# Parallel Plate Capacitor: Electric field strength, flux & magnetic field

1. Jan 21, 2010

### geoffreythelm

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

This is a question on a past paper of a second-year undergraduate physics paper.

A parallel plate capacitor is charged and the voltage increases at a rate of dV/dt. The plate radius is R and the distance between the plates is d.

(a) What is the electric field strength E(V,d) inside a parallel plate capacitor? (4 marks)

(b) Find the electric flux for a circular area of radius r around the central axis (4 marks)

(c) Derive the magnetic field strength B(r, R, d, dV/dt) (9 marks)

3. The attempt at a solution

(a) Isn't this just E = V/d? Surely that's not worth 4 marks, but I can't imagine what else it could be.

(b) Similarly, isn't this phi = 4 pi k q? (or q/epsilon 0 r^2) O_O Or is it phi = E*d, so E*pi*r^2? SURELY not??

(c) Got a bit stuck with this one. As I wasn't sure about phi, especially. Ended up with

E = (μ0*I)/d + μ0*ε0*(q/ε0*r^2*dt)

Not sure how to progress from there, and especially get it in terms of dV/dt.

Any help would be great.