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Homework Help: RL circuit inductance problem

  1. Feb 26, 2010 #1
    1. The problem statement, all variables and given/known data

    An RL circuit has a 9.0 V battery and a 5.5 ohm resistor. When the switch is closed, the current in the circuit is observed to increase from 0 to 0.30 A in 0.17 s.
    A) What is the inductance?
    B) How long after the switch is closed does the current have the value 0.47 A?

    2. Relevant equations
    τ = L/R
    EMF = L*I / t
    I = EMF / R *(1-e^-tR/L)

    3. The attempt at a solution
    I don't know how to find a time constant, my text doesn't explain it, which makes me think that this question may not even use this equation. I can't get to part B without an answer to part A. By using the second equation I got L = 5.1, but that isn't correct.
  2. jcsd
  3. Feb 26, 2010 #2
    are you sure you don't know how?
  4. Feb 28, 2010 #3
    No, I don't. Its asking for inductance, I have resistance, I have no idea how to get the time constant.
  5. Mar 1, 2010 #4
    [tex]I(t) = \frac{v}{R} e^{\frac{-tR}{L}}[/tex]
    You know the value of current = 300mA at t = .17. v = 9 and r = 5.5. This leaves 1 unknown. Solve for it.
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