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Homework Help: Transient analysis (RC) question problem

  1. Jun 5, 2010 #1
    hello friends....

    i am having following problem in this question.

    first i dont understand how they have calculated R equivalent=5/6.i think its value should be 25Kohms. will it be different at 1ms and 4ms or same ????\\

    i shall be thanks to u .
    in one picture is the problem n in other is solution itself.


    Attached Files:

  2. jcsd
  3. Jun 5, 2010 #2
    Yes, it is different before 1 ms and after that, because switch is opened at 1ms. At t=0, the voltage source becomes dead (i.e., it will be short circuited) and so the capacitor sees three resistors connected in parallel to it (effectively it is 5/6 kohm). But at t=1 ms switch is opened, the capacitor thereafter sees only one resistor (25kohm) connected to it.
  4. Jun 5, 2010 #3
    thanks for the reply sir, you have solved my problem.
    why is the voltage source dead at t=0. ????

    i m also confused with two differet time values..

    v(t)=25.e^-2000t for 0<t<1ms.

    tell me something about this equation....
  5. Jun 5, 2010 #4
    Because of the unit step function u(-t). Remember u(-t) is 1 for all t<=0 and is equal to zero for t>0.
    Upto t=0, the voltage source is active and at t>0, capacitor starts discharging to all three resistors (the above equation is valid), but at t=1ms switch is opened and so after that capacitor discharges only through 25kohm.
  6. Jun 6, 2010 #5
    after seeing such a marvellous reply , it came to my mouth ,wow !!!

    thanks friend , u have helped me a lot.:smile:
  7. Jun 6, 2010 #6
    CAN u provied me some pdf files on study on transient analysis ?.
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