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I would like to verify these answers please

  1. Apr 11, 2008 #1
    I would like to verify these answers please..

    What is the inductance of a series RL circuit in which R=1.0 K ohms if the current increases to 1/3 of it's final value in 30 Ms (M is the greek symbol Mu).

    Is the answer 30 (none of these) or is it 74mH?? I calculated 74mH

    i(t) = I (1-1/e^t/τ)

    given i(t) = 1/3*I and t =30uS and τ =L/R

    so,,substitutin these values

    we get t/τ = 0.41

    30*10^-6 / τ = 0.41

    τ ˜ 74 μ sec

    L/R ˜ 74usec

    R = 1K

    so,,L ˜74mH




    The choices for answers are.

    1.) 49
    2.) none of these
    3.) 99
    4.)74
    5.) 62

    thankyou!
     
    Last edited: Apr 11, 2008
  2. jcsd
  3. Apr 12, 2008 #2

    Tom Mattson

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    Staff Emeritus
    Science Advisor
    Gold Member

    I get 74mH too.
     
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