Homework Question (Calculating current by taking charge derivative)

Discussion in 'Homework Help' started by fishguts, Sep 10, 2009.

  1. fishguts

    Thread Starter Member

    Sep 3, 2007
    22
    0
    I have a question about the following homework problem.

    The charge enters the positive terminal of an ekement is given by q(t) = -12e^-2t mC. The power delivered to the element is p(t) = 2.4e^3t W. Computer current in the element the voltage across the element and energy delivered to the element in the time 0 < t < 100ms

    I know that current is equal to the derivative of charge

    i = dq(t) / dt

    But when I take the derivative of q(t) i get 24e^-2t which does not give me a constant. Do I plug in 100 ms into 24e^-2t and subtract that from 24e^-2t evaluated at 0?

    Thanks for any help!
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Integrate voltage*current between 0s and 100ms to find the energy.
     
  3. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    What leads you to believe the current should be a constant? Once you have the answer to that, you'll realize you already have the correct answer to the question about the current.
     
  4. jit26

    New Member

    Sep 12, 2009
    11
    0
    Hi,
    I am not sure but try this way.

    p(t)=v(t)*i(t)
    p(t)=v(t)*dq(t)/dt
    p(t)*dt=v(t)*dq(t)

    now intergrate t=0 to t=100ms with respect to t. :confused:
    not sure if i am right or wrong.
     
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