Question:
A cubical box of volume $216 \mathrm{~cm}^{3}$ is made up of $0.1 \mathrm{~cm}$ thick wood. The inside is heated electrically by a $100 \mathrm{~W}$ heater. It is found that the temperature difference between the inside and the outside surface is $5^{\circ} \mathrm{C}$ in steady state. Assuming that the entire electrical energy spent appears as heat, find the thermal conductivity of the material of the box.
Solution:
