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

Options

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

Answer

D

Explanation

Thermal resistance we use
`R=(l)/(KA)`
`implies (l_(1))/(k_(1)A_(1))=(l_(2))/(k_(2)A_(2))`
`implies (l_(1))/(l_(2))=(k_(1))/(k_(2))xx(A_(1))/(A_(2))`
`implies (l_(1))/(l_(2))=(5)/(4)`