1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Complex power question. What is S exactly?

  1. Oct 11, 2014 #1
    • Warning! Posting template must be used for homework questions.
    I know that S = P +jQ however I am kind of unsure of what S exactly is.

    Is S the power supplied to the circuit of which only P is used by a load (e.g. powering a light bulb) whilst the Q component is wasted?

  2. jcsd
  3. Oct 11, 2014 #2
    S = apparent power
    P = real power
    Q = reactive power

    Q is not wasted. It is not dissipated but circulates between source and load and back from load to source.
  4. Oct 11, 2014 #3


    User Avatar

    Staff: Mentor

    Q represents supply current that does nothing useful as far as the power company is concerned. A non-zero value of Q means the power company is having to supply you with more current than you really need, so their transformers have to be larger and cables have to be heavier, and powerline I2.R losses are greater.

    Ordinary power meters record your time usage of P and you aren't billed for Q. Power companies would prefer every consumer reduced their load's Q to zero (or else paid or it, I guess :DD )

    Over one cycle, Q is first +ve then goes -ve, so it's being taken from, then delivered back to, the mains. So it's no net flow.

    Have a read of: http://www.allaboutcircuits.com/vol_2/chpt_11/1.html
    Last edited: Oct 12, 2014
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted