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


    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.
  2. jcsd

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