# Homework Help: Question about displacement current and ampere's law

1. Feb 14, 2005

### andrew410

A 0.230-A current is charging a capacitor that has circular plates 10.0 cm in radius.

(a) If the plate separation is 4.00 mm, what is the time rate of increase of electric field between the plates?

(b) What is the magnitude of the magnetic field between the plates 5.00 cm from the center?

So...
I know the formula for displacement current, which is I = e(flux/dt) = e(EA/t).
I'm not sure how to get the electric field from the given information.
I need some help for this. Thx in advance!

2. Feb 14, 2005

### s_a

Use the following facts:
E = V/L, (E = electric field, V = voltage, L = separation)
Q = CV -> dQ/dt = C * dV/dt -> I = C * dV/dt,
C = εA/d (provable using Gauss' Law)

3. Feb 14, 2005

### Andrew Mason

a) Find the capacitance using $C = A\epsilon_0/d_{sep}$ (the permittivity of air is virtually the same as empty space). Then use $V = Ed_{sep} = Q/C$ and dQ/dt = I to get $dE/dt = I/Cd_{sep}$ to get the rate of change of electric field.

b) I don't know about the magnetic field. Are you sure it is not asking for the electric field? While there is a current with a particular directon leading to the plates, we don't know the direction(s) of the current(s) in the plates themselves.

AM