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Homework Help: Find charge given current and time?

  1. Jan 21, 2012 #1
    1. The problem statement, all variables and given/known data
    If a current of 3.5 A flows through a wire for 25 minutes, how much charge passes through the wire?

    I = 3.5
    t = 25 min = 1500 seconds
    2. Relevant equations
    P=W/t
    W=1/2qV
    P=VI

    3. The attempt at a solution

    If P = W/t
    and
    W = 1/2qV
    then
    P = qV/2t

    so (qV/2t) = VI

    qV = 2tVI

    q = 2tI

    q = 2(1500)(3.5)

    q = 10,500 C

    .......................

    the answer given by my teacher is 5250 C. So somewhere, I'm off by a factor of two. Can someone help me find my error? Or did my teacher make a mistake?
     
  2. jcsd
  3. Jan 21, 2012 #2

    SammyS

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    For what situation is your equation, W = (1/2)qV, valid?

    All you really need for this problem is the definition of electric current.
     
  4. Jan 21, 2012 #3
    I=q/t

    q=It

    q=(3.5)(1500)

    q=5250 C

    ...........................

    Okay so I get that, but now I'm struggling with...
    I'm confused... i.e. the difference between using
    W=(1/2)qV
    and
    W=qV

    One is the charge of the capacitor the other is the charge on the electron/proton (usually these particles are the one's being talked about). This is the W=qV equation... i think...

    I read online that "W = qV is more for moving just one or a set number of charges across a voltage difference V (ie a capacitor) while W = 1/2QV is the work required to fully charge, or move ALL of the charges, across a capacitor to fully charge it"

    ...is that right? I don't really understand...

    ................................
     
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