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Homework Help: Current of RC Circuit After Switch is Opened

  1. Mar 9, 2017 #1
    1. The problem statement, all variables and given/known data
    See attached image for circuit diagram and givens.
    The switch has been closed for a very long time.
    a) What is the charge on the capacitor?
    b) The switch is opened at t=0 s. What current initially flows?
    2. Relevant equations
    V = IR
    3. The attempt at a solution
    Since the switch has been open for a long time, no current flows through the 10 Ohm resistor. Therefore,
    I = 100V/(60 Ohm + 40 Ohm) = 1 Amp, V = 1Amp*40 Ohm, and Q = 2uF * 40V = 80uC.

    So my question is: Is the current that initially flows after the switch opens at t=0 s just due to the battery? So it would just be 1Amp? I'm having trouble understanding how to find the current at part b.

    Attached Files:

  2. jcsd
  3. Mar 9, 2017 #2


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    Hello MUU, :welcome:

    First step is to establish where the current flows. Through an open switch ? Or elsewhere ?
  4. Mar 9, 2017 #3


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    Hello @Math-U-Up. Welcome to PF!

    I think you might have misread the problem statement. According to the wording in the problem statement, initially, the switch has been closed for a very long time. That means that current has been flowing through the battery for a very long time.

    At time [itex] t = 0 [/itex], the switch is opened, meaning that after [itex] t = 0 [/itex] no current flows through the battery, and any current flowing in the circuit is the result of whatever residual charge is left in the capacitor.

    At least that's the way that I interpret the problem.

    (A "closed" switch means that the switch can conduct current. An "open" switch does not conduct current.)
  5. Mar 9, 2017 #4


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    The idea in PF is that the thread poster discovers that after a small nudge from a helper ...
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