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Circuit with capacitor, current source, voltagr source and a switch

  1. Feb 19, 2007 #1
    1. The problem statement, all variables and given/known data
    the switch in the circuit is closed at t=0 for a very long time.

    find the current across the 2 ohm resistor before the switch is closed.
    find the current across the 5 ohm resistor before the switch is closed.
    find the current across the 2 ohm resistor RIGHT at t=0.
    find the current across the 5 ohm resistor RIGHT at t=0.
    explain why the current across the 2 ohm resistor at 0- is equal to 0+
    explain why the current across the 5 ohm resistor at 0- is not equal to 0+

    here is the circuit i made in LTSpice (and a little with paint):
    [​IMG]

    2. Relevant equations
    when the switch is open, everything to the right of the switch isin parallel with the switch... everything to the right of the switch is shunted when the switch is closed. the circuit is really then, two seperate circuits.

    ohm;s law:
    V=iR

    relationship btwn current, voltage, and capacitance:
    i=C(dV/dt)

    KVL and KCL


    3. The attempt at a solution

    this is as far as i have gotten:

    i used (for the left side of the circuit when the switch is closed):
    V4=i(R4)
    R4 = 1 volt / 20 ohms = 0.05 amps

    so, when the switch was open, the initial current through R3====>
    0.1amps + 0.05 amps = 0.15 amps

    the right side of the circuit is a voltage divider when the switch is closed

    While the switch is open, the capacitor fully charges up to 1 volt. right at t=0, there is no instantanious change in voltage, therefore dV/dt = 0 therefore current through the capacitor is also 0.

    any ideas?
    (btw is there a way to make this image smaller without having to re-upload it?)
     
  2. jcsd
  3. Feb 20, 2007 #2
    Analyze the circuit at t = -0, t= +0, t=0 separately. I saw that you kind of did it that way, but just label just label it so that its easier to follow your work.

    And what's your question?
     
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