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Homework Help: About kirchoff?

  1. May 13, 2008 #1
    about kirchoff????

    1. The problem statement, all variables and given/known data

    can anybody explain why we write kirchoff laws on RL circuit like

    -Ldı/dt -iR=0

    shouldn't it be written like

    -Ldı/dt +iR=0

    2. Relevant equations

    -Ldı/dt -iR=0
    -Ldı/dt +iR=0

    it is about signs and how we know inductans' poles + or - while writing these equations
  2. jcsd
  3. May 13, 2008 #2
    maybe the inductor can be considered as a normal resisitor...therefore its written as -iR
  4. May 13, 2008 #3


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    Homework Helper

    Hi cherev,

    From your equation, I guess your RL circuit is just a resistor and an inductor attached together (no voltage source).

    In that case, Kirchoff's loop rule indicates that the magnitudes of the potential differences must be the same for the resistor and inductor, and that one must be positive and one must be negative. (That's the only way two potential differences would cancel as you travel around the loop.) Which one is positive and which one is negative depends on whether you go clockwise or counterclockwise around the loop.

    So let's say we travel around the loop in the same direction as the current. Then the potential difference across the resistor is negative, and across the inductor is positive:

    {V}_L -I R =0

    But [itex]V_L[/itex] is the back emf of the inductor, and is therefore [itex]V_L = -L (dI/dt)[/itex], giving your result.

    If you go the other way around the loop, [itex]V_R=IR[/itex] is positive and [itex]V_L[/itex] is negative, which leads to the same result except with both terms positive.
  5. May 14, 2008 #4
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