Introduction to Capacitor

 What is Capacitor?

Capacitor is also known as condenser. This is one of the passive components like resistor. Capacitor is generally used to store the charge. In capacitor the charge is stored in the form of “electrical field”. Capacitors play a major role in many  electrical and electronic circuits.

Generally, a capacitor has two parallel metal plates which are not connected to each other. The two plates in the capacitor are separated by non conducting medium (insulating medium) this medium is commonly known as Dielectric.

There are different types and different shapes of capacitors  available , from very small capacitors which are used in resonance circuits to large capacitors for  stabilising HVDC lines. But all capacitors are doing the same work that is storing the electrical charge.

The shape of a capacitor is rectangular, square, circular, cylindrical or spherical shape. Unlike a resistor, an ideal capacitor does not dissipate energy.As the different types of capacitors are available different symbols were available to represent them which are shown below.

Why capacitors are important?

Capacitors have many properties like

1. They can store the energy and it can dissipate this energy to the circuit when ever required.
2. They can block DC and allow AC to flow through it, and this can couple one part of the circuit with the other.
3. Circuits with capacitors depend on the frequency, so can be used to amplify certain frequencies.
4. As the capacitors when applied with AC input , the current leads the voltage and thus in power applications it increases the pay load power and makes it more economical.
5. It allows high frequencies and so it can be used as a filters either to filter low frequencies or to collect high frequencies.
6. As the reactance and frequency of the capacitor are inversely related, this can be used to increase or decrease the circuit impedance at certain frequency and can be used as filter.

Likewise, capacitors exhibit many properties , when used in AC or DC circuits and hence they play important role in electrical and electronic circuits.

Construction of a Capacitor

As said before , there are different types of capacitors. These different types will have different type of construction. A Parallel plate capacitor is the simplest capacitor. Let us understand the construction of this capacitor.

It consists of two metal plate separated by a distance. The space between these two plates is filled with a dielectric material. The two leads of the capacitor are taken from these two plates.

The capacitance of the capacitor depends on the distance between the plates and area of the plates. Capacitance value can be changed by varying any of these parameters.

A variable capacitor can be constructed by making one of these plates fixed and other moving.

Dielectric Of a Capacitor

Dielectric acts as an insulating material between the plates . Dielectric can be any non conducting material such as ceramic, waxed paper, mica, plastic or some form of a liquid gel.

Dielectric also plays an important in deciding the value of capacitance. As the dielectric is introduced between the plates of the capacitor ,its value increases.

Different dielectric materials will have different dielectric constants ,however this value is >1.

Below table gives value of dielectric constant for each dielectric material

Dielectric can be of two types

Polar dielectrics: These dielectrics will have permanent dielectric movement

Non Polar dielectrics: These will have temporary dielectric moment. By placing them in a electric field they can be induced with dipole moments.

Working

As said before capacitor consists of two conductor separated by a dielectric , when there is any potential difference between the two conductors electric potential is developed.This causes the capacitor to charge and discharge.

Let us understand this in a  practical way. When the capacitor is connected to a battery(a DC source) , current starts flowing through the circuit .

Thus negative charge is accumulated on one plate and positive charge is accumulated on the other plate. This process continuous until the capacitor voltage reaches supply voltage.

When the charging voltage is equal to the supply voltage capacitor stops charging further even though the battery is connected. When the battery is removed two plates will be accumulated with positive and negative charges. Thus the charge is stored in the capacitor.

But when the supply voltage is from an AC source it charges and discharges continuously .The rate of charging and discharging depends on the frequency of the source.

Example

Working can be understood using simple example here. Below circuit shows two switches A and B. When switch 1 is closed , current starts flowing from from the battery to the capacitor. When the capacitor voltage reaches the supply voltage ,it stops charging further.