Question

The dielectric strength of impregnated paper is about
A. $30\,kV/mm$
B. $20\,kV/mm$
C. $15\,kV/mm$
D. $5\,kV/mm$

Answer 1

To calculate the dielectric strength of the material, we perform a series of experiments with different values of voltages and thickness of the material. We connect the material to a voltage source and galvanometer to indicate the presence of current in the circuit. Further we increase the voltage and the material would resist the flow of current. As and when the supply voltage would reach the breakdown value, the current would start flowing. At this voltage, the material is said to lose its insulator properties. We will divide this value of voltage source with the thickness of the material to get the dielectric strength.

Complete answer:
Dielectric strength of a material is that maximum voltage which the material can withstand before breakdown. The dielectric strength depends upon the thickness of the material and on the surrounding conditions. The dielectric strength is calculated by dividing the breakdown voltage with the thickness of the material.

The higher value of dielectric strength corresponds to a better-quality insulator whereas a lower value of dielectric strength corresponds to a weaker insulator.Impregnated paper is made up of cotton and wood pulp. It possesses appreciable water retaining and absorbing properties. This paper is widely used in industrial applications since it is a good insulator. The cellulose present in the pulp is a significant insulator which is responsible for the electric insulation that the impregnated paper offers.This paper is used in capacitors, transmission lines and transformer boards.

To calculate the dielectric strength of impregnated paper, we connect it to a voltage source and a galvanometer that would show a deflection if the current passes through the paper.Now as we increase the voltage, the paper would resist the flow of current. As and when the supply voltage would reach the breakdown value, the current would start flowing.At this voltage, the paper would lose its insulator properties. We will divide this value of voltage source with the thickness of the paper to get the dielectric strength.From many such experiments, the calculated value of dielectric strength of impregnated paper is $30\,kV/mm$.

Therefore, the correct answer is option A.

Note: While we supply the voltage to the material, the positive and the negative charges get separated due to distorted electric fields of the charges on applying the external voltage. At a particular voltage, the separation becomes sufficient to allow the passage of the current. This is said to be the development of potential gradients. Breakdown value is that value of applied voltage which is sufficient enough to carry the current within the insulator.