# 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:

(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)

If the thermal resistances of the two plates are R1 and R2, then as the plates are in series.
Rs = R1+ R2 = L1/AK1 + L2/AK2 (as R=L/KA)
So, H = dQ/dt = ∆θ/R = (T1 – T2)/(R1+R2) = A(T1 – T2) / [L1/K1 + L2/K2]