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:
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.
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…
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.
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.)?
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
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.
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.
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.
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.
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 electrostatics (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.
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 electrostatics (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.
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?
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.)
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.
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.
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.
