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Homework Help: Voltage of capacitor

  1. Apr 19, 2014 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    v(t)=v(infinity)+ [v(0) - v(infinity)]^e(-t/RC)

    3. The attempt at a solution

    I found Vo(0) to be 8V, but I'm not sure about about v(infinity) and R

    If the switch is at B and the capacitor is an open circuit at steady state, then shouldn't the resistance be the addition of the parallel combination of 40k and 30k and the 20k
    however, the solution for R is given as 12

    and for v(infinity), I don't know know what to do
  2. jcsd
  3. Apr 19, 2014 #2


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    Staff: Mentor

    When the switch moves to position B the 40 k resistor will have one end open, so no current can flow through it and it will play no further role in the circuit.

    For v, remove the capacitor from the circuit and determine the potential across the points where it was connected.
  4. Apr 19, 2014 #3
    thanks, I have another question

    http://postimg.org/image/m6c1rf60h/ [Broken]

    why is there no mention of i(infinity) for this?
    is the current through the inductor zero at steady state with the switch closed?

    i(t)=i(infinity)+ [i(0) - i(infinity)]^e(-Lt/R)
    Last edited by a moderator: May 6, 2017
  5. Apr 19, 2014 #4


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    Staff: Mentor

    The inductor current falls away, becoming 0 at t=∞

    The switch directs to ground all current from the battery, so the battery has no influence over the inductor once the switch closes.
  6. Apr 19, 2014 #5


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    Staff: Mentor

    For a NEW question, why not make a NEW thread? Did you solve the problem of the current thread?
    Last edited by a moderator: May 6, 2017
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