As you
know, Joule heating is given by i2R and depends on i2
(which is always positive whether i is positive or negative)
and not on i Thus, there is Joule heating and
dissipation 
of 
electrical 
energy 
when 
an
ac current passes through a resistor The instantaneous power dissipated in the resistor is
2
2
sin2
m
p
i R
i R
ωt
=
=
(7 4)
The average value of p over a cycle is*
2
2
sin2
m
p
i R
i R
ωt
= <
> = <
>
  [7