Where Does Heat Come From?

When we talk about "hot" weather, we are referring to heat that comes from the sun. The sun's heat is created by nuclear fusion, in which hydrogen atoms transform into helium. What other sources of heat can you think of? In the winter, we usually heat our homes by burning a fuel, like oil, natural gas, or wood; this is a chemical reaction called combustion. You've probably also noticed that some electrical appliances, such as lamps and toasters, also get hot. The sun, oil, electricity - these seem to be very different. How can they all produce the same thing - heat?

You might think that they all contain heat, which just escapes when fusion, combustion, or electricity happens. Long ago, scientists believed that matter contained phlogiston (pronounced flow-jis-tun), a substance that had no taste, color, or weight. They thought that phlogiston came out of matter when it burned. There were a lot of problems with this idea, and by around 1800, scientists realized that phlogiston didn't really exist.

So, matter doesn't contain either phlogiston or heat, but it does contain thermal energy. "Heat" and "thermal energy" may sound like they mean the same thing, but to scientists, they don't. This might be easier to understand by a comparison. Think of the aroma of cookies baking in the oven. Where does the smell come from? "Smell" isn't something that the cookie dough releases as it cooks. Instead, we smell that wonderful odor when our nose senses certain molecules given off by the cookies. So, smell isn't something in itself, but it is what we experience. Usually, when we say that something is "hot", we mean that it feels warm to the touch. Thermal energy is sort of like the fragrant molecules. When it is released, we feel its effect, and call it heat. Of course, they're not exactly the same, because thermal energy doesn't contain actual molecules, but hopefully you get the idea.

When you burn wood, the fire feels hot because the original wood has more thermal energy that the ashes, water, and other stuff that's left over after the fire. The heat you feel is the thermal energy that is lost during combustion. The same thing happens during the fusion reactions in the sun. You can also feel heat when some other type of energy is transformed into thermal energy. That's why electricity creates heat - the electrical energy is changed into thermal energy. In the next experiment, you'll learn how to turn mechanical energy into thermal energy - using your very own hands!