# Find current for circuit with capacitor, resistor, capacitor in series

1. Apr 5, 2006

### Cyrus

I have to solve this problem for a circuit. Its a capacitor, a resistor and another capacitor all in series.

Both capacitances are the same, 1uF, and the resistor is 100k-Ohm.

I have to find the current function of time.

What I did was use KVL to get:

$$- \frac{1}{c_1} \int i(t)dt + v_1 (t_0) + \frac{1}{c_2} \int i(t)dt + v_2 (t_0) + R i(t) = 0$$

Then I took the derivative to get:

$$- \frac{1}{c_1} i(t) + R \frac {di}{dt} + \frac{1}{c_2}i(t) =0$$

Which simplifies to:

$$R( \frac{1}{c_2} - \frac {1}{c_1} )^{-1} \frac {di}{dt} +i(t) = 0$$

But the two capacitances have the same value, which means that RC-eq is zero. That's wrong....hmmmmmmmmm

Last edited: Apr 5, 2006
2. Apr 5, 2006

### barob1n

WTF. Why not change the signs of one of the capacitances. They should have the same sign. Or just let the combined capacitance equal to .5uF.

3. Apr 5, 2006

### Cyrus

Because based on the diagram, they are in the passive configuration which is why it is negative.