# Basic Electrical Engineering Questions and Answers – Temperature Coefficient of Resistance

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This set of Basic Electrical Engineering Interview Questions and Answers focuses on “Temperature Coefficient of Resistance”.

1. The resistance of pure metals ___________
a) Increases with an increase in temperature
b) Decreases with an increase in temperature
c) Remains the same with an increase in temperature
d) Becomes zero with an increase in temperature

Explanation: In a conductor, the valence band and conduction band overlap each other, there is an excess of electrons in the conduction band. When the temperature increases, there is an overcrowding of electrons in the conduction band hence reducing the mobility and hence resistance increases.

2. The resistance of insulators __________
a) Increases with an increase in temperature
b) Decreases with an increase in temperature
c) Remains the same with an increase in temperature
d) Becomes zero with an increase in temperature

Explanation: In the case of an insulator, the energy gap between the conduction band and the valence band is very large. When the temperature is increased, the electrons move from the conduction band to the valence band and hence it starts conducting. When conductance increases, resistance decreases, since C=1/R. Thus, when the temperature increases, resistance decreases in insulators.

3. Which of the following statements are true about metals?
a) Metals have a positive temperature coefficient
b) Metals have a negative temperature coefficient
c) Metals have zero temperature coefficient
d) Metals have infinite temperature coefficient

Explanation: The resistance of metals increases with an increase in temperature thus, it has a positive temperature coefficient.

4. Which of the following statements are true about insulators?
a) Insulators have a positive temperature coefficient
b) Insulators have a negative temperature coefficient
c) Insulators have zero temperature coefficient
d) Insulators have infinite temperature coefficient

Explanation: Insulators have a negative temperature coefficient because as temperature increases, the resistance of insulators decreases.

5. What is the unit of temperature coefficient?

Explanation: R=Reff[1+temp. coeff(T-Teff)].
From the given expression: (R/Reff-1)/(T-Teff) = temp. coeff. Hence, the unit is the reciprocal of that of temperature = centigrade-1.

6. A copper coil has a resistance of 200 ohms when its mean temperature is 0 degree centigrade. Calculate the resistance of the coil when its mean temperature is 80 degree centigrade. Temperature coefficient of copper is 0.004041 centigrade-1
a) 264.65 ohm
b) 264.65 kilo-ohm
c) 286.65 ohm
d) 286.65 kilo-ohm

Explanation: R=R0(1+α dT) = 200(1+ 0.004041*80) = 264.65 ohm.

7. The temperature of a coil cannot be measured by which of the following methods?
a) Thermometer
b) Increase in resistance of the coil
c) Thermo-junctions embedded in the coil
d) Calorimeter

Explanation: Calorimeter measures the amount of heat and not the temperature of the coil. The temperature of a coil is mainly measured by thermometer. Resistance of coil increase with an increase in temperature of coil so we can measure temperature using this method. Another method is the formation of thermocouple inside coil due to high temperature at one end and low temperature at other ends.

8. The rise or fall in resistance with the rise in temperature depends on ________
a) The property of the conductor material
b) The current in the metal
c) Property of material as well current in that material
d) Does not depend on any factor

Explanation: The rise or fall in resistance with a rise in temperature depends upon the property of the material. Hence it rises with temperature in metals and falls with temperature in insulators and semi-conductors.

9. If the temperature is increased in semi-conductors such that the resistance incessantly falls, it is termed as _______
a) Avalanche breakdown
b) Zener breakdown
c) Thermal runway
d) Avalanche runway

Explanation: When the temperature keeps increasing, the resistance keeps falling continuously and hence the current to increase. This causes the heat in the semi-conductor to rise. This causes the temperature to increase further and the resistance to further decrease. This process continues and until there is sufficient heat to destroy the structure of the semi-conductor completely. This is known as a thermal runway.

10. Materials having resistance almost equal to zero is _______
a) Semi-conductor
b) Conductor
c) Superconductors
d) Insulators

Explanation: When the temperature of the material falls to absolute zero, the resistance falls to zero and hence there are no I2R losses. Since resistance is zero, conductance is almost infinity and hence these materials are known as superconductors.

Sanfoundry Global Education & Learning Series – Basic Electrical Engineering.

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