Question

If one junction of a thermocouple is heated and other junction is cooled continuously in the same steps, the e.m.f. will

• A. first increase and then decrease
• B. first increase and then decrease and finally reverse
• C. not change at all
• D. change appreciably

Answer 1

Answer: (B)

first increase and then decrease and finally reverse

Let initial temperature of one end be $T_1$ and that of the other end is $T_2$. After $n$ steps, the temperature of first end will be $T_1+n\Delta T$ and that of other end will be $T_2-n\Delta T$. New temperature difference will be $T_1-T_2+2n\Delta T$. Hence if $T_2>T_1$, the temperature difference will reverse after some time and so will the emf.