Question

At neutral temperature, thermoelectric power is
A. infinite
B. zero
C. minimum
D. maximum

Answer 1

Remind the concept of thermoelectric power, thermo emf and neutral temperature. Use the relation between the thermoelectric power and the slope of the graph of variation of thermo emf with the temperature of the hot junction.

Formula used:
The thermoelectric power is given by
$S = \dfrac{{dE}}{{dT}}$
Here, is the thermoelectric power, $E$ is the thermo emf and $T$ is the temperature of the hot junction.

Complete step by step answer:
The graph of thermo emf versus temperature of hot junction is as follows:

In the above diagram, ${T_c}$ is the temperature of the cold junction, ${T_n}$ is the neutral temperature and ${T_i}$ is the increased temperature of the hot junction.In a thermocouple, the temperature of the hot junction is increased with the temperature of the cold junction at a constant value.The thermo emf of the thermocouple increases with the increase in the temperature of the hot junction upto a maximum value. This temperature at which the value of the thermo emf is maximum is known as the neutral temperature.
The rate of change of the thermo emf with the temperature is known as the thermoelectric power. This rate is known as Seebeck coefficient.So, the expression for the thermoelectric power is
$S = \dfrac{{dE}}{{dT}}$
The thermoelectric power can also be defined as the slope of the graph of thermo emf versus temperature of the hot junction.In the above figure, the slope of the graph of the thermo emf versus the temperature of the hot junction at the neutral temperature is a straight horizontal line parallel to the axis named temperature of the hot junction.Hence, the slope of the curve of the thermo emf versus temperature of the hot junction at the neutral temperature is zero.Hence, the thermoelectric power is zero at the neutral temperature.
Hence, the correct option is A.

Note: The thermoelectric power at neutral temperature can also be determined by using the mathematical relation between the thermo emf and the temperature of the hot junction.Also remember that in thermocouple the flow of current is occur due to the temperature differences between the two dissimilar metals.