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Help with RC circuits SPDT switch

  1. Mar 22, 2013 #1
    1. The problem statement, all variables and given/known data
    Find v(t) after t=0, when the switch moves up
    mj7pqg.png

    need to know if i did this right.. V(C1)=V(C2)=10V (this is same for t=0- and 0+)
    V(infinity)=0V (no voltage source)

    T=RC=100k(ohm)*(4uF)=0.4 sec. (i got the 4uF by putting the 2 caps. in parallel with each other, because they are in series after t=0.)

    v(t)=v(infinity)+[v(0)-v(infinity)]*e^(-t/T) <-- general equation
    V(t)=10e^(-2.5t)V
    where i got -2.5t by dividing -t/0.4

    please help!!


    2. Relevant equations



    3. The attempt at a solution
     
    Last edited: Mar 22, 2013
  2. jcsd
  3. Mar 22, 2013 #2

    gneill

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    Staff: Mentor

    Rethink your V(infinity). If both VC1(0+) and VC2(0+) are the same (10V at t = 0+), what potential difference exists to drive a current through the 100 kΩ resistor?
     
  4. Mar 22, 2013 #3
    well there wouldnt be any current would there? and i thought V(infinity) would go to 0V once the voltage source gets disconected. the caps. in dc are replaced by an open circuit so i believe current would also be 0A.
     
  5. Mar 22, 2013 #4

    gneill

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    Staff: Mentor

    With no current flow, the charges on the caps cannot change...
     
  6. Mar 28, 2013 #5
    sorry for the long delay, was waiting for my teacher to post homework answers because i was unsure myself.
    20g1sug.png

    according to this, i(t) is 0. because i think the 2 capacitors voltage cancel out? 10V-10V=0V.
     
  7. Mar 28, 2013 #6

    gneill

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    Staff: Mentor

    Yup. A more technically satisfying way to put it would be that because the potentials across the two capacitors are identical, the potential difference across the 100kΩ resistor is zero so that no current will flow through it. And since no current flows, the charges (and thus the potentials) on the capacitors will remain constant.
     
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