The temperature of the outer surfaces of a composite slab consisting of two materials having coefficients of thermal conductivity K and 2K and thickness x and 4x respectively are T2 and T1(T2>T1). The rate of heat transfer through the slab in steady state is [{AK(T2-T1)}/x]f, where f is equal to

a) 1

b) 1/2

c) 2/3

d) 1/3

Solution: When two materials are joined in series, then the reciprocal of their conductanes are added.

Here the individual conductances of these two slabs are = KA/x and 2KA/4x 

the equivalent conducatnce =1/ (x/KA + 4x/2KA) = 1/( x/KA + 2x/KA )= 1/ (3x/KA) = KA/3x

The the rate of transfer of heta through the slab = KA(T₂-T₁)/3x ---------(ans)

In this way the answer is 1/3