# Capacitor

joe shmo
Posers and Puzzles 09 Aug '12 15:12
1. joe shmo
Strange Egg
09 Aug '12 15:12
This is just a physics problem that I can't put together, I'm not in the course anymore...just reading over the material to occupy myself until I find some employment. :-)

Anyhow, the problem as stated

Determine the rate at which the electric field changes between the round plates of a capacitor, 6.0 cm in diameter, if the plates are spaced 1.1 mm apart and the voltage across them is changing at a rate of 120 V/s

The equation i'm trying to use, which gives the electric field between to closely spaced conductors is as follows

E = Q/(e*A)

where,
E = Electric field
Q = Electric Charge
e = permittivity of free space
A= surface Area of the plates

So if we are looking for the rate at which the electric field changes, differenciate the above equation with respect to time

dE/dt = (1/(e*A))*dQ/dt

So the problem is that I'm having trouble relating the change in voltage per unit time given in the problem to the current (dQ/dt) in the working equation. I don't really care about the numbers. If you could just explain the above relationship to me I would be happy.

Thanks