Question

The rate of diffusion of electrons in a thermocouple is
(A) Greater at hot junction and less at cold junction
(B) Equal at both junctions
(C) Lesser at hot junction greater at cold junction
(D) Diffusion of electrons does not take place

Answer 1

Hint
We should know that diffusion current is a current in a semiconductor caused by the diffusion of charge carriers’ holes and/or electrons. This is the current which is due to the transport of charges occurring because of non-uniform concentration of charged particles in a semiconductor. Drift current is electric current due to the motion of charge carriers under the influence of an external electric field while diffusion current is electric current due to the diffusion of carriers leading to a change in carrier concentration. Atomic diffusion in semiconductors refers to the migration of atoms, including host, dopant and impurities. Diffusion occurs in all thermodynamic phases, but the solid phase is the most important in semiconductors. The diffusion of atoms and defects both native and dopant is at the heart of semiconductor processing.

Complete step by step answer
From the given question, we can determine that, when two dissimilar metals are brought in contact while one metal junction is heated and the other junction is cooled a net ems flows through the circuit. This effect is called the setback effect. The rate at which electrons diffuse from one conductor into the other depends on the nature of the material and the temperature at the interface. The metal losing a net charge becomes positively charged while the metal gaining electrons become negatively charged and these charges build up a contact potential difference across the interface. Thus, the contact potential at a hot junction becomes higher at the hot junction than at the cold junction.
Hence, the correct answer is Option (A).

Note
For solving such questions, we need to know that a thermocouple is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature. The thermocouple working principle is based on the See back 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. 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.