# Homework Help: Electrostatical energy of charged surface in applied voltage

1. Dec 17, 2012

### toni_2374

1. The problem statement, all variables and given/known data

1. Given a coloidal polystyrene particle with a diameter of 350 nm and a surface charge of 7.73. µC cm-2 evaluate: a) the electrostatic energy under and applied voltage of 100 mV

2. Relevant equations
particle charge density:
ρ = (3Q)/(4πR^3)
U0 =∫V (r)dq

3. The attempt at a solution
V(r)=q(r)/4πε0r
U0 =∫V (r)dq=(4π(ρ^2)R^5)/15ε0

BUT I DO NOT UNDERSTAND APPLIED VOLTAGE WHAT IT MEANS?

2. Dec 17, 2012

### Staff: Mentor

Hi toni_2374, Welcome to Physics Forums.

Perhaps "applied voltage" means an electric potential of 100mV where the particle is located; perhaps an awkward translation? If so they're probably looking for the electric potential energy associated with that charge being located there.

Also, do you think the polystyrene particle will be uniformly charged throughout its volume, or just on its surface? If just its surface, look carefully at your expression for charge density; what's the expression for the surface area of a sphere?

3. Dec 17, 2012

### toni_2374

yes particle will be uniformly charged on surface,and we consider particle as sphere to be able to use A=4πr^2,HOW should i calculate electric potential energy?

Last edited: Dec 17, 2012
4. Dec 17, 2012

### Staff: Mentor

Electric potential energy of a charge Q sitting at a location where the electric potential is V is just Q*V. Q is in Coulombs and V is in Volts, which is also Joules/Coulomb