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Circuits question

  1. May 26, 2013 #1
    1. The problem statement, all variables and given/known data

    Why is it when referring to the power delivered through a circuit the value is negative, and the power absorbed would be positive?

    Logically it doesn't make sense to me that negative power value is being delivered through out the circuit.

    Thanks

    Higgenz



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. May 26, 2013 #2
    Because when the power is being delivered, its being "taken away" from the circuit. When it's being absorbed, it's "gaining" power, thus positive.

    P= V x I, so if you think about it the electric potential is positive, and the current is positive. It's converting, or charging or something. When it's releasing (or delivering) power, like a battery would do to an associated item, the power in the battery's circuit would be negative (because the current is flowing out, and thus negative)
     
  4. May 26, 2013 #3
    awesome explanation thanks!
     
  5. May 26, 2013 #4
    Whether a positive number for power means the element is "producing" or "consuming" power depends on the sign convention used.

    For the passive sign convention, positive power means the circuit is consuming power. For the active sign convention, positive power means the circuit is producing power:
    http://en.wikipedia.org/wiki/Passive_sign_convention#Sign_conventions
     
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