# How can a conductor have zero field inside when it is not in equilbrium

## Homework Statement

My book draws this picture where there is an external field outside a conductor and inside has charges where an opposite field takes place inside and there a net zero field occurs. But the book then generalizes that ALL conductors have zero field inside.

My question is why? What if there is no external field? How can they say ALL?

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Andrew Mason
Homework Helper

## Homework Statement

My book draws this picture where there is an external field outside a conductor and inside has charges where an opposite field takes place inside and there a net zero field occurs. But the book then generalizes that ALL conductors have zero field inside.

My question is why? What if there is no external field? How can they say ALL?
Only perfect conductors would have zero field inside (eg. a superconductor) at all times. If current flows through a copper wire, there is a small field inside the wire as a small amount of work is required to sustain the current (resistance is not zero).

For practical purposes, however, in the presence of an external electric field (no potential difference maintained between ends of the conductor) the field inside a conductor is effectively zero - once the charges settle down after a few pico seconds. Let's assume that the field inside the conductor is not zero - what would happen? Since electrons are able to move freely within the conductor, charges would immediately move in response to the electric field (actually opposite to the direction of the field), until the field became 0.

AM

But I am saying what if there was no external field in the beginning.

SammyS
Staff Emeritus
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Gold Member
The book is saying that there is no field inside a conductor, even though there is a non-zero external field. If there is NO external field, then of course the author must have reasoned that nobody would expect there to be a field inside.

(I believe OP is referring to a static field - so a perfect conductor is not needed.)

I pulled it out from my book (part of it because it is too big)

[PLAIN]http://img96.imageshack.us/img96/2802/79188880.jpg [Broken]

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Not sure what the confusion is. If there were no external field, the charges wouldn't need to rearrange themselves to create the opposing field.