Capacitors and Capacitance

The capacitor is a passive electronic component with two terminals. It is a device that stores electrical energy in the form of an electric field. A capacitor is one of the widely used electronics components in various electronic circuits.

The capacitor is an energy storing element and this potential of storing a charge is known as capacitance. The value of the capacitor is specified in terms of capacitance.

All capacitors consist of two parallel conducting plates. These plates are separated by an insulating material. And these insulating materials are known as a dielectric.

Types of Dielectric Materials

Dielectric materials used for capacitors can be grouped into the following main classes.

• Mica, glass, ceramic
• High permittivity ceramic
• Paper and metalized paper
• Electrolytic: Oxide film, tantalum
• Dielectrics such as polystyrene, polythene polytetrafluoroethylene

Reactance of Capacitor

A capacitor tries to oppose the flow of current in the AC circuits. The ability to oppose the flow of current is known as capacitive reactance. The reactance of a capacitor is denoted by Xc and its expression is given by,

Xc = 1/(2πfC)

From the above equation, we can say that the capacitive reactance is inversely proportional to the frequency. Therefore, the reactance decreases with an increase in frequency.

Capacitance

The amount of charge required to develop a 1 Volt potential difference between its plates is known as capacitance. And the unit of capacitance is Farad (F).

The Capacitance the ratio of charge 'Q' to the voltage 'V' and expressed as

C = Q/C

Now if, Q = I Coulomb, V = 1 Volt and C = 1 Farad

The capacitance of a parallel plate capacitor is given by,

C = ε (A/d)

Where,

ε = Permittivity,

A = Area of each plate and

d = Separation between the plates

According to the above equation, the capacitance is directly proportional to the permittivity and area of each plate. And it is inversely proportional to the separation between plates.

The above figure shows the symbols of fixed and variable capacitors.

Units of Capacitance

Capacitance is measured in Farads (F). But Farad is a very large unit. So, smaller values are expressed in terms of microfarad (μF) or nano farad (nF). In some electric circuits, it is also defined in terms of picofarad (pF).

1 microfarad = 1x10^-6 farad

1 nenofarad = 1x10^-9 farad

1 picofarad = 1x10^-12 farad