A thermocouple of negligible resistance produces an emf of $40 \mu \mathrm{V} /{ }^{\circ} \mathrm{C}$ in…

A thermocouple of negligible resistance produces an emf of $40 \mu \mathrm{V} /{ }^{\circ} \mathrm{C}$ in the linear range of temperature. A galvanometer of resistance $10 \Omega$ whose sensitivity is $1 \mu \mathrm{A} /$ div, is employed with the termocouple. The smallest value of temperature difference that can be detected by the system will be
  1. $0.5^{\circ} \mathrm{C}$
  2. $1^{\circ} \mathrm{C}$
  3. $0.1^{\circ} \mathrm{C}$
  4. $0.25^{\circ} \mathrm{C}$

Solution

For minimum deflection of 1 division require current $=1 \mu \mathrm{A}$ voltage required $=i R=1 \times 10=10 \mu \mathrm{V}$ $\begin{aligned} \therefore \quad 40 \mu \mathrm{V} & \cong 1^{\circ} \mathrm{C} \\ 10 \mu \mathrm{V} & \cong \frac{1}{4}{ }^{\circ} \mathrm{C} \\ & =0.25^{\circ} \mathrm{C} \end{aligned}$ .

Asked in: NEET 2011 (Mains)

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