Capacitor discharging + Transients

Discussion in 'Homework Help' started by techie, May 1, 2008.

  1. techie

    Thread Starter New Member

    Jan 2, 2008
    Hello , i 'm skeptical about some things and would appreciate your help .
    At the first drawing of the image i attached there is a charged capacitor (was charged from the circuit when the switch was at position 1 ) which is currently part of an open circuit(switch at position 3 ) and after a specific amount of time will make a loop with the voltage source ( switch position 2).
    My queries are these :
    a) Will there be current flowing through the Resistance? (there is a voltage difference across the resistance so i think there should be current flowing , (V1-Vc)/R
    b) Will the capacitor discharge or not? Should it discharge if there is no current flowing as it is an open circuit?(assuming we are close to the settling time )

    At the second drawing of the image ( Z is an inductor , couldn't draw it good enough :p ) the switch is at position 1 and after some time goes at position 2.
    I have found the complete response of the circuit at position 1 and i 'm wondering , since the time it takes for the switch to switch positions is not bigger than the settling time of the circuit (not at steady state yet ) what will the initial conditions at 2 be?
    I obviously shouldn't just use the forced response to find them because its not a steady circuit yet , but shall i use the complete response or just the natural response ?
    • Q1.jpg
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  2. Caveman

    Senior Member

    Apr 15, 2008
    While in position 3, no current will flow through the resistor.
    When in position 2, current will flow to further charge the capacitor until it reaches 100V.

    In position 3, an ideal capacitor will not discharge. However, in a real capacitor, it will slowly discharge due to leakage resistance across the capacitance.

    If you assume that the switch time is instantaneous, then the current through the inductor will not change. So the voltage across the resistor will not change. Therefore, the voltage difference between V1 and V2 will develop across the inductor.

    If you assume a non-zero switch time, you will not be able to solve it, since during the switch time, the inductor current is being forced to zero instantaneously, which will lead to an infinite voltage. Of course in real life, there would be some capacitance across the inductor as well as resistance.
  3. techie

    Thread Starter New Member

    Jan 2, 2008
    Thank you for your answer and time ! ;D