Capacitance Basic Theory:

Capacitance is a device which opposes the sudden change in the voltage in a circuit. Simplest type of capacitor can be constructed by placing two metal plates which are separated by small distance. The gap between the metal plates is filled with insulating material called dielectric.

For a given capacitor, factors which capacitance value of the capacitor depends are:

• The area of the plates
• The distance between the plates
• The type of dielectric used between the plates

Greater or larger the surface area of the plates, the more will be the capacitance value of the capacitor. Larger the distance between the plates, less will be capacitance value of the capacitor.

Capacitor in DC Circuit:

When a capacitor is charged with battery, electrons from the positive plate (connected to positive terminal of battery) is removed and deposited on the negative plate (connected to negative terminal of battery) of the capacitor. The flow of electrons (current) until equal voltage establish between the positive and negative plates of the capacitor which is equal to battery voltage. When the voltages becomes equal, the capacitor is fully charged and flow of current in the circuit stops. When the dc supply (battery) to the circuit is removed, the charged capacitor will discharge if a closed path is provided and current flows in the circuit.

Thus in dc circuit, capacitor charges until both the plates reaches equal potential. After charging it acts as open circuit.

Dielectric:

Energy stored in the capacitor is in the dielectric and the type of dielectric is important to determine the amount of capacitance of a capacitor. Dielectric materials are assigned with dielectric constants (numbers). Air is assigned as 1 and is taken as reference dielectric.

Capacitance Unit:

The basic unit of capacitor is Farad and its symbol id “F”. A Capacitor has a capacitance of one farad when a charge of one volt across its plate results in movement of one coulomb.

Q = C X V

Where

• Q = Charge in Coulomb
• C = Capacitance in farads
• V = Charging Voltage

Some Important Points Related to Capacitor:

• Capacitor oppose the sudden change in the voltage in a circuit
• Capacitors charge and discharge at an exponential rate
• In a capacitor circuit current leads the voltage by 90^o
• Capacitors are used for power factor improvement in industrial applications, to produce phase shift for thyristors (SCR and Traic Circuits), to filter pulsating dc  and in RC timing circuits