By Sunil Bhardwaj

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The device used for storing electric charge is called capacitor. It stores electric energy. It is used to provide uniform electric field. The capacitor allows to pass ac signal but blocks dc signal. Various types of capacitor like variable capacitors, spherical capacitors, cylindrical capacitors, tubular capacitors and miniature capacitors etc are used to meet the needs of modern technology.

A capacitor is constructed from two conductors called plates which are separated by an insulator (dielectric). Its basic function is to concentrate the electric field. The capacitor is charged with current source. It can be discharged when a current source is replaced with conducting path.

CHARGING AND DISCHARGING OF A CAPACITOR

The electrons from negative terminal of battery gather on the plate of the capacitor connected to it when capacitor plates are connected to a battery. The other plate of the capacitor connected to positive terminal of battery loses electrons. The excess of electrons produces negative charge on one plate while other plate having deficiency of electrons gets positive charge.The charge on one plate needs for consideration because number of electrons gathered on one plate is equal to number of electrons lost by other plate. This redistribution of electrons from one plate to other plate by current source is called charging of capacitor.The capacitor discharges when conducting path is provided between the plates.

MATHEMATICAL TREATMENT

Consider a parallel plate capacitor connected with a battery of voltage $$V$$ as shown in fig. It is experimentally observed thatWith more charging voltage, the electric field between the plates is stronger, hence more charge is stored.

The amount of charge stored in the capacitor is directly proportional to $$V$$ of battery.$$Q \propto V$$ $$Q=CV$$ Where ($$C$$) is constant of proportionality called capacity or capacitance of capacitor.

CAPACITANCE

The charge stored on one of the capacitor plates per unit potential difference of battery is called capacitance of capacitor.

UNIT OF CAPACITANCE

The SI unit of capacitance is called farad and defined as "the capacitance of a capacitor will be one farad when one coulomb charge is stored on one of the capacitor plates by applying Pd of one volt". $$1 F= \frac{C}{V}$$ Some multiple units of capacitance are $$\mu F= {10}^{-6}F$$ and $$pF= {10}^{-12}F$$ FACTORS WHICH AFFECT CAPACITANCE

The capacitance of a capacitor depends upon following factors.

1. AREA OF PLATES: The larger plate area increases the capacitance of capacitor.

2. SEPARATION OF PLATES: The smaller separation between plates increases the capacitance of capacitor.

3. DIELECTRIC MEDIUM: The introduction of dielectric medium increases the capacitance of capacitor.

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