# EEs & Physicists: Why does current flow in a wire?
**URL:** https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453
**Category:** Factual Questions
**Created:** [March 31, 2003, 3:29pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453 "2003-03-31T15:29:03Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [March 31, 2003, 3:29pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/1 "2003-03-31T15:29:03Z")
I thought electrons moved inside a conductor due to an electric field existing _inside_ the conductor. But a coworker (with a MSEE) says the electric field inside a conductor (even at DC) _ **must** _ be zero at all times, by definition. If that’s true, how/why do the electrons move? Is it due to diffusion?
This link from the MIT Physics Department says that there **is** an electric field inside a conductor:
[http://web.mit.edu/8.02t/www/modules/VI.%20Current%20and%20Resistance/ppt/currents.pdf](http://web.mit.edu/8.02t/www/modules/VI.%20Current%20and%20Resistance/ppt/currents.pdf)
Could my co-worker be correct only when there’s no current (i.e. when a voltage is not placed across the conductor)?
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### Author:  [@Nametag](https://boards.straightdope.com/u/Nametag)
#### Post date: [March 31, 2003, 3:44pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/2 "2003-03-31T15:44:09Z")
Your coworker needs to go back to school. The rule he is thinking of is a cosequence of Gauss’s Law, and applies only to conductors _at equilibrium_. Conductors carrying current are not at equilibrium, and do have an internal electric field.
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### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [March 31, 2003, 3:48pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/3 "2003-03-31T15:48:35Z")
During our discussion, he said (in defense), “If an electric field existing inside the conductor, then there would be a significant potential (voltage) drop along the conductor. This isn’t the case.” But I think he might be confusing “potential energy” with “voltage.”
For the record, I am also an EE, but it’s been a while since I had this field stuff…
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### Author:  [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)
#### Post date: [March 31, 2003, 3:51pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/4 "2003-03-31T15:51:39Z")
Take a conductor and apply a voltage to it. A current flows through it per Ohm’s law. If you measure the resistance of _half_ the conductor, and the voltage across that half, you can calulate the current and it will be the same as that flowing through the entire conductor since I=V/R = (V/2)/(R/2). Keep doing that and you’ll see that at any two points on the conductor there is proportional V/R driving the current. To me, that suggests there is certainly an electric field in the conductor when a current is flowing, at least. I can’t say for the case when a voltage is applied, but no current flows, but my gut says no, there isn’t.
I’ve greatly simplified here, for the sake of clarity, not to be condescending to the OP.
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### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [March 31, 2003, 3:58pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/5 "2003-03-31T15:58:30Z")
Thanks **Q.E.D.** , but I’m all too familiar with Ohm’s Law. Just wondering about the existence of an E-field inside a good conductor.
Is the magnitude of the electric field a function of resistance? For example, would a silver conductor have an especially weak E-field? Or is the E-field fairly strong in all good conductors (e.g. copper, aluminum, silver, gold, etc.)?
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### Author:  [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)
#### Post date: [March 31, 2003, 4:02pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/6 "2003-03-31T16:02:25Z")
Yes, I knew you were. I was more attemting a simplified explanation for the less electrically-inclined who might wander in. Apologies if you felt I was talking down to you, **Crafter\_Man**
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### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [March 31, 2003, 4:04pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/7 "2003-03-31T16:04:27Z")
No apologies necessary, **Q.E.D.** Thanks for the input.
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### Author:  [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)
#### Post date: [March 31, 2003, 4:36pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/8 "2003-03-31T16:36:13Z")
Incidentally, it’s my understand that e-field strength is directly proportional to the voltage. I don’t believe the resistance of the conductor is a factor, but if anyone has better information, I’d be interested in hearing it. I can’t find anything specifically related to the strength of an electric field _within_ a conductor, only _between_ conductors.
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### Author:  [@kenibbling\_pin](https://boards.straightdope.com/u/kenibbling_pin)
#### Post date: [March 31, 2003, 4:47pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/9 "2003-03-31T16:47:13Z")
This might be a little less tech but I have been taught that a wire does not conduct electricity through it, rather the electicity travels in a field on the surface of the wire.
Inside the wire you might not ‘see’ the current. The current is carried on a very very thin ‘skin’ on the surface of the wire.
Of course we don’t usually have instruments that can distingish between the middle of the wire and the surfice of the wire.
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### Author:  [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)
#### Post date: [March 31, 2003, 4:55pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/10 "2003-03-31T16:55:11Z")
> [@](#):
>
> \*Originally posted by kenibbling pin \*
> **This might be a little less tech but I have been taught that a wire does not conduct electricity through it, rather the electicity travels in a field on the surface of the wire.**
I believe what you are thinking of is the “skin effect”, which becomes increasingly pronounced as the frequency of the AC current rises. At DC, the current most certainly _does_ flow through the entire thickness of the conductor.
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### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [March 31, 2003, 5:00pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/11 "2003-03-31T17:00:37Z")
**kenibbling pin** : The skin effect is frequency-dependent; as frequency increases the effect becomes more pronounced. As far as I’m aware, electrons flow through the conductor’s entire cross-sectional area at DC.
So can anyone help me here? Let’s say I have a 22 AWG solid silver wire with 3 amps DC flowing in it. Is that current due to a strong E-field in the wire?? Is the direction of the E-field parallel with the wire at every point? I’ve always understood this to be the case. But my co-worker insists an E-field of any appreciable magnitude _cannot_ exist in a good conductor, even at DC. I want to be cautious here because he’s usually not wrong about things like this.
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### Author:  [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)
#### Post date: [March 31, 2003, 5:05pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/12 "2003-03-31T17:05:16Z")
> [@](#):
>
> A conductor contains at least some charges which are free to move within it. When initially placed in an external E-field the field will penetrate into the conductor and cause the free charges to move (a). However these can only move as far as the surface of the conductor (assuming it is of finite size). Charge collects at the surface and **produces an E-field within the conductor** which opposes the external field. Equilibrium is quickly reached where the displaced charges produce an internal field which exactly cancels the external one (and hence there is no further movement of charge) (b).
From[this article](http://www.shef.ac.uk/uni/academic/N-Q/phys/teaching/phy205/lectures_5_and_6.htm) Most of the mathematics is completely beyond me, I’m embarassed to say, but hopefully this will be of some help to you.
[fixed coding]
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### Author:  [@tool462](https://boards.straightdope.com/u/tool462)
#### Post date: [March 31, 2003, 5:52pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/13 "2003-03-31T17:52:50Z")
Ohm’s Law states:
J = sigma \* E
where J is the current density, sigma the conductivity (sigma^-1 = resistivity), and E is the electric field.
Now, if a current is flowing in a wire, J is obviously not equal to zero. Further, the conductivity is not zero (it’s a conductor). Therefore it stands to reason that E is not equal to zero. Incidentally, J and E are both vectors. This further implies that the E-field is pointed exactly along the path that current travels, and vice-versa.
What your friend is thinking is that in electro **statics** (an important distinction, AC is an entirely different case) E-fields have to vanish at the surface of a conductor, not necessarily inside.
Ultimately, the only force that can move electrons is an electromagnetic field. If there is no electric field, the electrons can’t move, ergo no current.
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### Author:  [@tool462](https://boards.straightdope.com/u/tool462)
#### Post date: [March 31, 2003, 5:59pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/14 "2003-03-31T17:59:06Z")
Ohm’s Law states:
J = sigma \* E
where J is the current density, sigma the conductivity (sigma^-1 = resistivity), and E is the electric field.
Now, if a current is flowing in a wire, J is obviously not equal to zero. Further, the conductivity is not zero (it’s a conductor). Therefore it stands to reason that E is not equal to zero. Incidentally, J and E are both vectors. This further implies that the E-field is pointed exactly along the path that current travels, and vice-versa.
What your friend is thinking is that in electro **statics** (an important distinction, AC is an entirely different case) E-fields have to vanish at the surface of a conductor, not necessarily inside.
Ultimately, the only force that can move electrons is an electromagnetic field. If there is no electric field, the electrons can’t move, ergo no current.
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### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [March 31, 2003, 7:46pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/15 "2003-03-31T19:46:11Z")
I’m beginning to think my co-worker is correct. The SI unit for electric field is volts/meter, correct? A good conductor has a very low voltage drop per meter. So wouldn’t the E-field also be very low compared to, say, the E-field in a resistor? Would the E-field in a superconductor be E = 0 no matter what the current?
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### Author:  [@TGWATY](https://boards.straightdope.com/u/TGWATY)
#### Post date: [March 31, 2003, 7:58pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/16 "2003-03-31T19:58:38Z")
> [@](#):
>
> \*Originally posted by tool462 \*
> \*\*
> What your friend is thinking is that in electro **statics** (an important distinction, AC is an entirely different case) E-fields have to vanish at the surface of a conductor, not necessarily inside.\*\*
fyi, **tool462** probably made a typo here as it is backwards. In electrostatics, the field vanishes inside a conductor and is nonzero at the surface of a (charged) conductor. Also of note is that the field (forces) are perpendicular to the surface of the conductor. (Otherwise the charges would be pushed around the surface and it wouldn’t be static.)
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### Author:  [@Ring](https://boards.straightdope.com/u/Ring)
#### Post date: [March 31, 2003, 8:19pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/17 "2003-03-31T20:19:54Z")
If you place a stationary conductor in a static E field the charges in the conductor will align themselves in such a manner to cancel the field within the conductor.
If you place a voltage source across a loop of wire you will establish an E field within the conductor that will cause a current to flow. No E field, no current. Of course without a load this would be called a short circuit.
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### Author:  [@tool462](https://boards.straightdope.com/u/tool462)
#### Post date: [March 31, 2003, 8:26pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/18 "2003-03-31T20:26:46Z")
> [@](#):
>
> \*Originally posted by TGWATY \*
> \*\*fyi, **tool462** probably made a typo here as it is backwards. In electrostatics, the field vanishes inside a conductor and is nonzero at the surface of a (charged) conductor. Also of note is that the field (forces) are perpendicular to the surface of the conductor. (Otherwise the charges would be pushed around the surface and it wouldn’t be static.) \*\*
Yeah, sorry. Thanks for pointing that out 🙂
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### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [March 31, 2003, 8:34pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/19 "2003-03-31T20:34:32Z")
> [@](#):
>
> \*Originally posted by Ring \*
> **No E field, no current.**
This is what I’m having a problem with.
Let’s say I connect a battery to a regular 'ol resistor and a current of 500 mA flows. Let’s say I use superconductors for wires. A superconductor has zero resistance, correct? Which means it has zero voltage drop, correct?
The unit for electric field is volts/meter. Because there’s no voltage drop between any two points in a superconductor, would the electric field be 0 volts/meter inside the wires? _Yet there’s still 500 mA flowing through them_. This would seem to imply that you do not need an electric field for current to flow.
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### Author:  [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)
#### Post date: [March 31, 2003, 8:39pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/20 "2003-03-31T20:39:46Z")
**Crafter\_Man** , your co-worker is incorrect. **tool462** has the right equation, but mixed up the explanation a bit. Conductors have a large _but finite_ conductivity. For current to flow, there must be an electric field present in the wire. Since the conductivity is large, the electric field is small, but it isn’t zero.
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