# Charges in conductor and insulator

1. Aug 28, 2007

### ArielGenesis

by definition, a charge cannot move inside an insulator
and by a similar definition, a charge can move inside a conductor.

so, I don't understand how do we could charge an insulator, the charge cannot move, or there is something that i missed?

2. Aug 28, 2007

### ZapperZ

Staff Emeritus
But there's nothing to prevent something from removing charges from the insulator. You can either strip some of the electrons, especially from the surface of the insulator, or add electrons to it. That's what you do when you rub a piece of plastic with a cloth when you do those high school experiments with an electroscope.

Zz.

3. Aug 28, 2007

### ArielGenesis

I am thinking that if i take or add charges from an insulator, which is most probably done at a section of the surface, the charge have to move to the insulator, along the surface and spread themselves. and the same will apply for removing the charges. they have to move, dont they?

4. Aug 28, 2007

### mgb_phys

Charges do move in an insulator - just more slowly than a conductor.
Like most things these labels are just extremes, there is a continuous scale between very good conductors and very good insulators. Charges can move in all of them just more easily in good conductors.

5. Aug 28, 2007

### ArielGenesis

that way, could i conclude that in electro static, an insulator behave in a same way as a conductor?

6. Aug 28, 2007

### mgb_phys

If you rub an insulator you will build up a surface charge, this will slowly leak through the insulator and away to whatever the insulator is fixed to.
If you rub a conductor you will very briefly create a surface charge before it is dissapated through the rest of the conductor.

7. Aug 28, 2007

### Gokul43201

Staff Emeritus
I've got a related question. From Resnick to Jackson, you'll find problems involving an insulating sphere with a uniform (volumetric) charge density. How does one make such a thing? Is there a simpler way than say, irradiating with a beta emitter?

8. Aug 28, 2007

### mgb_phys

The charge would naturally diffuse to be uniform (in volume) as the individual charges repel each other. You can put the charge on it any way you like, eg. a Van der Graaf generator.

9. Aug 28, 2007

### ArielGenesis

I dont think emitting with beta radiation will make it uniform, unless the object is very thin.
so when i charge anything, i only give a surface charge which any object possess, in conductor, the object itself absorb the charge in the surface, hum... thats better.

I think it can be done using chemical reaction. like having a charged water and then freezing it. the ice will be uniformly charged volumetrically. or something similar. the point is that the object have to be a non insulator at the time we insert the charge then we could convert the object to an insulator so the charge stays there...

10. Aug 29, 2007

### Gokul43201

Staff Emeritus
mgb, if the only difference (between a conductor and an insulator) of relevance here is the transport time constant, how do you get different steady state charge distributions (i.e., uniform surface charge for a metal*, and uniform volumetric charge for an insulator) for the two cases with identical initial conditions?

*Now, I'm not sure what exactly you mean by the second sentence in post#6.

11. Aug 29, 2007

### ArielGenesis

so its a fuzzy logic thing? that conductivity is about how quick the charge could move?

12. Aug 30, 2007

### bernhard.rothenstein

charge motion in a dielectric

Think please about the polarization of an dielectric in order to have an ideea about the distance an electric charge can move inside it.