Flux leakage happens when the magnetic lines of primary coil are not completely linked with secondary coil. Energy loss due to this flux leakage is minimized by winding coils one over the other.

(i) principle of transformer is the mutual induction between two coils.
(ii) That is, when an electric current passing through a coil changes with time, an emf is induced in the neighbouring coil.

(i) Transformer is a stationary device used to transform electrical power from one circuit to another without changing its frequency.
(ii) The applied alternating voltage is either increased or decreast with corresponding decrease or increase of current in the circuit.

The mutual inductance between two neighbouring coils is one henry, if a current changing at the rate of 1 A s^-1 in one coil induces an opposing emf of 1 V in neighbouring coil.

It is defined as the ratio of true power to the apparent power of an a.c circuit. It is equal to the cosine of the phase angle between current and voltage in the a.c circuit.
It is given by
cos Φ = \(\frac{True \ power}{Apparent \ power}\)
= \(\frac { { P }_{ average } }{ { V }_{ rms }-{ I }_{ rms } } \)

(i) The total resistance offered to the flow of current due to resistance R, inductive resistance X_L, and capacitive reactance X_c in a circuit is called impedance.
It is given by \(z=\sqrt { { R }^{ 2 }+({ { X }_{ L }-{ X }_{ C }) }^{ 2 } } \)
(ii) At resonance, when X_L= X_C

(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.

Current lags behind the applied voltage  \(\frac{\pi}{2}\) in an inductive circuit. This fact is depicted in the phasor diagram. In the wave diagram also, it is seen that current lags the voltage by 90°.

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.