RC circuit question

Discussion in 'Homework Help' started by epsilonjon, Jul 30, 2011.

  1. epsilonjon

    Thread Starter Member

    Feb 15, 2011

    The circuit shown is at steady state before the switch opens at time t=0. Determine the voltage accross the inductor for t>0. The op-amp is assumed to be ideal.


    I'm following the method the book uses for these types of questions which is to first convert the circuit connected to the capacitor to its Thévenin equivalent. So first I find the open circuit voltage and I get this to be 10V. Next I want the short circuit current so that I can get the Thévenin resistance. But if I replace the capacitor by a short circuit then I have a problem. The op-amp is ideal so no current flows along the inputs, which means that the short circuit current must flow around the outer loop. But then i'm going to get 3 voltage drops and no voltage rise :confused:

    Thanks for any help!
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    If switch is close at t < 0 (steady state before the switch opens)
    So the capacitor voltage is equal 5V. And since we have the ideal opamp in the circuit Rout=0.


    So the only resistance it is seen by the capacitor is 20K resistor.
    And at time T=0 (switch opens) the opamp voltage rise to 10V.