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Homework Help: Inductance RL Circuit

  1. Mar 19, 2008 #1
    [SOLVED] Inductance RL Circuit

    Calculate the inductance in an RL circuit in which R = 0.300 and the current increases to one fourth its final value in 1.40 s.

    I tried doing this with V=IR, (I know, the easy way didn't work).
    I also tried I= V/R(1-e^(Rt/L)) but I don't know what V would be in this equation. Can someone help me out here?
  2. jcsd
  3. Mar 19, 2008 #2


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    Staff: Mentor

    The value of V itself will not matter -- you are dealing with ratios and the time constant equation that you show. Just leave it as a variable "V", and see if you can solve for the inductance....
  4. Mar 19, 2008 #3
    Okay, so what you said got me thinking, and I remembered that the time constant equals 1/t and that equals L/R so I set 1/1.4 = L/0.3 but that didn't work. Am I on the right track?
    P.S. is inductance negative?
  5. Mar 19, 2008 #4
    Never mind. I got it.
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