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Homework Help: RC Circuit - Post-Lab Questions

  1. Mar 27, 2010 #1
    1. The problem statement, all variables and given/known data

    What is the voltage across a capacitor after a time of two constants when charging from zero voltage. When discharging from a fully charged condition?

    2. Relevant equations

    VCapacitor = VBattery(1 - e-t/2T)

    VCapacitor = VMaximum * e-t/2T

    3. The attempt at a solution

    I'm confused at how to start. I can do the algebra...

    Charging from zero voltage - so would I set VMaximum to 0? Wouldn't I need to know VBattery? That wouldn't make sense to me. If I can get an idea of how to approach this problem, I should be able to solve it no problem. Thanks!


    1. The problem statement, all variables and given/known data

    With V = Vo*e-t/RC, it mathematically takes an infinite time for a capacitor in an RC circuit to discharge. Practically, how many time constants does it take for a capacitor to discharge to less than 1% of its initial voltage?

    2. Relevant equations

    V = Vo*e-t/RC

    3. The attempt at a solution

    0.01*Vo = Vo*e-t/RC
    0.01 = e-t/RC
    100 = et/RC
    ln(100) = t/RC
    2*ln(10)*RC = t

    So, I'm saying that it takes approximately 4.61 time constants.

    Would this be correct? Thanks!
  2. jcsd
  3. Mar 27, 2010 #2
    For the first problem, I think your teacher wants a symbolic answer such as ".5V"

    Your 4.61 time constants is right.
  4. Mar 27, 2010 #3
    I'm not even sure how to work either of the first two problems? It seems like I should need to know more information...
  5. Mar 28, 2010 #4
    Anyone? I've been trying to work the first two still, and I just can't get past the fact that I think that I need more information.
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