Its average is zero (the positive half of the cosine cancels the negative
half) Therefore,
cos
2
m m
v i
P
φ
=
cos
2
2
m
m
v
i
φ
=
V Icos
φ
=
[7 30(a)]
This can also be written as,
2
cos
P
I Z
φ
=
[7 30(b)]
So, the average power dissipated depends not only on the voltage and
current but also on the cosine of the phase angle f between them