How Accurate is the Calculated Inductance of 74mH in a Series RL Circuit?

[snipaj9696]

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!

 

[quantumdude]

I get 74mH too.