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Homework Help: RC time constant lab, Current vs time

  1. Jan 24, 2010 #1
    1. The problem statement, all variables and given/known data

    In my lab we where given a simple circuit and told to get some data of a capacitor charging and discharging. Thus I now have a graph of current vs time, we used semi-log paper for this so the graph would be a straight line.

    We are given the value of the resistor and capacitor from the professor.

    We are now asked to find the RC time constant of both charge the discharge, then find the C of our capacitor from our data and compare it with the given value from our professor.

    C=(62x10^-6) farads
    I=(11x10^-6) amps
    Our lab asked us to turn up the voltage until we got 11uA therefore I can use ohms law to find my voltage using V=IR
    V=(11x10^-6)*(0.94x10^6) = 10.34V

    2. Relevant equations


    3. The attempt at a solution

    I know one value of C simply because my professor gave it to me.
    Given C = (62x10^-6) farads

    Now I must use the data I recorded to calculate C. This is where I'm stuck, I have a current vs time graph but cannot see how to get a C value with it and can't find the RC time constant of my data.

  2. jcsd
  3. Jan 24, 2010 #2


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    Welcome to Physics Forums :smile:

    What equation relates current to time, for your circuit? It is necessary to know that, in order to proceed.
  4. Jan 24, 2010 #3
    Current = charge / time
    I = q / t
    Solve for the unknown q

    Then I can use Q=CV right?
    Plug in Q and V and solve for C.

    C= Q / V

    This seems to make sense...
    Last edited: Jan 24, 2010
  5. Jan 25, 2010 #4


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    This would work if I were constant, but it isn't constant.

    The actual the relation is
    I = dQ/dt

    At any rate, you should have an equation something like
    I = I0e-t/t0
    in your textbook or class notes. That is the one you would need to use.
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