inductor question

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

  1. Vanush

    Thread Starter Active Member

    Apr 19, 2008

    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

    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
    I mean the current flowing through the 10 ohm resistor that i want to know the voltage of