Q. Two identical rods of copper and iron are coated with wax uniformly. When one end of each is kept at temperature of boiling water, the length upto which wax melts are $8.4cm$ and $4.2cm$ respectively. If thermal conductivity of copper is 0.92, then thermal conductivity of iron is
A) 0.23
B) 0.46
C) 0.115
D) 0.69
Explanation
(a) $\frac{K_{1}}{K_{2}} = \frac{l_{1}^{2}}{l_{2}^{2}}\ \therefore K_{2} = \frac{K_{1}l_{2}^{2}}{l_{1}^{2}} \approx \frac{0.92 \times (4.2)^{2}}{(8.4)^{2}} = 0.23$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.