Question

Three conducting rods of same material and cross-section are shown in figure. Temperatures of $A,DandC$ are maintained at ${20}^{\circ }C,{90}^{\circ }Cand{0}^{\circ }C$ respectively. The ratio of lengths of $BD$ and $BC$ 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)

$\frac{7}{2}$

no heat flow in AB so $T_B={20}^{o}C$
$\therefore \frac{kA(90-20)}{l_{BD}}=\frac{kA(20-0)}{l_{BC}}$
$\therefore \frac{l_{BD}}{l_{BC}}=\frac{7}{2}$