# Homework Help: A simple capacitor problem

1. Dec 29, 2013

### Noirro

1. The problem statement, all variables and given/known data
Thin parallel plate capacitor is connected to a U=600 V current source. Find the electric current flowing in the circuit if one of the electrodes is pulled at v=6 m/s velocity. The electrodes are square (l=10 cm), the distance between them is d=1 mm.

2. Relevant equations
C=ε0$\frac{A}{d+vt}$, C=$\frac{q}{U}$, I=$\frac{dq}{dt}$

3. The attempt at a solution
I tried finding the current by first expressing q from those capacitor equations and plugging it into the dq/dt equation, but there is still time t left. I am not sure if the answer depends on time or is it possible to find some constant value of I?

2. Dec 29, 2013

### ehild

No, the current is not constant. Find the time dependence of the current, and the current at t=0.

ehild

3. Dec 29, 2013

### Noirro

OK thanks!

4. Dec 30, 2013

### CWatters

I assume you mean a voltage source.

5. Dec 30, 2013

### rude man

Hint: i = C dV/dt is not the complete story.

The complete story is i = d/dt (CV).