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Homework Help: Calculating total charge

  1. May 18, 2010 #1
    1. The problem statement, all variables and given/known data
    The current at the terminals of the element in an ideal basic circuit element is
    i = 0, t < 0;

    i = 20e(-5000t) A, t [tex]\leq[/tex] 0

    Calculate the total charge ( in microcoulombs) entering the element at its upper terminal.


    2. Relevant equations



    3. The attempt at a solution I'm just starting a Circuit Analysis course next week and I'm looking ahead in the text, trying to hit the ground running. There are no sample problems that even give me a starting place??
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. May 18, 2010 #2

    rock.freak667

    User Avatar
    Homework Helper

    Current can be expressed at the rate of flow of charge, so that i =dQ/dt.

    So you can integrate over time and get the total charge. Though why it time t < 0?
     
  4. May 18, 2010 #3
    I suppose that is just a way of saying that there was no current flowing before the reference time, t = 0.
     
  5. May 18, 2010 #4
    I tried integrating and came up with -0.04e-5000t + C coulombs.

    Answer is supposed to be 4000 microcoulombs. My integration is a little rusty. Help?
     
  6. May 19, 2010 #5

    rock.freak667

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    Homework Helper

    The -0.04 should be 0.004, but remember your time is t≥0. So you are really integrating from 0 to ∞ so you need to compute

    [tex]\left[ -0.004e^{-5000t} \right]_0 ^{\infty}[/tex]
     
  7. May 19, 2010 #6
    Thanks for the help, I knew the Integral to be solved and the limits of integration, what I was missing was the negative exponent of e. You helped me, thank you.
     
  8. May 19, 2010 #7
    Thanks rock.freak, I was overlooking the negative exponent of "e" when I was integrating.
    You helped me, thanks. Sometimes you just need to bounce it off someone else.
     
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