# Electrostatic repulsion formula for plates

1. Oct 5, 2009

### joshste86

Hey all

I read somewhere online that Coulomb's Law only applies to spheres and point charges, and was wondering what formula is used to calculate the force between 2 plates whilst charged the same polarity

Also, what is the formula to determine how much voltage is required to maintain a charge within the plates?

cheers

2. Oct 5, 2009

### kanato

Coulomb's law applies to any charge distribution, but if it's complex you have to integrate over the charge density to get the correct result. This can lead to more or less complex expressions depending on the charge distribution.

For a parallel plate capacitor, the equation is Q = CV where C is the capacitance, Q is the charge and V is the voltage.

The easy way to calculate the force between to parallel plates is to calculate the energy, and then differentiate with respect to the distance between them. You can do this using $$U = \frac12 CV^2$$ and $$C = \epsilon A/d$$. (Take care about whether you are keeping voltage or charge constant.)

Last edited: Oct 5, 2009