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Homework Help: RC Circuit

  1. Feb 19, 2005 #1
    A capacitor in an RC circuit is charged to 68.0% of its maximum value in 0.980 s. What is the time constant of the circuit?

    >> I know the time constant is equal to Q/(Q/t) and that t = 0.980 s. I also know that Q = CE and E is a constant. But they don't tell me the capacitance to use... Bah I'm really stuck and confused :( Does anyone know how to solve this?
  2. jcsd
  3. Feb 19, 2005 #2


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  4. Feb 19, 2005 #3


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    The time dependence of voltage across a capacitor when it is charged from a source of emf E is

    [tex] U=E(1-\exp(-t/\tau)) [/tex] ,

    [tex] \tau [/tex] being the time constant and the maximum voltage is E across the capacitor, so

    [tex] U/U_{max}=1-\exp(-t/\tau)[/tex]

    Just plug in the values and solve for [tex] \tau [/tex].

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