# Can someone help explain me a problem?

1. Jan 26, 2012

### xdrgnh

1. The problem statement, all variables and given/known data

A sphere of radius R has a charge Q distributed uniformly over its surface. How large
a sphere contains 90 percent of the energy stored in the electrostatic ﬁeld of this charge
distribution?

2. Relevant equations

U= 1/8pi integral E^2dv.

3. The attempt at a solution

I was able to derive that the energy to assemble a shell is Q^2/2R. The problem is idk what kind of sphere they are talking about? Do they mean a hollow sphere or one with a charge density?

2. Jan 26, 2012

### Staff: Mentor

I think they mean solid sphere because otherwise they would say hollow sphere in the question.

So you have to compute the energy in the initial sphere, take 90% of it and then use it to get a new radius, right?

3. Jan 26, 2012

### xdrgnh

I guess it is a solid one. The thing that confused me was that the initial sphere has to be hollow because there is no charge density on the inside just on the surface. Thus I know I can treat that as a shell.

4. Jan 26, 2012

### netgypsy

Solid or hollow the charge is on the surface only.

5. Jan 26, 2012

### Staff: Mentor

It could be that they mean what size of a spherical volume of space surrounding the charged sphere contains 90% of the energy of the field. So the "sphere" would be insubstantial, like a Gaussian surface, a designated boundary.

Last edited: Jan 26, 2012
6. Jan 26, 2012

### xdrgnh

I think I understand now thanks a lot gneill I'll post my answer tmr. I'm pretty tired from working all day.