Question

Three conducting rods of same material and

cross-sectional area are joined as shown. Temperatures at points A, D and C are maintained at 20°C, 90°C and 0°C respectively. The ratio of lengths BD to BC if there is no heat flow in AB is-

• A. $\frac{2}{7}$
• B. $\frac{7}{2}$
• C. $\frac{9}{2}$
• D. $\frac{2}{9}$

Answer 1

Answer: (B)

$\frac{7}{2}$

Answer 2

As no heat flows in rod AB

Therefore T_A = T_B = 20⁰C

Rate of heat flow through CB and BD are same.

Hence, $\frac{90 - 20}{\frac{BC}{KA}}$= $\frac{20 - 0}{\frac{BD}{KA}}$

$\frac{7}{BC} = \frac{2}{BD}$

$\frac{BD}{BC}$ = $\frac{7}{2}$