Homework Help: Electrostatics problem

1. Sep 17, 2009

harini_5

1. The problem statement, all variables and given/known data
A spherical shell of radius R with uniform charge Q is expanded to a radius H.find the work done by electric forces during the shell expansion
given answer is 1/(8pi epsilon) *Q^2(1/R-1/H)
my attempt
work=-change in P E
=Q(V1-V2)
=1/(4pi epsilon) *Q^2(1/R-1/H)
plz explain the additional 1/2 factor

2. Sep 17, 2009

rl.bhat

Here you have to consider the energy stored in a spherical capacitor.
What is the capacity of a spherical conductor?
What is the energy stored in a capacitor?

3. Sep 18, 2009

harini_5

what is the mistake in my approach?
why not take change in potential energy as work done?

4. Sep 18, 2009

rl.bhat

Because the electric field in continuously changing during the expansion of the spherical conductor. To find the total work done you have to take the integration.
Energy stored in the capacitor= 1/2*Q^2/C
Capacity of the spherical conductor = 4πεοR