My instructor asked me why have I written the power factor formula as
If \$\varphi\$ is the phase angle between the current and voltage, then the power factor is equal to the cosine of the angle, \$cos\,\varphi\$: $$|P| = |S|\,\cdot\,cos\,\varphi$$
with his reasoning that \$P\$ must always be positive with the exception when the resistance is negative, so I should not have the absolute value there.
Now I cannot remember the reason why, but I am pretty sure I have found it somewhere, but since I did a poor job with references, now I cannot find the origin of this formula.
I have found it on wikipedia.
Is this formula wrong? If yes, how can it be written correctly in this form. If no, can you forward me to some more reading on this? Many thanks.