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Homework Help: Finding voltage in circuit with capacitors (sequential)

  1. Nov 3, 2016 #1
    1. The problem statement, all variables and given/known data
    I need to find Vc(t) in the following circuit:

    2. Relevant equations

    3. The attempt at a solution
    I was trying to follow an example in my notes. They automatically consider that t>0 it becomes closed and therefore like a short circuit. Can I do that in this case? If I do that, will there still be current/voltage through the short circuit branch? My thought would be to then use Thevenin or Norton to calculate the voltage that I need. How should I go about solving this problem? upload_2016-11-3_18-23-28.png
  2. jcsd
  3. Nov 3, 2016 #2


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    Staff: Mentor

    The circuit as drawn is problematical. When the switch is closed the voltage source will be shorted. That is not a correct scenario as it would cause the voltage source to produce infinite current.

    Well, ignoring the circuit issue and assuming that the voltage source and the wiring somehow survives this mistreatment, I note that the diagram indicates that switch opens at t=0 and remains open for 1.25 ms when it closes again. This is not the same scenario as the example you mentioned where the switch closed at t=0 but some of the principles used to analyze the circuit will be the same.

    Presumably the switch was closed for some long period of time before t=0. So just before the switch opens, what voltage will be across the capacitor? What current will be flowing in the resistor R? In other words, what are the initial conditions of circuit just before time t=0?

    Forget all that. The incorrect circuit scenario makes it impossible to tell what the initial conditions will be . Even if we presumed some non-ideal components we don't have enough information to determine currents or voltage drops when the switch shorts the voltage source.
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