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Homework Help: Instantaneous current through an area

  1. Oct 6, 2011 #1
    1. The problem statement, all variables and given/known data

    Help with this question would be greatly appreciated as I have attempted it many times without success and I don't know where I'm going wrong.

    The quantity of charge q (in coulombs) that has passed through a surface of area 1.90 cm2 varies with time according to the equation q = 8t^3 + 7t + 6, where t is in seconds.

    What is the instantaneous current through the surface at t = 0.900 s?

    2. Relevant equations

    I = Q/t

    3. The attempt at a solution

    I would think that you find dQ/dt which would give 24t^2 + 7, substitute 0.9 into t which equals the charge and then divide that by the time to find instantaneous current. Current was = 20.15A, which was incorrect

    I also tried substituting 0.9 into 8t^3 + 7t + 6 and then dividing by time. Here the current was = 29.37A. Incorrect as well.
  2. jcsd
  3. Oct 6, 2011 #2


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    Staff: Mentor

    When you find dQ/dt and substitute in t=0.9s, that is the answer. You do not need to divide by time or anything else.
  4. Oct 6, 2011 #3


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    Staff: Mentor

    Why divide by time? If you take the derivative w.r.t. time, that's already dQ/dt.
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