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How Heat Moves

This experiment comes from sitting in front of a wonderful fireplace. As someone that has never been a big fan of winter, it is great to have a toasty fire to keep warm at night. Watching the fireplace lead to thinking about heat and how it moves.

To experience the ways that heat moves, you will need:

  • a metal spoon
  • a cup of hot water
  • a lamp with an incandescent bulb

We will begin with the metal spoon. Fill a cup with very hot water. Place the spoon into the cup. Touch the handle of the spoon, to see that it is still cool. Wait a minute and touch the handle again. It should be warmer now. The heat has moved up the handle by a process called conduction. This is so common that we don't even think about it most of the time. If one part of an object is hotter than another heat energy will move from the hotter part to the cooler part. The atoms in the hot part are moving very quickly, and they bump into nearby atoms, causing them to move faster. That increased movement of the atoms is what we sense as heat. Those atoms will bump into other nearby atoms, carrying the heat energy further along the object.

But, conduction is not the only way that heat can get from one place to another. The second way that heat travels is called convection. We can see that by using an incandescent light. Incandescent bulbs produce light by using electricity to heat a thin wire, called a filament. This filament gets so hot that it glows, giving us light to read by. The process also produces a lot of heat.

Turn on the lamp and wait a minute or so. Hold your hand about two inches from the side of the bulb and you will feel heat. Hold your hand about two inches above the bulb and you will feel quite a bit more heat. Why is it hotter above the bulb than beside it? Heat rises, right? Wrong. Heat does not move up any faster than it moves down or sideways. OK, so what we really meant was hot air rises, right? No. What we should say is that hotter air rises. What does that mean? Conduction lets heat move from the hot bulb to the air that is touching it. As this air is heated, it expands. That makes it less dense than the air around it, so it is pushed upwards by the cooler, denser air around it. Since there is a constant flow of hot air rising around the bulb, your hand picks up more heat energy when it is above the bulb. This process is called convection.

Why is not correct to say that hot air rises? Imagine that we are in the desert. It is a warm day, and the air around us is 100 degrees F. Will that hot air rise? No, because the air around it is just as hot and has the same density. Now imagine that we are in the Arctic. The air around us is -20 degrees F. I bring out a piece of metal that is 0 degrees F. Some of its heat moves to the surrounding air by conduction, heating the air to -10 degrees F. Will that air rise? Yes! Although it is very cold, it is warmer (and less dense) than the air around it. The colder, denser air pushes the warmer, less dense air upwards. So for convection, we should say that warmer air rises.

The third way that heat moves is less familiar to most people, although you experience it every day. Turn off the lamp and wait until the bulb is cool to the touch. Hold your hand about two inches from the bulb and turn the lamp on. Almost instantly, you will feel heat on your hand. Touch the bulb and you will find that it is still cool. It takes several seconds for the glass to get hot.

Now, wait a minute. For conduction, the heat has to move from atom to atom. If the heat you felt was from conduction, the glass would need to have been hot, so it could pass its heat to the air, and then to your hand. The heat you felt was not from conduction.

For convection, you still need conduction to move heat energy from the bulb to the air, so it can expand and rise. Since the bulb was not hot that could not be the answer. You can also test that by starting with a cool bulb and holding one hand beside the bulb and one hand above the bulb. When you turn on the lamp, both hands will almost instantly feel the same amount of heat. If convection was moving the heat to your hand, you would expect the hand above the bulb to feel more. Once the glass gets hot, convection plays a role and the hand above the bulb will begin to feel warmer.

It seems that the heat moved directly from the hot filament to your hand, without having to heat the objects in between. That is exactly what happened. The heat was traveling as radiant energy, just as light energy does. This is very important because it is the way that the heat from the sun reaches us. There is not enough matter in the space between us and the sun for conduction or convection to carry the sun's heat to us. We rely on the radiation of heat by the sun to keep the Earth warm.

That same radiation of heat is keeping me toasty warm, right through the glass in front of the fireplace. It is also making me quite sleepy, so I will tell you good night and have a wonder filled week.