First slide

Heat transfer

Question

There are three thermometers{one in contact with the skin of the man, other in between the vest and the shirt and third in between the shirt and coat. The readings of the thermometers are 30^oC,25^oC and 22^oC, respectively. If the vest and the shirt are of the same thickness, the ratio of their thermal conductivities is

Moderate

A

9:25
B

25:9
C

5:3
D

3:5

Solution

Rate of flow of heat will be equal in both vest and shirt
$\begin{array}{l} ∴ \frac{K_{vest} A \cdot {Δθ}_{vest} t}{l} = \frac{K_{shirt} {AΔθ}_{shirt} t}{l} \\ \Rightarrow \frac{K_{vest}}{K_{shirt}} = \frac{{Δθ}_{shirt}}{{Δθ}_{vest}} \Rightarrow \frac{K_{vest}}{K_{shirt}} = \frac{25 - 22}{30 - 25} = \frac{3}{5} \end{array}$

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