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Simple current and charge circuit question

  1. Mar 11, 2008 #1
    1. The problem statement, all variables and given/known data
    If the current in a wire is given by the following functions of time, what charge passes
    a given point from t=0 to t=10 seconds? (i is in Amps and t in seconds). Show units
    for charge.

    i(t) = 4t + 5


    2. Relevant equations

    Nothing really

    3. The attempt at a solution

    0 seconds: 5A
    10 seconds: 4x10 + 5 = 45A

    Therefore 50C have passed through the wire in total.
    But the answer at the back for my question says 250C, I don't get it.
     
  2. jcsd
  3. Mar 11, 2008 #2
    You have to integrate that formula with respect to time, then plug in a value for t.
     
  4. Mar 11, 2008 #3
    Ah...now I see that I get the right answer...but why do we have to integrate. I don't get it.
     
  5. Mar 11, 2008 #4
    Nvm I just realized. This is what happens when you stay out of touch with physics for 4 months.
     
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