How do you calculate inductance in an RL circuit?

[GreenLantern674]

[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?

 

[berkeman]

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?

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...

 

[GreenLantern674]

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?

 

[GreenLantern674]

Never mind. I got it.