Question

The thermocouple is based on the principle of:
A) Seebeck effect
B) Thomson effect
C) Peltier effect
D) Joule effect

Answer 1

Hint: A thermocouple is a sensor for measuring temperature. This sensor consists of two dissimilar metal wires, joined at one end, and connected to a thermocouple thermometer or other thermocouple-capable device at the other end.
A small thermoelectric current is generated when two different metal wires are put into contact at both ends with their junctions having a different temperature.

Complete answer:
Two dissimilar metals ‘A’ and ‘B’ are joined at the two junctions ‘P’ and ‘Q’. Here the ‘P’ junction is a hot junction whereas the junction ‘Q’ is a cold junction and a galvanometer is connected in this arrangement as shown in Figure.

When these junctions are kept at different temperatures, generally cold junction is kept at $0^\circ $C and the hot junction is kept at an unknown temperature which we want to measure (i.e. the temperature of the junction is raised by heating it). An e.m.f. will be generated in this circuit due to the temperature gradient along the wire across the junctions.
When both the junctions are at different temperatures, a current will flow through the meter and the meter will show the deflection.
As the generated e.m.f. is proportional to the temperature difference, the amount of current flow will also be proportional to the temperature difference.
The working principle: Seebeck Effect
Seebeck Effect: This effect states that when a closed circuit is formed by joining two dissimilar metals at two junctions, and junctions are maintained at different temperatures then an electromotive force (e.m.f.) is induced in this closed circuit.

Final answer is A, Seebeck Effect.

We can see that in a thermocouple two dissimilar wires or electrical conductors are joined and a temperature difference is created between the junctions by heating one junction thus this temperature gradient produces a voltage difference between two junctions and the current starts flowing. This is the Seebeck effect.

Note: At the hot junction, electrons get energy and start moving towards the cold junction and this results in the flow of current in the thermocouple
When both the junctions are at the same temperature, e.m.f. generated at both junctions will be the same. No current will flow through the circuit. And there will be no deflection in the meter.
The amount of induced e.m.f. is different for different metal combinations and is proportional to the temperature difference of the junctions.