Question

1. A refrigerator has a surface area of 5m², and the internal and external temperatures are 4⁰C and 25⁰C respectively. The heat gain through the insulation layer should be 60W maximum. The material for the insulation layer must be chosen from the candidates below. Which of the materials is not appropriate? From the remaining materials, determine the most cost effective.

Material	Yield stress (MPa)	Thermal Conductivity (W / m K)	Conductivity (Ohm m)	Density (kg / m3)	Cost (/kg)	Appropriate (Yes / No) If no, why?
Copper	180	390	1.68 x 10-8	8900	$5
Expanded Polystyrene	15	0.033	2 x 105	20	$17
Glass wool	n/a	0.040	3 x 1011	32	$9.5
Aluminium	100	195	2.65 x 10-8	2700	$1.5