# Electrostatic potential

1. Feb 20, 2010

### kihr

1. The problem statement, all variables and given/known data
A spherical conductor of radius 2 m is charged to 120V. It is now placed inside another hollow spherical conductor of radius 6m. Calculate the potential to which the bigger sphere would be raised if the smaller sphere were made to touch the bigger sphere.

2. Relevant equations
Potential of a sphere = Q / 4*pi*E*r [ E = permittivity of medium]

3. The attempt at a solution

The potential of the smaller sphere = Q / 4*pi*E*2 Volts
Q= 4*pi*E*2*120
The same value of charge Q is induced on the outer surface of the larger sphere ( and -Q is induced on its inner surface).
Therefore, the potential of the outer sphere (before the two spheres touch each other) = Q / 4*pi*E*6 + Q / 4*pi*E*6 Volts [the first value is due to the charge on the inner sphere, and the second value is due to the charge on the outer sphere]

When the two spheres touch each other, charge flows from the inner sphere (which is at a higher potential) to the outer sphere till both the spheres reach the same value of potential.

I am, however, unable to proceed further to find out the potential to which the bigger sphere would be raised. Could someone please help with a few clues? Thanks.

2. Feb 20, 2010

### Shivpal

...Net charge is conserved...
The sum of the final charges on the spheres will be equal to the initial one...

3. Feb 20, 2010

### kihr

Yes, but this is still not helping me to find the potential of the bigger sphere! This is because when I calculate the potentials of the two spheres and equate them (since the potentials are the same when they are in contact with each other) the potential of the bigger sphere cancels out. I think the problem lies elsewhere.