In problem 21(see attached image) i need to find the inductor voltage at a certain time. I tried using V=L(di/dt) but i dont think that formula is correct.

Then on problem 22 i am totally confused on. Current equals i=v/r. but what am i given the times for then?

Thank you in advance for your help!

 

Hi markp912,
For question no 21) the voltage across the inductor is given by VL =V(exp(-t/tau)
where V is the dc supply

and tau ->time constant = L/R
iinitially the inductor would have 15 volts
(since it acts as OPEN CIRCUIT)

at steady state VL = 0 (since it acts as short circuit)

Inbetween the voltage decreases exponentially

see this page for further explanation
http://www.allaboutcircuits.com/vol_1/chpt_16/3.html

 

thank you for your help. i got it.