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Homework Help: LR Circuit Problem

  1. Nov 18, 2007 #1
    1. The problem statement, all variables and given/known dataIf You want to turn on the current through a coil of self inductance L in a controlled manner, so you place it in series with a resistor R= 2200 ohm, a switch, and a dc voltage source Vo=240 V. After closing the switch, you find that the current through the coil builds up to its steady=state value with a time constant T. You are pleased with the current's steady-state, but want T to be half as long. What new values should you use for Rand Vo.?

    2. Relevant equations
    I= Vo/R (1-e^(-t/T))

    3. The attempt at a solution
    I= 240/2200(1-e^(-t/T))
    I'= Vo'/R'(1-e^(-2t/T))

    Since, I=I'

    Can somebody point out what I am missing here? Thanks
  2. jcsd
  3. Nov 19, 2007 #2


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    Homework Helper

    You have the formula for the time constant T = L/R. You need the time constant to become half... so what do you need the R to become?

    The steady state value of the current is Vo/R. considering the change in R above, what do you need to do to Vo to keep the same steady state current as before? We don't want I = I'... we only want the steady state I to be equal to the steady state I'.
  4. Nov 19, 2007 #3
    Thanks...Got it...I was making it more complex than it was.
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