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Homework Help: RL Circuits

  1. Mar 14, 2010 #1
    1. The problem statement, all variables and given/known data
    A coil of resistance 8.0 ohms and self-inductance 4.0 mH is suddenly connected across a constant potential difference of 100 V. Let t = 0 be the time of connection, at which the current is zero. Find the current I and its rate of change dI/dt at times (a) t = 0, (b) t = 0.1 s, (c) t = 0.5 s, and (d) t = 1.0 s.

    2. Relevant equations

    If=E_o / R = 100V/8.00ohms = 12.5A

    T(time constant)= L/R = 4.0 mH/8ohms = .500 ms

    3. The attempt at a solution
    Substitute If and T to obtain
    I = (12.5A) (1-e^(-t/.500ms))

    Express dI/dt :

    dI/dt = 12.5A(-e^(-t/.500ms)) <--- here is where im stuck. Seems like there should be a numer in the thousands to multiply with units s^-1 to cancel out the other units

    which in return would yiels aan anser like:
    = (##kA)e^(-t/.500ms)

    Then substitute all the reset of the values in.

    Not sure what i am missing here Please help me!!!
  2. jcsd
  3. Mar 14, 2010 #2
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