1
$\begingroup$

If one applies an electric field to a conductor, then the electrons inside it will start moving around, until the field is balanced out to 0. It is said in Griffiths’ “Intro to E&M” that this happens almost instantaneously. In contrast, electrons in a wire under an electric field move painfully slowly. Why don’t they move from one part of the wire to another “almost instantaneously”? He does mention that there are collisions happening between electrons in a wire, which slows them down, but then why don’t these collisions slow down the process of field removal in a general conductor?

Also, he says that the source of EMF can be anything from, obviously, an electric field within a battery to a temperature gradient or a repeated crossbow shooting electrons. In the last two cases then, as it seems to me, there is no electric field caused by the source, so what is the source of an electric field that makes the electrons move across the $\textbf{entire}$ circuit?

Improve this question
$\endgroup$
5
  • $\begingroup$ Individual electrons move slowly. That does not contradict them moving collectively to quickly react to fields. $\endgroup$
    – Jon Custer
    Commented Aug 12 at 17:10
  • $\begingroup$ You are seeing this in the semi-classical sense. When you finally learn about what electrons actually do in a metal, you will realise that quite a lot of the schizophrenic behaviour is wonderfully explained by quantum theory. It is only slow because you are taking an average of a thing that is classically bonkers. $\endgroup$
    – naturallyInconsistent
    Commented Aug 12 at 17:16
  • $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Commented Aug 12 at 17:23
  • $\begingroup$ @JonCuster They drift slowly, but they move very fast. $\endgroup$
    – my2cts
    Commented Aug 12 at 17:56
  • $\begingroup$ @my2cts - sure, but what most lay people think of 'move' is actually 'drift' in the context of solid state physics. And 10,000,000 electrons each taking a small step to the left is the equivalent of one electron running a long long ways. $\endgroup$
    – Jon Custer
    Commented Aug 12 at 18:11

1 Answer 1

Reset to default
0
$\begingroup$

Imagine you have a narrow circular trough (like a moat), and to make visualization easier, imagine it's filled with a line of ping-pong balls instead of water. You push one of the balls a short distance. All of the other balls shift in the trough by the same amount, and when you stop pushing they come to a stop. This all happens much more quickly than the time it would take for a single ball to circle the moat at that speed.

Collisions between the ping-pong balls do slow them down, but are also the mechanism by which your force is transmitted to the other balls. If they didn't interact with each other (passed through each other like ghosts) then there would be no current at the other side of the moat unless/until the one ball traveled all that distance. With interaction, there is a current at the other side almost instantly.

what is the source of an electric field that makes the electrons move across the entire circuit?

The electrons themselves are the source. If you manage to force electrons from point A to point B by any means, there will be a net positive [negative] charge at point A [B] and a resultant electric field that attracts [repels] electrons, resulting in a net charge elsewhere, etc.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.