1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
    50/(10+0.2S)=I(s)
    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

    gneill

    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)=250/s(s+50)
    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
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Simple RL question with laplace
  1. Simple RL circuit (Replies: 1)

  2. Simple RL Circuit (Replies: 6)

  3. Simple RL circuit (Replies: 11)

Loading...