# Homework Help: Ampere's Law and charge on the capacitor

1. Apr 28, 2015

### jsko

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

Number 9

2. Relevant equations

Ampere's law; B*dl= mu(I_enclosed + I_displace)
I_displace=e_0 * d(E*dA)/dt

3. The attempt at a solution

There's no current enclosed, only displacement current. I don't know how to find the change in electric flux, so I don't know how I can compare B1 and B2. I know B1 is just = mu*I/(pi*r)

2. Apr 29, 2015

### ehild

You know how the charge on the capacitor and the current flowing to it are related. You also know the relations between voltage and charge, and voltage and electric field. From all of those, you get the displacement current - how is it related to the current flowing in and out of the capacitor?

3. Apr 29, 2015

### jsko

Will displacement current = current flowing in and out of capacitor?

If I relate them to area ->
B_2*pi*r = mu*e_0* (pi/4*r^2)/(pi*r^2) * I <-------- ratio of small disk (radius = r/2) over total area of capacitor radius r
B_2= e_0/4 * mu*I/(pi*r)
B_1,3= mu*I/(pi*r)

Except we still have e_0 as a factor, which isn't one of the options....

4. Apr 29, 2015