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 rate of flow of heat:

Question

(A) A (T1 – T2) / [L1/K1 + L2/K2]

(B) A(T1 + T2) / [L1/K1 - L2/K2]

(C) A[L1/K1 + L2/K2] / (T1 – T2)

(D) A[L1/K1 - L2/K2] / (T1 + T2)