(i) 180⁰ (or) π radian
(ii) Zero.

(i) The electromagnetic oscillations of LC system can be compared with the mechanical oscillations of a spring-mass system.
(ii) There are two forms of energy involved in LC oscillations. One is electrical energy of the charged capacitor; the other magnetic energy of the inductor carrying current.
(iii) Likewise, the mechanical energy of the spring-mass system exists in two forms; the potential energy of the compressed or extended spring and the kinetic energy of the mass. The Table lists these two pairs of energy.
(iv) By examining, the analogies between the various quantities can be understood and these correspondences.
(v) The angular frequency of oscillations of a spring-mass is given by equation
\(\omega =\sqrt { \frac { k }{ m } } \)
k ⟶ \(\frac { 1 }{ C } \) and m ⟶ L. Therefore, the angular frequency of LC oscillations is given by
ω = \(\frac { 1 }{ \sqrt { LC } } \)

Current lags behind the applied voltage by \(\frac{\pi}{2}\) in an inductive circuit.

Current leads the applied voltage by \(\frac{\pi}{2}\) in a capacitive circuit.
This is the resistance offered by the capacitor, called capacitive reactance (X_c). It measured in ohm.
\({ X }_{ c }=\frac { 1 }{ \omega C } \)
The capacitive reactance (X_c) varies inversely as the frequency. For a steady current, f = 0
∴\({ X }_{ c }=\frac { 1 }{ \omega C } -\frac { 1 }{ 2\pi fC } =\frac { 1 }{ 0 } =\infty \)
Thus a capacitive circuit offers infinite resistance to the steady current.

Phasor: A sinusoidal alternating voltage (or current) can be represented by a vector that rotates about the origin in an anti-clockwise direction at a constant angular velocity ω. Such a rotating vector is called a phasor. A phasor is drawn in such a way that
(i) the length of the line segment equals the peak value Vm (or I_m) of the alternating voltage (or current)
(ii) its angular velocity w is equal to the angular frequency of the alternating voltage (or current)
(iii) the projection of phasor on any vertical axis gives the instantaneous value of the alternating voltage (or current)
(iv) the angle between the phasor and the axis of reference (positive x-axis) indicates the phase of the alternating voltage (or current).
The notion of phasors is introduced to analyze the phase relationship between voltage and current in different AC circuits.
Phasor diagram
The diagram which shows various phasors and their phase relations is called the phasor diagram.