# Pressure on charged spherical shell, alternative solution

1. Jun 6, 2012

### Pifagor

1. The problem statement, all variables and given/known data
Find the pressure on a uniformly charged spherical conducting shell of Radius R and total charge Q. The answer is (Q^2) / (32*π*ε*R^4)

I´m fine doing this using the derivative of the energy as the sphere grows to get the force.

My question is: Why do I get twice the answer if I think of a small surface element dA and calculate the force on it as the field E = Q/(4πεR^2) multiplied by the charge on that surface element, dQ = dA*(Q/(4πR^2) ?

I cannot see where I double the force. How should this approach be mended?