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Homework Help: Inductance formulas

  1. Mar 25, 2010 #1

    The current in a 4.70 mH inductor decays over time according to the following function, where t is in seconds.

    I(t) = (.943A) e-(396 s-1) t)

    (a) What is the induced emf across the inductor at t= 5.00 ms? (b) How long does it take the emf to drop below 1.25 mV?

    3. The attempt at a solution

    Since emf = -L(dI/dt) , I can take the derivative of the function above and plug in t=.005, right?

    I' = (.943A) e-(396 s-1) t) (-1/396s)

    Plug in t=.005 and I get I' = -.00238

    emf = -L(dI/dt)

    emf = -(.0047)(-.00238) = 1.119x10-5

    I tried this answer but it was incorrect. What am I doing wrong?
  2. jcsd
  3. Mar 25, 2010 #2


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    You have not taken into account the term e^-396*t.