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Homework Help: Electric Circuit Question PLEASE HELP!

  1. Feb 15, 2010 #1
    1. The problem statement, all variables and given/known data
    20fyjkn.jpg

    V = 1.2 kV
    C = 65 micro Farads
    R1 = R2 = R3 = 0.75 MegaOhms

    With C uncharged, switch S is closed (t = 0). At t = 0, what is the current through each resistor? At t = infinity, what is the current through each resistor?

    2. Relevant equations

    V= IR
    q = qo[1 - e^-t(RC)] (charging of capacitor
    q = qoe^-t/(RC)] (discharging of capacitor)

    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
    Last edited: Feb 15, 2010
  2. jcsd
  3. Feb 15, 2010 #2
    umm... how exactly are those components arranged and what is the problem asking you to find?
     
  4. Feb 15, 2010 #3
    sorry guys. the question is:
    With C uncharged, switch S is closed (t = 0). At t = 0, what is the current through each resistor? At t = infinity, what is the current through each resistor?
     
  5. Feb 15, 2010 #4
    do you see the picture? if not here is the link:
    http://tinypic.com/r/20fyjkn/6
     
  6. Feb 15, 2010 #5
    At t = 0, an uncharged capacitor has zero charge on it and zero voltage drop across it. At t = infinity, a capacitor's plates are fully charged and have no current going "through" them.
     
  7. Feb 15, 2010 #6
    but its not asking for the current through the capacitor but the current through the resistor
     
  8. Feb 15, 2010 #7
    At t=0 the circuit is equivalent to a circuit with C replaced by a wire.
    At t=infinity the circuit is equivalent to a circuit with C replaced by an open switch.

    You should be able to figure out the currents in these resistor only circuits.
     
  9. Feb 15, 2010 #8
    Yes, but if it has zero current through it, it looks like an open circuit, and if it has a drop of zero volts, it looks like an short circuit. I was trying to get you to arrive to the answer instead of stating it.
     
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