Question

The dielectric strength of air is nearly ________ V/mm
A. 1
B. infinite
C. 1000
D. 3000

Answer 1

Hint: A dielectric is a poor conductor or an insulator. The valence electrons of dielectrics are tightly bound and there is no loose electron available for conduction. If the voltage or the potential difference across the dielectric keeps on increasing then there comes a point when the voltage becomes strong enough to pull out the tightly bound electrons. This voltage is called dielectric strength. Air, being a poor conductor, has high dielectric strength.

Complete answer:
A dielectric is a poor conductor or an insulator. When these dielectrics are placed in an electric field almost no current flows through then. The valence electrons of dielectrics are tightly bound and there is no loose electron available for conduction. Dielectrics act as insulators for the most part, but if the voltage or the potential difference across the dielectric keeps on increasing then there comes a point when the voltage becomes strong enough to pull out the tightly bound electrons of these dielectrics.
This potential difference is called the breakdown voltage. This is also called dielectric strength. It is measured in kV/mm or V/mm or kV/cm. It means that if the voltage exceeds the dielectric strength then the dielectric would rupture and the tightly bound electron could come loose and a huge correct will flow in the dielectric.
Air is a very bad conductor of electricity and has high dielectric strength. The dielectric strength of air is nearly 3000V/mm.

Option D is the correct answer.

Note:
The insulators and dielectric with high dielectric strengths are found in many real-life appliances. Mica, which is an insulator, is used to electric motor windings. Glass is also a dielectric and is used in transformers. Naphtha is one of the liquid dielectrics and is used in circuit breakers.