Spherical capacitor energy problem

[AgPIper]

Show that the energy associated with a conducting sphere of radius R and charge Q surrounded by a vacuum is

U = k*Q^2 / (2R)

thanks

 

[Gza]

Where are you having difficulty with the problem?

 

[turin]

Consider how much work you would have to do to bring all of the charge together from an infinite separation. In other words, how much work does it take to contract an infinitely large sphere of charge Q down to a small sphere of radius R?