# inductor question

Discussion in 'Homework Help' started by Vanush, Jun 15, 2008.

1. ### Vanush Thread Starter Active Member

Apr 19, 2008
46
0

Can someone please tell me if my logic is right regarding the above circuit? I'm trying to find the voltage, v for t > 0

i(t) = i(inf) + (i(0) - i(inf))*exp(-t/(R/L))

At i(0), the current across the 10ohm resistor will be 2 A as the inductor causes the current in its branch to be zero -> current can't change instantaneously.
At t(inf), the inductor will be short circuit to steady state dc, so circuit acts like a current divider and the current across the resistor in question will be

Ix = (Rt/Rx)*(Is) = (5/10)*2 = 1.

so the current flowing throug the resistor in question for t > 0 will be

i(t) = 1 + (2 - 1)*exp(-t/20) = 1 + 1*exp(-t/20)

thus the voltage will be 10 + 10*exp(-t/20)

answer says i'm right, but when i showed my working to my tutor, he didn't know what the hell i was on about.

2. ### Ratch New Member

Mar 20, 2007
1,068
4
Vanush,

You defined the current source as 2u(t), which means constant 2 amps after t>0. Why do you show it as variable with time?

Current cannot change instantaneously in the coil, you mean. That value of zero is only true at the t=0 instant.

Yes, your logic is correct, but you probably need to learn a systematic method of calculation for loop circuits like these in order to solve more complicated problems that are not so intuitive. Ratch

3. ### Vanush Thread Starter Active Member

Apr 19, 2008
46
0
I mean the current flowing through the 10 ohm resistor that i want to know the voltage of