## Question:

One experimental method of measuring an insulating material's thermal conductivity is to construct a box of the material and measure the power input to an electric heater inside the box that maintains the interior at a measured temperature above the outside surface. Suppose that in such an apparatus a power input of 180 W is required to keep the interior surface of the box 65.0 Co (about 120 Fo) above the temperature of the outer surface. The total area of the box is 2.18 m2, and the wall thickness is 3.90 cm. Find the thermal conductivity of the material in SI units.