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General Question about circuits

  1. Nov 29, 2013 #1
    1. The problem statement, all variables and given/known data
    I have a general question concerning circuits. I am a bit confused as to how to determine when the current is negative. For instance, on circuit diagrams, we are shown arrows that indicate the positive direction of current. Then, when we open the switch on the circuit, the current through an inductor all of a sudden turns negative. I thought that such a discontinuous change was not possible. In general, how can I determine when the sign of the current turns negative?

    2. Relevant equations
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    3. The attempt at a solution
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  2. jcsd
  3. Nov 29, 2013 #2


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    It is not the current that all of a sudden turns negative through an inductor when a switch gets opened. Just the opposite: If current flowed through a coil earlier, and we break the circuit so the current would cease, an induced electromotive force appears in the coil which maintains the current for a while: opposes the change of current. (Lenz's Law) Sometimes you even see a spark when you plug out a heater or hair-dryer from the socket.

    Last edited: Nov 30, 2013
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