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Homework Help: Simple RL question with laplace

  1. Oct 21, 2014 #1
    1. The problem statement, all variables and given/known data
    I have a circuit with resistor and inductor (RL) that is placed serial and a power supply.
    the voltage is constant 50 V at t=0 , R=10 ohm, and L=0.2 H.
    Find the I using laplace transformation
    2. Relevant equations

    3. The attempt at a solution
    my attempt is transform everything to S-domain and with nodal analysis
    so the equation go like this
    so, 250/(S+50)=I(s)
    then i transform it to t-domain
    I(t)=250 e^(-50t)
    at t=0
    I(0)=250 e^0
    so I(0)= 250
    is this right?
    i have V at 50 Volt, but the current become 250 A
    am i doing it right?
    thanks for the reply
  2. jcsd
  3. Oct 21, 2014 #2


    User Avatar

    Staff: Mentor

    If you recall how inductors behave, you wouldn't expect the current to instantaneously go from zero to 250 Amps at time t = 0 when the voltage is switched on. So clearly something has gone awry in your workings.

    Your voltage source should "turn on" at time t = 0, so it's really 50u(t) volts. In the s-domain that becomes 50/s ;)
  4. Oct 21, 2014 #3
    ooh, so the equation go like this?

    I(s)=5/s - 5/(s+50)
    I(t)=5 - 5 e^ -50 t
    then because t=0
    then I(0) = 5-5 = 0
    so because it haven't been started so the current would be 0
    okay, thanks
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