# [E&M] Concentrical metalic spheres

1. Aug 11, 2015

### Xsnac

The problem statement, all variables and given/known data

I two concentric metalical spheres r1 =/= r2.
What to do to have:
a) potential energy = 0 and electric charge =/= 0
b) potential energy =/= 0 and electric charge =0.

In eighter one of the spheres.

Relevant equations
Gauss law.

3. The attempt at a solution
a)
If I charge the outside sphere, I will have potential energy = 0 on it and electric charge =/= 0.

b)
I tought If I charge the one inside with Q, I will have a potential =/= 0 on the bigger sphere and 0 charge.

Is this correct? The problem does not give correct answer.

2. Aug 13, 2015

### Physicist97

Do any of the spheres have an initial charge on them? And are you at a point in between the two spheres?

3. Aug 14, 2015

### Xsnac

I'm outside and I can charge eighter the bigger or the smaller or both of them. I have to have the specs from eighter a) or b) on 1 of them.

4. Aug 14, 2015

### Physicist97

Well, when you have a net positive or negative charge the pontential will approach zero only at infinity (unless otherwise specified such as the ground on a circuit). If you charge the outside sphere, why would potential energy be zero on it? May you show us what equation you are using for electric potential energy?

Last edited: Aug 14, 2015
5. Aug 14, 2015

### SammyS

Staff Emeritus
Electric potential

or

Potential energy stored in the system?

6. Aug 15, 2015

### Xsnac

Electric potential

7. Aug 15, 2015

### haruspex

That makes more sense. Electric potential is different from electrical potential energy.
So, what equation do you have for finding the electric potential at a given distance from a uniformly charged sphere?

8. Aug 15, 2015

### Xsnac

k * q1 / r ( where K is 1/4 pi epsilon )

9. Aug 15, 2015

### SammyS

Staff Emeritus
What does "r" represent ?

10. Aug 15, 2015

### Xsnac

the distance from where the electric potential is 0 to where you want to measure the electric potential.

11. Aug 15, 2015

### haruspex

12. Aug 16, 2015

### Xsnac

that's what I have written in the manual. Electric potential V = Q/(4*pi*epsilon) * 1/r. Should I upload a picture of the manual?

13. Aug 16, 2015

### haruspex

I'm not querying your equation. It's your definition of r that is wrong.

14. Aug 16, 2015

### Xsnac

then I'm lost. whats the definition? I always tought it is the distance between the place where electric potential is 0 to where you want to measure the electric potential..

15. Aug 16, 2015

### haruspex

The equation you quoted can be applied in two contexts.
In the most basic one it gives the potential at distance r caused by a point charge Q, if taking the potential at infinity to be zero.
It also works if the charge is spread uniformly over a spherical shell or through a solid sphere. In these cases, r is the distance from the centre of the sphere, and the equation only works if r is greater than the radius of the sphere.