CAPE Physics Unit 1 · 2005 · Paper 2 · Question 8(b)(ii)

A home-made cooler is constructed from plywood and foam plastic. The walls consist of two pieces of plywood 1.0 cm thick with a 1 cm layer of foam plastic in between. When in use the temperature inside the box is 2 °C while the outside temperature is 30 °C. Figure VII shows a cross-section of the wall.

The thermal conductivity of plywood is 0.24 W m⁻¹ K⁻¹ whilst that of the plastic foam is 0.012 W m⁻¹ K⁻¹. Calculate the thickness of a piece of plywood which would have the same insulating effect as 1 cm of foam.