Further physics - The operation of a refrigerator
Cheung Kai-chung (Translation by Janny Leung)  

refrigerator
Fig. 1  The structure of a refrigerator.
This is one of the best things to do in summer: take out an ice cream from the refrigerator and enjoy it! However, have you ever paid attention to the structure of a refrigerator? Why is it always black at the back?

The operation of a refrigerator is based on two physical principles. First, when a substance changes from liquid state to gaseous state, its temperature would remain at its boiling point until the liquid is all evaporated. The substance has to absorb certain amount of energy, called "latent heat", during the change. On the other hand, the boiling point of a substance rises under high pressure, thus the gasified substance may return to its liquid state and releases the latent heat.

There is a pipe partly placed inside a refrigerator and partly outside it. A special substance inside the pipe has a very low boiling point. It is continuously circulated within the pipe by a pump at the bottom of the pipe (Fig. 1). There is a tiny hole at the upper part of the pipe. It slows down the speed of the substance inside the outer pipe, and thus the substance there would be under high pressure. As a result, the boiling point of the substance in this area would rise, and hence the substance would change from gaseous state to liquid state, releasing the latent heat. When the substance enters into the refrigerator through the tiny hole, its pressure would be reduced and its boiling point would return to its previous low level. The substance would then change from liquid state to gaseous state; when the heat (latent heat) inside the refrigerator is absorbed during the change, the temperature inside the refrigerator would be lowered, resulting in a freezing effect.

In addition, since the substance outside the refrigerator has to release its heat, it is painted black - a substance in black emits more radiation - to facilitate the release. Besides, many large metal pieces are placed around the pipe for the same purpose.