The heat flow rate through parallel walls of thickness $L_1, L_2$ and $L_3$ and having surface areas $A_1, A_2$ and $A_3$, thermal conductivities $k_1, k_2$ and $k_3$, respectively and first and last walls maintained at temperature $t_1$ and $t_2$ will be
- A. $\frac{t_1-t_2}{\frac{L_1}{A_1 k_1} + \frac{L_2}{A_2 k_2} + \frac{L_3}{A_3 k_3}}$
- B. $\frac{t_1-t_2}{\frac{k_1}{A_1 L_1} + \frac{k_2}{A_2 L_2} + \frac{k_3}{A_3 L_3}}$
- C. $\frac{t_1-t_2}{\frac{L_1 L_2 L_3}{k_1 A_1 + k_2 A_2 + k_3 A_3}}$
- D. None of these
Answer: A. $\frac{t_1-t_2}{\frac{L_1}{A_1 k_1} + \frac{L_2}{A_2 k_2} + \frac{L_3}{A_3 k_3}}$
Explanation: For composite walls in series, the total thermal resistance is the sum of individual resistances ($R = L/kA$). $Q = \Delta T / \sum R$.