Voltage Source across Resistor

 

V(t) = V_m sin ωt

i(t) = I_m sin ωt

And P(t) = v(t) i(t)

P(t) = V_m sin ωt I_m sin ωt

Hence, average power is

It is clear that if current or voltage waveform has a frequency of 50 Hz then power waveform has a frequency of 100Hz.