**INTRODUCTION**

In many electrical circuits, resonance circuits are a very important phenomenon. In a study of resonance, circuits are very useful, particularly in the field of communication. The resonant circuit’s best example is radio receivers. here radio receiver has the ability to select certain desired frequencies transmitted by the station. The radio receives all unwanted frequencies transmitted by other stations.

### What is resonance?

Resonance is defined as a phenomenon in which applied voltage and resulting current are in phase which means an a.c circuit is said to be in resonance if it exhibits a unity power factor condition, which means applied voltage and resulting current are in phase.

**resonance in series circuits is referred to as series resonance or simply resonance.**

**resonance in parallel circuits is referred to as parallel resonance or antiresonance.**

**Q-factor**

Q-factor is also known as the quality factor. This phenomenon is observed in a.c circuits consisting of reactive elements such as inductors and capacitors. these two elements are basic passive elements of energy storing type.

The figure of merit or Q-factor is defined as

Thus the expression for the quality factor of the inductor and capacitor**I.quality factor of Inductor :**

considered that sinusoidal voltage V is applied to an inductor with leakage resistance R in series

from the above fig resistance R and inductance L are connected in series and I current flows through it.

Thus by definition quality factor of the inductor is given by,

**II.quality factor of capacitor:**

considered that sinusoidal voltage V is applied to a capacitor with a small resistance R in series.

thus by definition, the quality factor of the capacitor is given by,

In resonant circuits, the quality factor of the inductor or coil is the controlling factor. So, when the Q of a resonant circuit is stated, the frequency of the circuit is used for the measurement.

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