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Homework Help: Electric current function of time

  1. Dec 28, 2014 #1
    1. The problem statement, all variables and given/known data
    ##i=20\sin(377t)## is the current function. in the time interval 0-1/120 sec what is the charge that passed

    2. Relevant equations
    $$q=\int_0^t i dt$$

    3. The attempt at a solution
    $$q=\int_0^{1/120} 20\sin(377t) dt=\frac{20}{377}\int_0^{3.14} \sin x dx$$
    $$q=-\frac{20}{377}\cos x|_0^{3.14}=-\frac{20}{377}(-0.0015)=79.6\cdot10^{-6}[Coulomb]$$
    It should be q=0.106
  2. jcsd
  3. Dec 28, 2014 #2


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  4. Dec 28, 2014 #3
    One word is enough, thanks
  5. Dec 28, 2014 #4


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    Maybe, but I'll add few anyway.
    It's not a good idea to substitute numbers in prior to the final step, particularly if they're only approximations.
    ##\cos(\pi)## is exactly -1, and you should not be using a calculator for that.
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