Question

show that current leads the voltage in phase by pie/2 in an ac circuit containing an ideal capacitor

Answer 1

Phase Difference
tan(α) = Xc/R
now R=0
tan(α) = infinity
α =90
Therefore they current leads the voltage by π/2 radian

Answer 2

Hey !!

If                     V = V₀ sinωt

                     q = CV = CV₀sinωt

    ∴                 I = dq/dt = ωCV₀ cosωt

(or)               I = ωCV₀ sin(ωt + π/2)

So, the current leads the applied voltage in phase by π/2

Good luck !!