# Homework Help: Potential energy and electrostatic forces

1. Apr 20, 2006

### A_I_

show that the potential energy due to electrostatic forces of uniformly charged sphere of radius R and total charge Q is:

3Q^2/5R

i dont even know how to start??
i really don't want to hate physics

i was reading through in my textbook and they only gave us the formula, and they didn't say anything about how to derive it.
In the notes, the professor did NOT say anything about that...

HELP,
give me hints.. :)

HELP!!!!!

Thanks :)
Joe

2. Apr 20, 2006

### Gokul43201

Staff Emeritus
1. Carve the sphere up into thin spherical shells of thickness = dr and radius r (0<r<R)
2. Calculate the potential energy between one such shell and all (the rest) of the sphere - do all parts of the "rest of the sphere" contribute a potential at the position of the chosen shell ?
3. Integrate over all the shells in the sphere.

Make a start, and see if you can take it all the way.

3. Apr 22, 2006

### A_I_

the problem is that i don't know the formula of the potential energy when we are dealing with charges :(

i know E=mgh where mg is the force and h is the distance.
If we put F (coulomb force) to be the force in this problem, and r??? to be the distance, and then we integrate it??it still doesn't make any sense? :(

Thanks for the help again :)

4. Apr 22, 2006

### Gokul43201

Staff Emeritus
This is covered in your text. Look it up. Do you know the eqution for the force between charges ?

5. Apr 22, 2006

### A_I_

yes F = kQ1Q2/r where K is the coulomb constant :)

But Still i didnt find the formula :(

any hint? :)

6. Apr 23, 2006

### Gokul43201

Staff Emeritus
Actually, F = kQ1Q2/r^2 is the equation for the force. The equation for the energy is given by
E = kQ1Q2/r
The best advice I can give you is start at the beginning of this chapter (Electrostatics) and study it carefully solving all the worked examples along the way.

PS : What is your textbook ?