# How to uniformly charge an insulating sphere?

#### amiras

In my Physics book there was this problem of finding electric field produced by the sphere, such that electric charge is distributed uniformly throughout the volume of an insulating sphere.

I know that excess charge tends to distribute itself on the surfaces, but since this sphere is made from insulating materials excess charge cannot leave individual molecules to do so?

Anyway, how it is possible to charge an insulating sphere throughout the volume?

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#### mfb

Mentor
Those spheres are just hypothetical objects - there are no perfect insulators, if you wait long enough the charge will be at the surface only.
You can place electrons at specific regions with electron beams, but I doubt that the result will give a uniform charge distribution.

#### Johnahh

is it not possible to uniformly heat the "insulating sphere" as to release conduction electrons?

#### sophiecentaur

Gold Member
is it not possible to uniformly heat the "insulating sphere" as to release conduction electrons?
You could take a charged 'wand' and touch the surface of the sphere at all points over its surface. Alternatively, you could put the sphere in a vacuum and bombard it with electrons whilst tumbling it at a steady rate. Electrons would then be spread (painted) over the surface. Of course, you would need to adjust the energy and focus of the electron beam because it would be deflected by electrons already on the surface. Woops - just read the previous post which says more or less the same thing.

What about covering the sphere with a plastic coating and ripping the coating off. You would then get a charge all over the surface as you do with a roll of cling film. That. I think, is a method that could actually deliver some sort of a practical result.

#### mfb

Mentor
What about covering the sphere with a plastic coating and ripping the coating off. You would then get a charge all over the surface as you do with a roll of cling film. That. I think, is a method that could actually deliver some sort of a practical result.
As far as I know, those induced charges are quite unpredicable, and not constant.

In addition, it charges the surface only, not the full volume.

#### sophiecentaur

Gold Member
You're a hard man to please.

#### mfb

Mentor
I would use something more space-filling, like cubes (with special parts for the surface?), but that is possible, sure.

Staff Emeritus
You're unhappy with perfectly insulating, but are fine with perfect spheres?

Like a lot of problems in physics, these are idealizations or approximations. Just like frictionless planes, massless and stretchless ropes, etc.

#### sophiecentaur

Gold Member
Perfect 'enough'?

#### technician

In my Physics book there was this problem of finding electric field produced by the sphere, such that electric charge is distributed uniformly throughout the volume of an insulating sphere.

I know that excess charge tends to distribute itself on the surfaces, but since this sphere is made from insulating materials excess charge cannot leave individual molecules to do so?

Anyway, how it is possible to charge an insulating sphere throughout the volume?
What physics book?

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