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Homework Help: Charge or Discharge?

  1. May 25, 2014 #1

    Why are they using the i(t) = Vth/RTh .... formula to work out the current? I mean in the image on the right (in the above link), the inductor is de-energising through the resistors so why are they using an equation that is used when energising?
  2. jcsd
  3. May 26, 2014 #2


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    Staff: Mentor

    They are showing the general equation. If the network included a voltage source, then that equation would apply for the current through the inductor. It simplifies here because the voltage source is zero.

    While you can say the inductor is de-energizing, you could equally consider it to be energizing to a new steady state current (of VTH/RTH), and so keep the formula as general as possible until substituting data values at the last line.
  4. May 26, 2014 #3


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    Staff: Mentor

    My guess would be that somewhere in your course materials they derive an equation that uses the Thevenin equivalent as part of a general solution.

    In RL or RC circuits the responses of voltages and currents always follow an exponential curve, one that either decays from one level to another, or increases from a starting level to a higher level (plateau). With appropriate signed constants a general equation produce both "versions".
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