Question

The thermo e.m.f. of a hypothetical thermocouple varies with the temperature $\theta$ of hot junction as $E=a\theta +b{\theta }^2$ in volts, where the ratio $\frac{a}{b}$ is ${700}^{\circ }C$. If the cold junction is kept at $0^{\circ }C$, then the neutral temperature is:

• A. ${700}^{\circ }C$
• B. ${1400}^{\circ }C$
• C. ${390}^{\circ }C$
• D. No neutral temperature is possible for this thermocouple

Answer 1

The correct option is D No neutral temperature is possible for this thermocouple

$E=a\theta +b{\theta }^2$ so $\frac{dE}{d\theta }=a+2b\theta$

For neutral temperature $\theta ={\theta }_n,\frac{dE}{d\theta }=0$

or $a+2b{\theta }_n=0$

or ${\theta }_n=-a/2b=-700/2=-{350}^{o}C$

As cold junction is at $0^{o}C$ so negative neutral temperature can not be possible.