Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Find current for circuit with capacitor, resistor, capacitor in series

  1. Apr 5, 2006 #1
    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:

    [tex] - \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 [/tex]

    Then I took the derivative to get:

    [tex] - \frac{1}{c_1} i(t) + R \frac {di}{dt} + \frac{1}{c_2}i(t) =0[/tex]

    Which simplifies to:

    [tex] R( \frac{1}{c_2} - \frac {1}{c_1} )^{-1} \frac {di}{dt} +i(t) = 0 [/tex]

    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. jcsd
  3. Apr 5, 2006 #2
    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.
  4. Apr 5, 2006 #3
    Because based on the diagram, they are in the passive configuration which is why it is negative.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook