Electric field inside conductor?

[wangdang]

Hi all,

I need some help understanding why exactly there is no electric field inside any conductor (charged or uncharged). I am in year 12, and I would like to understand the reasoning behind it in terms of the MOTION OF CHARGES in the conductor.

What I know so far:
- Charges (electrons) inside a conductor will repel (Coulomb's law).
- The charges will experience repulsion which results in maximum separation distances between the charges.
- The charges will then redistribute along the surface of the conductor in order to achieve electrostatic equilibrium (ie. net force of zero on each charge)
- Thus, no electric field exists inside the conductor.

Please feel free to correct me if I am wrong in any of the above statements.

Appreciate the help. Thanks.

 

[tiny-tim]

hi wangdang!

I need some help understanding why exactly there is no electric field inside any conductor (charged or uncharged).

Often quoted, but not true: it only applies in equilibrium.

When a charge is first brought near a conductor, the free charge inside the conductor rearranges itself extremely quickly to cancel out the field … while this is happening, of course there is a field!

And of course there is an electric field inside every current-carrying wire.​

In equilibrium, by definition, a free charge inside the conductor will not move …

that is simply another way of saying that there is no electric field

 

[wangdang]

Often quoted, but not true:

High school physics is wrong yet again :(

I am not sure I quite understand you though. Since there is free charge inside a conductor, wouldn't it redistribute to achieve equilibrium. Why must a charge first be brought near the conductor?

 

[tiny-tim]

Since there is free charge inside a conductor, wouldn't it redistribute to achieve equilibrium.

if there is no external field, it doesn't need to redistribute

when you apply an external field (bringing a charge near the conductor is just an example), then yes the charge will redistribute

when it stops redistributing, by definition there is no electric field, and there is equilibrium

 

[wangdang]

if there is no external field, it doesn't need to redistribute

But don't the charges (electrons) inside the conductor repel; thus redistribute?

 

[tiny-tim]

But don't the charges (electrons) inside the conductor repel; thus redistribute?

i don't understand what you mean by "redistribute" …

the conductor contains billions of negatively-charged electrons and positively-charged nuclei …

in the natural state, the electrons are attracted to the nuclei, and they stay there …

the repulsive force from other electrons all around one electron is roughly the same as the attractive force of all the other nuclei …

i think you're assuming that the electrons are the only free charges inside the conductor

 

[wangdang]

i think you're assuming that the electrons are the only free charges inside the conductor

That was exactly was I was assuming. Thanks a lot for clearing that up!

 

[dreamLord]

Hi, sorry for bumping this, but my question is related to this.

Isn't the above explanation only true for an uncharged conductor? Supposing the conductor had a net positive charge Q. How would the explanation proceed then?

 

[vanhees71]

If you charge a conductor, these additional charges indeed redistribute themselves within short times such that there's no electric field inside the conductor (Faraday-cage effect).

 

[dreamLord]

^What's the reason? I get it that it's happening. What I need to know is WHY the net positive charge Q is moving to the boundary of the conductor.

 

[tiny-tim]

WHY the net positive charge Q is moving to the boundary of the conductor.

because it can't move any further …

if it could, it would! ​

 

[dreamLord]

My question isn't why the boundary - my question is why is it moving at all? When there's no net electric field OUTSIDE, why do the charges rearrange at all such that the field inside is zero ?

 

[Born2bwire]

My question isn't why the boundary - my question is why is it moving at all? When there's no net electric field OUTSIDE, why do the charges rearrange at all such that the field inside is zero ?

When you initially apply an electric field across a conductor then there is a net field on the interior of the conductor. This induces a force on the electrons in the conductor via the Lorentz force which causes the electrons to move in response. As time passes, the electrons will arrange themselves so that the interior of the conductor has no net electric field. Since we generally discuss this topic in terms of electrostatics, there is no transient behavior. When we look at the problem the electric field has already been applied for a "very very long" time and the conductor's charges have already reached their equilibrium state.