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Homework Help: Want to find the current of inductor .

  1. Oct 8, 2011 #1
    1. The problem statement, all variables and given/known data

    Consider a 50 mH inductor .
    find i at t=10 ms if v= 20t^2 V for t>-0.1 ms and i(-0.02)=5 mA.
  2. jcsd
  3. Oct 8, 2011 #2


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    Staff: Mentor

    Forum rules: You should follow the posting template; you need to show some effort before help can be given; We don't do your homework for you.
  4. Oct 8, 2011 #3
    Sorry it's my first time here ;)
    by the way it's not my homework :D i just try to answer some problems with myself !
  5. Oct 8, 2011 #4


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    Science Advisor

    Tell us what you've tried so far. Do you know the integral relation between inductor voltage and current?

    BTW. Does "-0.02" represent -0.02 ms or -20 ms?
  6. Oct 8, 2011 #5
    I think that I should use this equation
    i(t)= 1/L ∫ V(t)dt + i(t)→(intial current )
    ... the problem tell me i(-0.02)=5 mA , i think it's -0.02 ms !...
    the final answer is 6.2 mA but i couldn't get it :(
  7. Oct 8, 2011 #6


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    Yes that's the correct way to approach it. The integral gives the change of current over the interval, so you just need to know the current at some point in time and then start you're interval from there. So t=-0.02 is ideal, as long as that's -0.02ms and not -20 ms, because we don't know what the voltage is prior to 0.1ms (hence we can't start the integral from that point).

    If you want a numerical answer then the details are important. V = 20 t^2, but you've used ms everywhere else so how do I know if "t" is ms or seconds in this formula? It only makes a difference of a factor of one million in the final result!
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