Q. An emf of 0.9 V is generated when the temperature difference hot and cold junction of thermocouple is 75 K. Assuming that the thermo emf is directly proportional to the temperature difference, the extent to which the thermo emf will change when the cold junction is heated up by 15 K is
A) 10%
B) 20%
C) 40%
D) 60%
Explanation
(b) Here, ${\ E}_{1} = 0.9V,\theta - \theta_{0} = 75E;E_{2} = ?when\ \theta_{0}^{'} = \theta_{0} + 15$
$\therefore\ \ $temperature difference $\theta - \theta_{0}^{'} = \theta - (\theta_{0} + 15)$
$= \left( \theta - \theta_{0} \right) - 15 = 75 - 15 = 60$ K
As $\frac{E_{2}}{E_{1}} = \frac{\theta - \theta_{0}^{'}}{\theta - \theta_{0}} = \frac{60}{75} = \frac{4}{5}$
or % decrease in thermo emf
$= \frac{E_{1} - E_{2}}{E_{1}} \times 100 = \left( \ 1 - \frac{E_{2}}{E_{1}} \right) \times 100$
$= \left( \ 1 - \frac{5}{4} \right) \times 100 = 20\%$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.