Question

The ratio of thermal conductivity of two rods is 5 : 3 and ratio of area of cross-section is 2 : 3. If they have same thermal resistance, then ratio of respective lengths of rods will be

Answer 1

thermal resistance L/(KA)

where L- lentgh of the rod

K - thermal conductivity

A - area of the rods

since two thermal resistance are equal.

L1/(K1 * A1) = L2 / ( K2 * A2 )

rearranging the terms we get like this

L1/L2 = (K1 * A1 )/ ( K2 * A2)

L1 / L2 = 5/3 * 2/3

= 10/9