Source across Capacitor

Where,  and is known as capacitive reactance

i (t) = I_m sin (ωt + 90⁰)

Instantaneous power

p(t) = v(t) i(t)

= V_m sinωt I_m cos ωt

Average power

Now vector diagrams are drawn below:

Hence in a capacitor current leads to voltage by 90° (or voltage lags to currents by90°).