active&reactive power

Thread Starter


Joined Mar 13, 2006
I have two questions to ask
what is difference between active power(P) and reactive power(Q). why dont arithmetic sum P+Q doesnt give apparent power S. Why should they be added (P^2+Q^2)^0.5. please give me the answer with physical interpretation.
what is differnce between resistance and reactance. why dont a resistor has reactance. while inductor and capacitor has internal resistance


Joined Nov 17, 2003
Active power is the true power loss from a component, i.e. power loss through the energy conversion process to heat due to current flow. This type of power loss is due to resistance. Inductors and capacitors do not exhibit this type of power loss since they store energy as opposed to dissipating heat, however they do produce a voltage drop across the devices giving the impression of true power loss - this is known as reactive power.

As for the reason that the apparent power is equal to (P^2+Q^2)^0.5 is due to the concept of the power triangle, where the apparent power is the magnitude of the hypotheneuse of a right angled triangle, and the other two sides are the true and reactive powers respectively (positioned at right-angles to each other).

A good diagramatical explanation can be found here.