Lorie Wackerbauer, age 12, of Calgary, Alberta, Canada, for her question:

How can electricity keep refrigerators cold?

We expect electricity to light the lamps and heat the stove. It seems like expecting a bit too much when we ask it also to cool the refrigerator. Nevertheless, the miracle works because liquids steal heat when they change into gases.

Maybe you have wondered why a dash of water cools off your skin, especially when you wave your wet hands in the air. Notice that the water, or some of it, evaporates. The liquid moisture changes into separate molecules of gaseous vapor. It so happens that a substance must use energy to make this miraculous change and the energy it uses it heat. Whenever a liquid vaporizes, it absorbs heat from its surroundings and when these objects lose heat, naturally they become cooler.

This is the secret that makes any kind of a refrigerator work. The first trick is to find a substance that easily changes itself from a liquid to a gas and back again to liquid. This is the refrigerant and most refrigerators use a dependable refrigerant called Freon 12. The next task is to seal the refrigerant in a system of pipes concealed between the double walls of the refrigerator box.

Now we need several cunning devices to change the refrigerant back and forth from a liquid to a gas. Like all motors, they need some form of energy to keep them working. Gas refrigerators use gas to run their motors. The motors that run an electric refrigerator, naturally, are kept going by the same electric current that heats the stove, runs the toaster and so on.

The refrigerant is pumped through the pipe circuit in its liquid form. However, this special substance boils and turns to vapor at a very low temperature. It changes to vapor as it circulates through the walls. As this happens, it absorbs heat from inside the refrigerator and as the food inside loses heat it becomes cool.

Of course the main trick is to keep the food cool and once the refrigerant becomes vaporized it absorbs no more heat. So, let's think about changing the gaseous vapor back again into its liquid state ¬and sending it around the circuit again to gather more heat.

This sounds impossible. But it can be done by compression, by exerting very high pressure on the returning gas. The gadget for this job is called the compressor and electricity is needed to keep it working. It squeezes the gaseous refrigerant, which concentrates its store of heat. Now the trick is to change the hot compressed gas back into a cool liquid. It is squirted into an air cooled condenser that condenses the gas to drops of liquid and gets rid of the heat.

The liquid refrigerant cools the contents of the refrigerator as it vaporizes. The closed system works by changing the gaseous vapor back into the heat absorbing liquid and sending it through the pipes again. In an electric refrigerator, electricity is used to run the little motors that compress, condense and circulate the refrigerant.