Question

Assertion:
Electrical conductivity of copper increases with increase in temperature.
Reason:
The electrical conductivity of metal is due to motion of electrons.
A.Both assertion and reason are correct and reason is the correct explanation for assertion.
B.Both assertion and reason are correct but reason is not the correct explanation for assertion.
C.Assertion is correct but reason is incorrect.
D.assertion and reason are incorrect.

Answer 1

A material which conducts electricity in presence of free electrons is known as a conductor. When the temperature increases, the electrons gain more energy. The conductivity of the material depends on the number of free electrons.

Complete step by step answer:
The conductivity of a material depends on the valence band, conduction band and the forbidden energy gap.
The band in which electrons having low energy are present is known as the valence band. The band in which the free electrons having high energy are present is known as the conduction band.
The energy gap between the valence band and the conduction band is known as the forbidden energy gap.
On the basis of conductivity, materials are classified into three types:
Insulator: In an insulator, the forbidden energy gap is greater. The electrons cannot jump from the valence band to the conduction band. Thus, the insulator does not conduct electricity.
Conductor: In a conductor, the forbidden energy gap is zero and the valence band and the conduction band overlap each other. Thus, the conductor conducts electricity.
Semiconductor: In a semiconductor, the forbidden energy gap is smaller. The electrons can jump from the valence band to the conduction band. Thus, the conductivity of a semiconductor lies between that of insulator and conductor.
At lower temperatures, the valence band of conductors is occupied by the electrons and the conduction band is empty.
When the temperature of the metal increases, the electrons in the valence absorb the energy and jump to the conduction band thus, decreasing the forbidden energy gap.
The valence electron becomes a free electron and thus, the metal can conduct electricity.
Thus, electrical conductivity of copper increases with increase in temperature because the electrical conductivity of metal is due to motion of electrons.
Thus, both assertion and reason are correct and reason is the correct explanation for assertion

Thus, the correct option is (A) both assertion and reason are correct and reason is the correct explanation for assertion. .

Note: With an increase in temperature, the electrons jump from the valence band to the conduction band. Thus, positively charged holes are created in the valence band. Thus, the number of holes in the valence band are equal to the number of electrons in the conduction band.