic flux.
Answer the following questions:
(1) Is it possible to measure the total emf directly with a voltmeter? If so, explain how to do this. If not, propose how to measure the total emf.
It is my opinion that a voltmeter will read zero volts between any two points on the ring as long as the ring is unbroken. This result is the only result consistent with KVL and question 2. Thus, I believe that your 'second try' answer is correct. Alfred Centauri 23:54, 6 September 2005 (UTC)
I disagree. The voltage at all points is the same, but that doesn' mean that is what the voltmeter will read. Put one probe on a spot on the ring. Put the second probe on the same spot, as expected the voltage will be zero. Now slowly move the second probe around the ring, and the voltage will slowly increase until the probe gets back to the beginning. Now you don't need the ring anymore, but note that the flux is going through a loop made up of the voltmeter probe leads. This solution is similar to the problem of contour integration around a singularity. Gah4 ( talk) 21:42, 15 October 2009 (UTC)
(2) KVL says that the sum of the voltage rises and voltage drops around a circuit must equal zero. Does KVL apply here? Explain your answer.
It is true that emf is not a point function precisely because it is defined as the integral of the electric field along a closed path. However, KVL is directly aimed at emf because it is emf that drives charge around a closed circuit. A conservative electric field cannot do this! Nonetheless, your answer is correct otherwise and consistent with the idea that a voltmeter reads zero volts between any two points on the ring for the very reason you gave that the emf is exactly balanced by Idr. Alfred Centauri 00:16, 7 September 2005 (UTC)
(3) Assume the current in the ring is anti-clockwise. If a voltage source were instantly inserted in series with the ring, what is the value and polarity of the voltage required to stop the current in the ring?
Correct. The voltage source should generate a current equal and opposite to the current driven by the emf. Now, doesn't this bother you? After all, you can now measure a voltage across the resistive ring but there is zero current!!!. What happened to Ohm's law? Alfred Centauri 00:19, 7 September 2005 (UTC)
(4) With this voltage source in circuit and with zero current in the ring, magically adjust the resistivity of the material to one-half its former value. Does the current change? Explain. If the current does change, what is the new value of the current?
Correct. Alfred Centauri 00:22, 7 September 2005 (UTC)
(5) Answer (4) again with the resistivity magically adjusted to zero.
Again, correct. But once again, doesn't this bother you? You have a voltage across a short circuit and zero current! How can you explain this? Alfred Centauri 00:22, 7 September 2005 (UTC)
You have to be very careful measuring voltages with changing currents around. The meter leads are unavoidably part of the measuring circuit. Gah4 ( talk) 21:42, 15 October 2009 (UTC)
Doesn't bother me as much as it would have afew days ago! There is an emf generated in this short cct by the changing mag flux. If this emf were not cancelled by my inserted voltage source, then I presume an infinite current would flow. Is this the same thing as a s/c turn in a transformer? The only weird thing to me now is that you can have an emf exist in s/c! Light current 00:49, 7 September 2005 (UTC)
Alfred Centauri 13:49, 5 September 2005 (UTC)
Now that you have become comfortable with the balance of forces, let me thow you a curve ball. [BTW I though all balls were curved!! ;-) Light current 17:26, 7 September 2005 (UTC)
According to Einstein's theory of gravity (General Relativity), the only force on the apple is the push from the deformation. This force causes the apple to be accelerated which is why the apple has weight. So, according to GR, the forces aren't in balance. Enjoy! Alfred Centauri 16:06, 7 September 2005 (UTC)
All the previous arguments (on induction) seem to be leading us down a path of saying that:
Whatever force can exist in the universe, whether mechanical, electromotive (emf), magnetic(changing or not), electric (changing or not), gravitational etc, that force, by its very presence, INDUCES an equal and opposite force on any object to which it is applied. If it is not applied to an object, then the force just exists on its own like an emf in an insulator. But, when applied to an object, that force MUST be resisted by the object (whatever it is) and it must be resisted in a way that leads to the continued existence of the object (ie its not crushed by gravity, it creates an induced voltage to stop current rising to infinity etc,etc)
Would anyone agree with this extreme generalistation of mine?? Light current 15:46, 5 September 2005 (UTC)
In your order. Yes!, Yes, (I knew someone would mention black holes!!) and Yes. OK, SCZenz THanks for your input :-) Light current 01:22, 7 September 2005 (UTC)
Yes I agree this wording needs a lot of work. Unfortunately I am no expert in nuclear physics (or physics in general for that matter - I'm an engineer). I just thought that after my discussions with [User:Alfred Centauri], this business of actions being equal and opposite seems to make sense to me within my limited sphere of knowledge. If it turned out to be a (fairly) general principle, I'm sure it would aid science understanding tremendously. One example I can quote is that of Newtons 3rd? law -to every action there is an equal and opposite reaction. It took me years to see how this could be true in the case (lets say) of an apple resting on the ground. THe apple has the force of gravity acting upon it,and is therfore attracted to the centre of mass of the earth. Since the apple doesnt move, there must be an equal and opposite force acting upwards on the apple. So far so good. THe big question is: how does the earth know what force to exert in response to the apple. The answer came to me one day. When the apple rests on the earth, the earth is slightly deformed by the attractive forces. The more the earth (and apple I suppose)is deformed, the greater its 'upthrust'. Hence the earth is deformed just enough so that the reaction force exerted by the earth is exactly equal to the 'weight' of the apple and a state of mechanical equilibrum then exists. The 'bigger question' is: does this sort of thing happen between other forces and objects or between two different forces like emf and induced voltage. I'm sure people could think of many more examples. Food for thought!!! Light current 02:58, 7 September 2005 (UTC)
Are you implying here that the reaction force is generated by electrostatic repulsion? Light current 05:09, 7 September 2005 (UTC)
OK then, but in an insulating material, is it not true that electrons are tightly bound to their atoms. If so how can there be an electron cloud? Light current 13:40, 7 September 2005 (UTC)
Ok. So regardless of how it happens, in the macroscopic case of the apple on the ground, one could say that the reaction force is induced by the gravitational force. Yes?. The upthrust of a fluid is induced by the dispalcement of that fluid (Archimedes principle). Voltage is induced to counteract emf in a changing magnetic field. Charges are induced on the plate of an electroscope to balance the electric field of a charged object brought close by. Heres a biggie -- electric flux is induced by the presence of charge???. Do you agree with all the above?. If not - say why not Light current 15:24, 7 September 2005 (UTC) Just noticed in my college notes that electric flux used to be called the electric induction. Light current 17:02, 7 September 2005 (UTC)
Possibly Im using the word 'induction' too loosely here , but it will do until someone comes up with a better word!. Light current 00:54, 8 September 2005 (UTC)
If its not always or generally true, then the postulate is erroneous. However, I dont think we can say that yet-- Light current 17:09, 8 September 2005 (UTC)
Apply a force to anything. There is always a reaction (Newton was quite right). Scrub 'induces' if you like and substitute 'causes'. It makes no difference to my postulate. Light current 03:14, 8 September 2005 (UTC)
What it buys us is the notion that all forces in the universe have an equal and opposite reaction. I have yet to think of an application where a new insight might be revealed - but give me time!!. I was hoping that others may latch onto this idea and suggest applications of this postulate. Light current 01:04, 8 September 2005 (UTC)
All forces existing in the universe, cause an (induced)reaction to be generated by any object or force field or medium to which the original force is applied. This reaction force is equal in magnitude and opposite in direction to the original force.
Note on my counterexample above. Of course, it is true that the particle exerts an equal and opposite force on the plates producing the electric field, but that's just an example of Newton's Third Law. (Since your statement doesn't say what the force applies to, it might be read as synonymous with that law--which would mean it was correct, but not new.) -- SCZenz 18:51, 8 September 2005 (UTC)
Ok But were you aware that Newtons 3rd law can be applied to every physical force in the universe?. I was not. I was used to only thinkng of it in the terms of mechanics (as was Newton of course). Light current 22:23, 8 September 2005 (UTC)
I think that comment is a little harsh AC,. After all, it is merely a generalisation of Newtons 3rd law as SCZenz has pointed out and is therfore not nonsense. I'm sure you accept Newtons laws as valid. -- Light current 23:41, 8 September 2005 (UTC)
A) Apple on the ground. Ground reacts to gravity force by induced force pushing up. B) Fluids react to displacement by induced upthrust. C) Electric Flux is induced by a charge. -- Light current 22:38, 8 September 2005 (UTC)
Take a rocket in space. Force on rocket F=ma. this is also equal to the force on the exhausted material. Momentum is conserved in the system. since Force = rate of change of momentum and momentum is not changing, then yes there is no net force in the system but the rocket still accelerates -- Light current 22:51, 8 September 2005 (UTC)
Correct. Would you care to expand on that?-- Light current 22:57, 8 September 2005 (UTC)
No I think I was trying to say that all forces are matched. SCZenz puts it very well in his last post. -- Light current 22:55, 8 September 2005 (UTC)
Newtons third law is applicable to every conceivable force in the universe. -- Light current 23:12, 8 September 2005 (UTC)
OR THe sum of forces in a closed system is zero -- Light current 23:29, 8 September 2005 (UTC)
Thanks for the compliment. Yes I do see what you mean. Correct: sum of all forces in universe is zero. I think I'll quit while I'm ahead! -- Light current 00:09, 9 September 2005 (UTC)
If I'd seen this [2] earlier then we could have made a few short cuts. C'est la vie!
Yes,you are correct. If it aint hurtin', it aint workin'. And no pain - no gain! etc -- Light current 01:22, 9 September 2005 (UTC)
Im sorry to disagree with you but planets in orbit do have a balancing force. Its called centrifugal force. This is what stops then falling into the sun!. Light current 01:08, 8 September 2005 (UTC)
You don't even have to use a rotating frame to see a fictitious force. When you step on the accelerator in your car, the acceleration is forward (hopefully) yet you feel as if a force pushes you backward into your seat. That is acceleration forward 'feels' like a force is pushing backwards. This force you 'feel' doesn't exist - it is fictitious. Now, to make the connection to GR, imagine that you are sitting in a car seat that is facing up. You feel as if you are being pushed back into your seat with a force equal to your weight. But, by the above reasoning, this implies that you are being accelerated up. Yet, you know that you are stationary on the surface of the Earth. According to GR, (this will raise some eyebrows) spacetime itself is accelerating downwards toward the center of the Earth. To be 'stationary' in spacetime would mean that you would also (free) fall towards the center of the Earth. Since the surface of the Earth prevents you from doing this, the surface of the Earth is, in effect, accelerating you. That is, your worldline is not on a geodesic of the spacetime. Wait -what's that I smell? Is it LC's brain short circuiting??? ;<) Alfred Centauri 02:52, 8 September 2005 (UTC)
I would like to avoid the quantum mechanical aspects at the moment if you are agreeable. The reason being that I know very little about quantum mechanics and I feel that the general law of induction should be established or debunked at the macroscopic level first. Light current 01:55, 8 September 2005 (UTC)
That doesn't sound right. When you squeeze something (a solid object, for example) it is the electrons that provide the reaction force. That is, the Exchange interaction that occurs on attempt to force electrons into the same quantum state. Gah4 ( talk) 01:47, 29 November 2009 (UTC)
http://fr.wikipedia.org/wiki/Inductance#Puissance_emmagasin.C3.A9e
pf:
-- HydrogenSu 10:52, 25 December 2005 (UTC)
I just wanted to put up a couple of my sources for putting up some transformer equations in the mutual inductance section. V_secondary = V_primary N_secondary/N_primary. And convsersely the curreent I_secondary = I_primary * N_primary/N_secondary
Fresheneesz 09:49, 14 April 2006 (UTC)
The symbol for current is (I), so why is (i) used here instead? GoldenBoar 18:40, 8 May 2006 (UTC)
I'm no expert, but the old book I have (1937) states that L stands for "Linked Flux", it is not homage to Lenz. I don't want to just go and arbitrarily edit the page, but here is my reference.
p.291 Electrical Engineering Vol 1 Chester L. Dawes, S.B., A.M. McGraw-Hill —The preceding unsigned comment was added by Ninthbit ( talk • contribs) 22:52, 9 February 2007 (UTC).
I have been playing with the formula for self-inductance L of a solenoid, and find that it can usefully be rearranged in terms of the lengths of the wire and the solenoid. Note that the formulae ignore end-effects, so are approximate.
which simplifies to
For an air-cored coil this simplifies further to:
where
Nice, eh? I don't suppose it's original but I haven't seen it elsewhere. I don't think I have made a mistake. GilesW 22:05, 31 May 2007 (UTC)
REAL WORLD PROBLEM! —Preceding unsigned comment added by 167.102.133.216 ( talk) 19:57, 18 January 2008 (UTC)
Yes, it's very nice, and exactly what I was looking for! Now, maybe you can help me with a real-world problem. I am erecting shortwave antennas. I find that my dipole antenna is resonant at too high a frequency. I know that I could add length to my dipole to lower the frequency. But that would mean getting out the soldering iron and doing repairs outside. I rather just add a lumped inductance somewhere in the middle of each arm of the dipole by taking the antenna wire and rolling it around a piece of plastic pipe. My question is will doing this lower the resonant frequency? Because as I roll the inductor, I am reducing the length of the antenna by the same amount. Specifically, the antenna is now resonant at about 5.0 MHz, and I would like it to be resonant at 3.8 MHz. If anyone takes up this challenge, please email me at jcotton@excite.com and let me know that this has been attempted. Thanks for looking —Preceding unsigned comment added by 167.102.133.216 ( talk) 19:55, 18 January 2008 (UTC)
I don't know specifically about that one, but in many cases the wire that goes into the inductor is exactly the right amount that you get the same frequency. Well, in the case of coaxial cables with helical center conductors, the inductance increase results in slower propagation of the EM wave. That comes out the same as if you consider the length of unwound helix. Gah4 ( talk) 01:53, 29 November 2009 (UTC)
Section: Inductance of a solenoid
The equation for inductance displays as L = (μ0 / μr)(N^2 * A) / l, which looks incorrect. When it confused my derivation of relative permeability (μr), I realized there might be a mistake. L = (μ0 * μr * N^2 * A) / l yields the correct results, which I confirmed by deriving μ directly from B/H. I consulted both electronic machinery (transformer) text and electromagnetic text. I am new to this, so I thought someone else should look at it and confirm this before any editing. Thank you for your help. Amanda.Tighe 18:14, 4 June 2007 (UTC)
The page is inconsistent about the use of vs. . I'd suggest explaining that distinction in the intro and getting mutual inductance out of the intro, to be explained later. Ccrrccrr 11:43, 3 August 2007 (UTC)
To be really useful, there should be a table with formulas for wire pair, wire plus wall, coaxial cable (with and without skin effect), rectangle, ring, solenoid ...
BTW, the style of some sections should be checked and corrected by someone with native english, it sounds queer to me. —Preceding unsigned comment added by 84.56.8.130 ( talk) 16:42, 4 November 2007 (UTC)
This is redundant to the section 'Vector field theory derivation', it is an intermediate step to derive the von Neumann formula, and the expression gets singular in the self inductance case. —Preceding unsigned comment added by Rdengler ( talk • contribs) 16:50, 24 November 2007 (UTC)
CONCERNING the formula for Single layer solenoid: I have no idea what the term with the O is? Is it a zero? If so, why include it at all? Is it another term? If so, what? —Preceding unsigned comment added by 167.102.133.216 ( talk) 20:27, 18 January 2008 (UTC)
This is a standard notation in mathematics, see Big O Notation. The complete expression is
This is the inductance of a cylinder with a constant current around its surface.
—Preceding unsigned comment added by Rdengler ( talk • contribs) 17:02, 21 January 2008 (UTC)
The formula for inductance of pair of parallel wires applies only for d>>2a, i.e. no proximity effect. The formula for inductance of parallel wires, high frequency, incorporates the proximity effect. 192.91.147.35 ( talk) 21:16, 30 January 2008 (UTC)
The inductance of a pair of parallel wires exactly is in the low frequency case for all d>2a - there is no proximity or skin effect.
I have removed 'per coil turn' from the definition of inductance.
What is the magnetig flux through a circuit consisting of a thin wire? It is proportional to the number of flux lines that have crossed the wire and now are "captured". This implies that the flux through a circuit with n turns is n times the flux through one turn. Defining the flux geometrically (area times field strenght) leads to inconsistencies and complications. Consider a circular loop with two turns in a constant magnetic field and deform the circuit to a shape looking like an 8. The flux doesn't change.
The magnetic flux through a circuit with a continuous current distribution may be defined by replacing the current distribution with a bunch of thin wires, each carrying a small current. —Preceding unsigned comment added by Rdengler ( talk • contribs) 07:41, 19 August 2008 (UTC)
Ths page does a great job explaining the complex concept of inductance, yet I beleive the most important thing to mention is that inductance is the ratio of magnetic flux created by an inductor to the change in current that induces it.
The formula
L = Φ/i
where L is the inductance in Henries, Φ is magnetic flux in Webers, and i is the current in Amperes, is the simplest formula I know to give a rudimentary understanding of inductance. Is this correct? If it is, I will add it to the page to allow beginners to understand better. -- Skyfinity ( talk) 23:07, 29 November 2008 (UTC)
Here is another comment to the same point. A definition in terms of magnetic flux and current turns the problem on its head. There is a simple definition for flux only in the case of thin wires - and even this case cannot be realized exactly experimentally. A generic definition of inductance is possible in terms of change of current and induced voltage or in terms of the energy of the magnetic field. The connection with the magnetic flux may be useful to illustrate some aspects of electromagnetism. It is not useful as a definition. —Preceding unsigned comment added by Rdengler ( talk • contribs) 16:51, 2 July 2009 (UTC)
In the diagram, the arcs that form the symbols of the inductors are the wrong way round. The convex sides of the arcs should face each other.
Where a single inductor with a core is depicted, the arcs face the core depicted by parallel lines.
I was brought up on BS3939, superseded many years ago by EN60617-4 and the BS version of it.
see: EN60617-4:1996 Graphical symbols for diagrams. Basic passive components ( ISBN 058026745 8). There are numerous engineering publications that use those symbols, mostly correctly(!). GilesW ( talk) 21:35, 15 January 2009 (UTC)
I think 'Coupled inductors' would be more technically correct than 'Mutually inducting inductors' in the text of the diagram (and anywhere else, if applicable). GilesW ( talk) 21:41, 15 January 2009 (UTC)
Could the page include a reference to the units of measurement, and practical help for those who might actually want to use the equations? I assumed MKS units, which seems reasonable in the context of Wikipedia. But when I calculated the inductance of a loop with a radius of 10mm I got 0.049 Henry - obviously a ridiculous result. I had to multiply this by mu-zero to get the much more reasonable answer of 6.2e-8 Henry. Jay.sinnett ( talk) 21:21, 26 June 2009 (UTC)
Can someone add a power series expansion of the solenoid expr terms of w^-1 in the limit w==>inf (i.e. short solenoid limit) Thanks! Woz2 ( talk) 14:30, 9 September 2009 (UTC)
There is a dispute about a superfluous factor -1 in the series expansion of the single layer solenoid inductance. First of all, the -1 (inside the square) is unnatural, useless and very confusing. More to point, the product in the numerator of the term with index m consists of (m-1) factors. For example, for m = 2 it simply is (2m-3) = 1, for m = 3 it is 1*(2m-3) = 1*3. For m = 1 the product thus consists of 0 factors, which is 1 by convention everywhere in mathematics. The only natural thing thus is to remove the factor -1. Of course, it arises if one sets m = 1 in (2m-3). However, there are 0 factors in the numerator of the (m=1)-term. The formalism is clever, and the result looks nice and natural!
Here is a link to a reference with details: http://home.arcor.de/rdengler/Transpo.pdf. Looking for official/original references for the article...
The article still is somewhat incoherent - the capacitance article is superior in this respect.
1) The section "Phasor circuit analysis and impedance" doesn't explain anything about inductance. Should be explained under "phasors" or elsewhere. If there are no objections then I shall remove it.
2) The section "Induced emf" has trivial and clumsy content. If there are no objections then I shall remove it.
3) The sub-sections under "Self-inductance of simple electrical circuits in air" (Inductance of a solenoid, Inductance of a coaxial line) should be moved to a new section "Solenoid and coaxial cable in detail" (or something like that), or, possibly, to other articles (coaxial cable, solenoid), with a link added here. Rdengler ( talk) 16:35, 7 November 2009 (UTC)
I have reverted this edit which has the edit summary changed "wire loop" to coil of wire. The quoted formula contains N respresenting a number of turns of wire. For a wire loop N is 1 and would not be included in the formula. I agree that the use of loop here is problematic and could , perhaps, be improved, but coil of wire is even more so. I think the definition only works if all the turns are essentially co-incident - that is, lying on the same loop. Furthermore, the change was not done consistently, the section continues to talk about wire loop further down. SpinningSpark 16:29, 9 August 2010 (UTC)
The equation for the inductance of a thin solenoid is not correct; one should use Babic and Akyel, Improvement in calculation of the self- and mutual inductance of thin-wall solenoids, eq. (8), IEEE Trans on Magnetics, Vol. 36, No. 4. July 2000 Prof. J.C. Compter —Preceding unsigned comment added by 194.25.102.189 ( talk) 10:04, 2 September 2010 (UTC)
The Lorenz expression for the inductance of a coil is the inductance of a cylinder with a current around its surface (might be indicated in a footnote), and as such is as exact as Maxwell's equations. Improvements (wire or coil thickness, wire spacing) are more complicated and less instructive. B&A use numerical methods. Appears that FEM and numerical methods should be mentioned (with references) under calculation techniques. —Preceding unsigned comment added by Rdengler ( talk • contribs) 08:21, 4 September 2010 (UTC)
In the opening paragraph, I find the following sentence to be confusing: "This is a linear relation between voltage and current akin to Ohm's law, but with an extra time derivate." In fact, the voltage across an inductor is not proportional to the current -- it is 90º out of phase with it if the reactance is perfectly inductive. Jdlawlis ( talk) 01:43, 18 December 2010 (UTC)
-Shouldn't be confusing, taking the time derivative is a linear operation, the statement is mathematically correct. radical_in_all_things ( talk) 08:30, 19 December 2010 (UTC)
The statement is not mathematically correct. It would be correct if it were written: "This is a linear relationship between voltage and the rate of change of current akin to Ohm's law, except that current is replaced by its derivative." The fact that the derivative is a linear operation does not relate to this particular issue. As an example, take an ideal RL circuit with an AC generator. Let be the voltage generated by the AC generator. It follows that the current , where the impedance and the phase constant . The voltage drop across the inductor, . If the voltage across the inductor were indeed proportional to the current, you would be able to multiply the current times a constant to achieve the voltage. Multiplying a cosine function times a constant will only change its amplitude -- it cannot transform it into a sine function. Hence the voltage across the inductor is not proportional to the current through the circuit. These equations can be found in any introductory E&M textbook such as Tipler or Purcell. Jdlawlis ( talk) 23:36, 20 April 2011 (UTC)
The section on "coupled inductors" has a paragraph at the bottom about tuned circuits, starting "When either side of the transformer is a tuned circuit, the amount...". This paragraph is not referenced and may be original research. Does anyone know where this material comes from? —Preceding unsigned comment added by 121.98.140.35 ( talk) 00:01, 20 May 2011 (UTC)
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