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Drift velocity not field propagation velocity. Is this why AC has less resistive losses? (the electrons not getting that "fast" in 1/200th 1/100th or 1/240th 1/120th of a second)
Sagittarian Milky Way (
talk) 00:54, 14 October 2016 (UTC)
Is this why AC has less resistive losses?No. Shameless self-promotion of a section I wrote: Electric_power_transmission#Advantage_of_high-voltage_power_transmission
The Drift velocity of electrons in wires is typically 1570 km/s (related to the Fermi energy concept in quantum mechanics). Please compare the values in bold as they relate to the OP's question. As derived in detail in the linked article [1]:
- 1A DC flowing in a small copper wire changes the electron velocity by 0.000 023 m/s. It's probable that the majority of electrons in a switch when it left the factory never move out of it throughout the life of the switch.
- Going from DC to AC means the change direction reverses 120 (US) or 100 (Europe) a second. Here the original electrons certainly never leave the switch or indeed any metallic conductors.
This may make it clear that electrons never "stop accelerating" and our use of them for electric power distribution has almost negligible effect on their individual paths.
Virtually all electronic equipment in the home, including the computer or phone you are using, uses DC supplies. Homes could be (have been) supplied by DC mains but AC distribution is universally preferred because of its advantages of low-cost voltage conversion by transformers, the possibility to balance generators and loads in a network by phase control, and other minor advantages in avoiding corrosion at connections between dissimilar metals, arc quenching and avoiding permanent magnetization.
The saving in cable weight by using a high voltage (low current) for long-distance distribution applies to both DC and AC. However AC conductors need more insulation to handle 41% higher peak voltage than DC for the same power level. The choice of AC frequency is a compromise between contradictory requirements.
Lower frequency | Higher frequency |
---|---|
More reliable rotating generators and converters | Smaller transformers for a given power :-) |
Negligible skin effect | Skin effect reduces effective cross section of conductors |
Negligible power factor loss | Power factor effects require correction and over-dimensioning of generators and network components to handle extra reactive current |
Negligible impact of cable capacitance | Cable capacitance hinders long-distance power transmission |
Tolerable audio hum from transformers or interference from mains wiring to audio equipment | Increasingly noticeable noise from transformers (see Magnetostriction) and potential for man-made EMI |
The main results of standardization are:
If we treat this as a high school level exercise where we pretend that Ohm's law is valid on any time scale (it only starts to be valid for time scales that are a bit larger than the typical collision time), we can argue as follows. Ohm's law says that the relation between the current and the drop in voltage across a piece of wire of resistance R is . The resistance R can be written as where is the resistivity of the wire, L the length and A the cross-section. The current can be written as where is the free electron number density, v the (average) velocity, and e the magnitude of the electron charge. We can then write the electric field E in the wire, given as the voltage drop divided by the length, as:
The force exerted on an electron due to the applied voltage is thus given by:
This is for a steady state situation where the electrons are moving at some constant average velocity. This means that the total force exerted in the electrons is zero, therefore there exists a friction force that is equal in magnitude but opposite in sign as the force due to the applied voltage. This friction force is thus given by:
it is caused by collisions of the electron with lattice vibrations and other electrons. If we suddenly cut the voltage, the friction force will still be there, slowing down the current. Newton's second law yields:
where m is the electron mass. The current will thus decay on a time scale of
This is of the order of for copper, which is similar to the collision time. So, it will take somewhat longer than this time scale for the current to dissipate. Count Iblis ( talk) 01:06, 17 October 2016 (UTC)
Please could you explain to me how the ingredients in "Metanium" cream actually treat nappy rash. According to this website the cream contains;
Titanium dioxide 20.0% w/w Titanium peroxide 5.0% w/w Titanium salicylate 3.0% w/w Dimethicone 350 Light liquid paraffin Tincture of benzoin White soft paraffin
I am only interested in the scientific understanding of how the ingredients in this cream treat nappy rash. I believe this question is acceptable on the Reference Desk and does not contravene the medical advice prohibition because I am not asking for any diagnosis or treatment plan. To make absolutely clear, I do not have nappy rash and I am not asking this question in relation to any specific case of nappy rash that has ever existed. This is purely a scientific question.
ありがとう — Preceding unsigned comment added by 2A01:430:D:0:2CC:B0FF:FE9B:CC73 ( talk) 12:57, 14 October 2016 (UTC)
it's not clear to me if the follicle itself is cell or something which composed of many epithelial cells 93.126.88.30 ( talk) 19:27, 14 October 2016 (UTC)
Science desk | ||
---|---|---|
< October 13 | << Sep | October | Nov >> | October 15 > |
Welcome to the Wikipedia Science Reference Desk Archives |
---|
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages. |
Drift velocity not field propagation velocity. Is this why AC has less resistive losses? (the electrons not getting that "fast" in 1/200th 1/100th or 1/240th 1/120th of a second)
Sagittarian Milky Way (
talk) 00:54, 14 October 2016 (UTC)
Is this why AC has less resistive losses?No. Shameless self-promotion of a section I wrote: Electric_power_transmission#Advantage_of_high-voltage_power_transmission
The Drift velocity of electrons in wires is typically 1570 km/s (related to the Fermi energy concept in quantum mechanics). Please compare the values in bold as they relate to the OP's question. As derived in detail in the linked article [1]:
- 1A DC flowing in a small copper wire changes the electron velocity by 0.000 023 m/s. It's probable that the majority of electrons in a switch when it left the factory never move out of it throughout the life of the switch.
- Going from DC to AC means the change direction reverses 120 (US) or 100 (Europe) a second. Here the original electrons certainly never leave the switch or indeed any metallic conductors.
This may make it clear that electrons never "stop accelerating" and our use of them for electric power distribution has almost negligible effect on their individual paths.
Virtually all electronic equipment in the home, including the computer or phone you are using, uses DC supplies. Homes could be (have been) supplied by DC mains but AC distribution is universally preferred because of its advantages of low-cost voltage conversion by transformers, the possibility to balance generators and loads in a network by phase control, and other minor advantages in avoiding corrosion at connections between dissimilar metals, arc quenching and avoiding permanent magnetization.
The saving in cable weight by using a high voltage (low current) for long-distance distribution applies to both DC and AC. However AC conductors need more insulation to handle 41% higher peak voltage than DC for the same power level. The choice of AC frequency is a compromise between contradictory requirements.
Lower frequency | Higher frequency |
---|---|
More reliable rotating generators and converters | Smaller transformers for a given power :-) |
Negligible skin effect | Skin effect reduces effective cross section of conductors |
Negligible power factor loss | Power factor effects require correction and over-dimensioning of generators and network components to handle extra reactive current |
Negligible impact of cable capacitance | Cable capacitance hinders long-distance power transmission |
Tolerable audio hum from transformers or interference from mains wiring to audio equipment | Increasingly noticeable noise from transformers (see Magnetostriction) and potential for man-made EMI |
The main results of standardization are:
If we treat this as a high school level exercise where we pretend that Ohm's law is valid on any time scale (it only starts to be valid for time scales that are a bit larger than the typical collision time), we can argue as follows. Ohm's law says that the relation between the current and the drop in voltage across a piece of wire of resistance R is . The resistance R can be written as where is the resistivity of the wire, L the length and A the cross-section. The current can be written as where is the free electron number density, v the (average) velocity, and e the magnitude of the electron charge. We can then write the electric field E in the wire, given as the voltage drop divided by the length, as:
The force exerted on an electron due to the applied voltage is thus given by:
This is for a steady state situation where the electrons are moving at some constant average velocity. This means that the total force exerted in the electrons is zero, therefore there exists a friction force that is equal in magnitude but opposite in sign as the force due to the applied voltage. This friction force is thus given by:
it is caused by collisions of the electron with lattice vibrations and other electrons. If we suddenly cut the voltage, the friction force will still be there, slowing down the current. Newton's second law yields:
where m is the electron mass. The current will thus decay on a time scale of
This is of the order of for copper, which is similar to the collision time. So, it will take somewhat longer than this time scale for the current to dissipate. Count Iblis ( talk) 01:06, 17 October 2016 (UTC)
Please could you explain to me how the ingredients in "Metanium" cream actually treat nappy rash. According to this website the cream contains;
Titanium dioxide 20.0% w/w Titanium peroxide 5.0% w/w Titanium salicylate 3.0% w/w Dimethicone 350 Light liquid paraffin Tincture of benzoin White soft paraffin
I am only interested in the scientific understanding of how the ingredients in this cream treat nappy rash. I believe this question is acceptable on the Reference Desk and does not contravene the medical advice prohibition because I am not asking for any diagnosis or treatment plan. To make absolutely clear, I do not have nappy rash and I am not asking this question in relation to any specific case of nappy rash that has ever existed. This is purely a scientific question.
ありがとう — Preceding unsigned comment added by 2A01:430:D:0:2CC:B0FF:FE9B:CC73 ( talk) 12:57, 14 October 2016 (UTC)
it's not clear to me if the follicle itself is cell or something which composed of many epithelial cells 93.126.88.30 ( talk) 19:27, 14 October 2016 (UTC)