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Homework Help: Rate of change of current in an R-L Circuit

  1. May 18, 2018 at 1:49 AM #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    Current cannot change suddenly through inductor unless input is impulse voltage source.

    3. The attempt at a solution
    since input is DC source we have i(0+) = i(0-) so we have di/dt at t = 0+ is 0.
    Is it correct? Book has mentioned answer as B which is weird cause there is no Ls in circuit.
  2. jcsd
  3. May 18, 2018 at 5:46 AM #2


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    It's probably a typo. They meant Is instead of Ls.

    How about using source transformation? It will make the circuit look more familiar.
  4. May 18, 2018 at 6:12 AM #3
    Wow.... I used laplace and then got the answer. You're right.
    But shouldn't it be zero as per concept? Unless the input is an impulse voltage source the current across inductor cannot change so quickly. V = L di/dt. So unless V is an impulse, di/dt has to be zero.
  5. May 18, 2018 at 6:43 AM #4


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    Well, if you use source transformation, you end up with a series RL circuit with Rtotal=Rs+R and inductance L, excited by a voltage source of emf IsRs.
    At t=0+, no current flows through the inductor, which means the entire source voltage appears across the inductor.
    So, IsRs=Ldi/dt, which gives di/dt=IsRs/L.
    No, here VL at t=0 is not zero, hence di/dt is also not zero. You can't compare the currents at t=0- and t=0+ to find di/dt at t=0+ because at t=0-, the inductor was not excited.
  6. May 18, 2018 at 6:49 AM #5
    Well you're method is way quicker than mine.
    Thanks a lot. Learnt new thing today.
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