Question

Thermo emf produced in a thermocouple depends on the
(A). The resistance of the elements
(B). Lengths of the elements
(C). The temperature of the junctions
(D). Size of the junctions

Answer 1

Hint: You can start by explaining thermocouple and thermo emf. When you are explaining thermo emf focus especially on the temperature difference between the junctions and also include the fact that the nature of the metal wires also affects thermo emf for clarifying further.

Complete step-by-step answer:
Before moving on to the actual solution, it will be really helpful to discuss what thermocouple and thermo emf means.
Thermocouple – In 1821 it was first discovered that when we join two different metals and there is a difference between the two joints then a magnetic field is observed around this arrangement. We already know that a magnetic field is observed around a metal wire only if the current is flowing through the circuit.
So, it was concluded that somehow due to the temperature difference between the two ends, a current started to flow through the circuit. The current starts to flow through the circuit due to the formation of thermo emf (voltage or potential difference) across the two junctions at different temperatures.
The value of thermo emf will also depend on the nature of the two metals in contact.
So, the thermo emf depends on the following factors:
1. The temperature of the junctions
2. Nature of the metals in contact
Hence, option C is the correct choice.

Note: The main use of thermocouple is to detect changes in temperature. As we discussed in the solution that the current starts to flow in the thermocouple if there is a difference in the temperature of the two junctions, this change can be detected by using an ammeter or a voltmeter. They are used to measure temperature in factories and fire alarms.