Problem 52-1    Source:  Problem 32 - page 440 -    BOYLESTAD, Robert -    Book: Introdução à Análise de Circuitos - 10ª edição - Ed. Pearson Education do Brasil - 2004.

A circuit dissipates 100 W (average power) at 150 V (rms input voltage) and 2 A (rms input current). What is the power factor? Repeat the problem for a power of 0 W and 300 W.

Solution of the Problem 52-1   

Item a -    P = 100 W

As V = 100 V and I = 2 A we can calculate the apparent power, or:

On the other hand, we know that the power factor can be expressed as the quotient between the average power and the apparent power. Soon:

Item b -    P = 0 W

In this case, since the average power is zero, it means that the circuit contains only reactive elements and, of course:

Item c -    P = 300 W

Note that in this case, P > S, that is, the average power is greater than apparent power and this is IMPOSSIBLE.