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Homework Help: RL circuit

  1. May 27, 2013 #1
    1. The problem statement, all variables and given/known data

    At the moment t = 0, a 24.0-V battery is connected
    to a 5.00-mH coil and a 6.00-V resistor. (a) Immediately
    thereafter, how does the potential difference across the
    resistor compare to the emf across the coil? (b) Answer
    the same question about the circuit several seconds later.
    (c) Is there an instant at which these two voltages are equal
    in magnitude? If so, when? Is there more than one such
    instant? (d) After a 4.00-A current is established in the
    resistor and coil, the battery is suddenly replaced by a short
    circuit. Answer parts (a), (b), and (c) again with reference
    to this new circuit.

    2. Relevant equations

    3. The attempt at a solution

    a) V_r = 0 at t=0
    b) V_r = ε at t --> ∞

    V_l = V_r at t=??

    I tried doing -L di/dt = IR to solve for t and it got me nowhere

    d) when the battery is removed,
    I got V_r = ε at t=0 and V_r = 0 at t-->∞
    , and still dont know if there is a time that V_r = V_l
  2. jcsd
  3. May 27, 2013 #2


    User Avatar

    Staff: Mentor

    The elements are in series, presumably.

    If you can't solve a first-order differential equation, then steal someone else's solution from a textbook. The response of a first order system (whether R+L or R+C ) is something you need to practise until you can sketch the graph in your sleep.
  4. May 27, 2013 #3
    I know how to do the differential equation

    the problem is that everything cancelled out and got me back to R = R.
  5. May 27, 2013 #4


    User Avatar

    Staff: Mentor

    What is the general solution to the DE with L, R and V?
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