Question

The ratio of thermal conductivity of two rods of different material is 5 : 4. The two rods of same area of cross-section and same thermal resistance will have the lengths in the ratio

• A. 4 : 5
• B. 9 : 1
• C. 1 : 9
• D. 5 : 4

Answer 1

Answer: (D)

5 : 4

Answer 2

Given $A_{1} = A_{2}$ and $\frac{K_{1}}{K_{2}} = \frac{5}{4}$

$R_{1} = R_{2} \Rightarrow$ $\frac{l_{1}}{K_{1}A} = \frac{l_{2}}{K_{2}A}$⇒ $\frac{l_{1}}{l_{2}} = \frac{K_{1}}{K_{2}} = \frac{5}{4}.$