Two plates each of area A, thicknesses L1 and L2 and thermal conductivities K1 and K2 are joined to form a single plate of thickness (L1 + L2). If the temperatures of the free surfaces are T1 and T2,Find the equivalent thermal conductivity:

Question

(A) L1 - L2 / [L1/K1 + L2/K2]

(B) L1+L2 / [L1/K1 - L2/K2]

(C) L1+L2 / [L1/K1 + L2/K2]

(D) L1+L2 / [L1/K2 + L2/K1]