An unnatural machine

This is why hot coffee cools down, and a cold drink heats up. Once either of the drinks reaches the same temperature as the surrounding air they stay at that temperature. There is no longer a hot place and a cold place when both are at the same temperature, so heat energy stops moving from one to the other. Scientists would say they are in "thermal equilibrium".

What's so special about this machine?

Thermal equilibrium is something which nature always moves towards - hot things cool down, cold things warm up. That's only natural.

Now a kettle moves water away from thermal equilibrium and is of course not natural, but at least it makes heat flow from hot to cold. I'm talking about a machine which makes heat flow from a cold place to a hot place!

What would such a machine be like? I'll tell you. Big, noisy and energy hungry. In case you haven't guessed, I'm talking about the refrigerator or fridge.

How does it work?

Before we switch on a fridge the temperature inside is the same as the temperature outside. However, as soon as we switch it on, it starts to take heat from inside and put it on the outside. This means the inside is now colder than the outside. So the heat tries to get back in. And the fridge has to continue to work to remove it. This is why it needs a constant supply of electrical energy.

A fridge reaches a steady temperature when the heat is being removed at the same rate as it is getting back in. If we want the temperature inside to be colder we have to make the fridge work harder, which takes more electrical energy.

Have you ever noticed how the back or sides of a fridge are warm? This is because of all the heat that's being removed from the inside and being put on the outside. The more heat energy we remove from the inside the colder it gets. But the outside gets hotter.

Be responsible

An air conditioned room is just like a big fridge. The compressor is the part of the air conditioning (A/C) system that is always outside. Have you ever noticed that the compressor noisily blows out hot air? The air itself is not from inside the room but the heat energy it contains is.

If you leave your air conditioning on with doors and windows open your air conditioner will use a lot more energy because you are helping the heat get back inside and so it has to work harder. In effect, when you leave doors and windows open your poor air conditioner will work very hard trying to cool down all of Bangkok!

Refrigeration and air conditioning are extremely useful in hot climates, but we should use them sensibly and remember that they turn a lot of useful energy into heat. After all, you have nothing to show for all the useful energy used up, because as soon as you switch the A/C off, nature works to bring back thermal equilibrium.

Refrigerators and air conditioners are now sold with energy efficiency ratings. If possible, you should buy the most efficient one you can afford.

If you want to know more about exactly how a fridge manages to perform this unnatural trick visit the following website: http://home.howstuffworks.com/refrigerator.htm

Last week I left you with the question "How does chemical energy get into an apple?" Well, the answer is something like this. Nuclear reactions in the Sun turn nuclear energy into light and heat. Some of the light escapes from the surface of the Sun and a little over eight minutes later is absorbed by a leaf on an apple tree on Earth. The leaf uses the light energy to turn some water and carbon dioxide into glucose by a process called photosynthesis. The glucose is transported to a growing apple where it is stored. Anyone eating the apple can digest it so the glucose enters the blood and can be used to provide chemical energy anywhere in the body.

Next week: The Awesome Power of Air - why an airplane is more natural than a fridge.