A device 'X' is connected to an ac source V=V0sinωt. The variation of voltage, current and power in one cycle is shown in the following graph: (a) Identify the device 'X'. (b) Which of the curves A, B, and C represent the voltage, current, and the power consumed in the circuit?

A) Phase difference between V and I is π/2. So the device can be a capacitor.
B) Curve A represents power P=VI, where the amplitude is equivalent to the multiplication of amplitudes of V and I curve. Curve B is a sine curve and represents voltage, and curve C is a cosine curve representing the current. Full cycle of the graph consists of two positive and two negative symmetrical areas. So the average power consumed in the circuit is zero.
C) The AC impedance of a capacitor is known as Reactance and, as we are dealing with capacitor circuits, more commonly called Capacitive Reactance, XC. The graph shows the variation of capacitive reactance with frequency.
D) V=V0sinωt
Q=CV
Where C is capacitance.
I=dQ/dt=CdV/dt=CV0ωcosωt
Thus there is a π/2 phase difference between current and voltage.