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Circuit and electromagnetism basics

In an AC circuit, with active power P [W] and apparent power S [VA], which expression gives the power factor?

  1. AP / S
  2. BS / P
  3. CP × S
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Answer: A. P / S

The power factor is the ratio P/S of active power P to apparent power S (RMS voltage × RMS current). For sinusoids it can be written cos φ, where φ is the phase difference between voltage and current. For example, S = 2000 VA and P = 1800 W give a power factor of 0.9. A low power factor means more current is needed to deliver the same active power, increasing losses in wiring and transformers.

  • A. Correct. The power factor is P/S, a value between 0 and 1.
  • B. S/P is the reciprocal of the power factor and is 1 or greater.
  • C. P × S is not the definition of power factor, and its units do not work out.
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