# Can electrons pile up in a conductor

1. May 21, 2012

### smiddleton

Hey All,
I'm new to the forum. I hope I put this in the right area of expertise. If not, I would appreciate it if you could direct me to the right place. I am just a curious person with a pretty good physics background but I came up with a question I couldnt answer. Hypothetically, if you had a 2 inch long straight copper wire, and some how created a positive force at localized at 1 end of the wire, would the electrons in the wire that are "mobile" move toward that end and sort of pile up in a large, well, pile, of electrons? Or would only the electrons on the surface of the wire move toward that point? I hope that makes sense. Thanks advance, I hope you don't mind me pondering physics as a hobby.
Sincerely,
SM

2. May 21, 2012

### yungman

I just join in the discussion. I believe it will. What you are asking is in any EM book where you put an +E field at one end of the wire. The electrons are attracted to that end of the wire. The other end is left with excess of protons and become +ve charged.

This should be same as a hollow conductor ball with a +ve charge in the middle. electrons will be pull the the inner surface and the outer surface has the +ve charge. The total +ve charge on the outer surface is equal to the charge inside the ball. The total charge in the conductor ball is neutral.

Last edited: May 21, 2012
3. May 21, 2012

### Kholdstare

Any constant electric field accompanies a linearly varying potential field. Its seen from Poisson's equation.

This reminds me of a question I was asked many years ago.
"Imagine a straight copper wire placed above a bowl. A current was flowing through the wire when the wire is suddenly cut in the middle. Will the electrons drop from one end of the wire into the bowl below?" :rofl:

4. May 21, 2012

### yungman

Not in a conductor, any field inside the conductor will cause electrons to move in direction to neutralize the potential. This is explained very clear in any of the EM text book.

5. May 21, 2012

### Kholdstare

Oops! I overlooked this thing. Electric field decays inside a conductor and reduces to 1/e at skin depth. Roughly yungman is correct.

6. May 21, 2012

### yungman

It is more than that, you are referring to varying E field that has skin depth. Even in total static E, there cannot be E inside a good conductor as any E will cause the electrons to drift to neutralize any the field.

Actually I made a mistake in my first post also. It's not potential, it's E field cannot exist inside a good conductor. I edited my first post already to cover the evidence!!!:rofl:

7. May 22, 2012

### Kholdstare

Well ! Years of lack of practice of EM is producing some effects. haha. Don't worry. I'll catch up.

8. May 22, 2012

### yungman

That's how it is. I could have answer better a year ago when I was hot on the trod with studying EM. I have been off for like a year, I swear I really forgot a lot of things.

9. May 22, 2012

### Bobbywhy

smiddleton, Welcome to Physics Forums!

Yes, a positive charge will attract free electrons in a conductor! You are exactly correct. If you Google "electrostatics" you can read all about this subject at the Wikipedia page. Especially check out the diagram about half-way down the page. It shows what you are describing graphically and clearly!

http://en.wikipedia.org/wiki/File:Electrostatic_induction.svg

10. May 22, 2012

### smiddleton

Wow, thanks for all the replies. I love physics, but I went in to materials science so my electrical engineering background is pretty basic. This really helps though, thanks everyone!
SM

11. May 23, 2012

### smiddleton

Hey guys, I have another quick question. How do you calculate the potential charge, in coulombs, of an electret? I can't find that one anywhere. Thanks again.
SM

12. May 23, 2012

### smiddleton

Also, how do they calculate the voltage? I think my understanding of distance in parallel plate caps is making it hard for me to understand this. Thanks.
SM

13. May 24, 2012

### DrZoidberg

If you bring an electric charge close to a copper wire the electrons in the entire wire would start moving, not just at the surface. That's why the resistance of a wire depends on it's cross sectional area and not on it's circumference. As soon as the field inside the wire has dropped to zero the electrons stop moving. One end of the wire will then be charged positive and the other negative. btw. there can be a field inside a conductor if there is a current flowing.

An electret is similar to a charged parallel plate capacitor, just without the plates. So voltage is equal to field strength * thickness. The charge is field strength * electric field constant * area * dielectric constant.