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Heat

Now look at a hot air balloon. When we heat up the gas inside a hot air balloon, we can make it expand and cause it to raise. When we cool it down, the balloon shrinks, which causes it to sink.

This is because when we add energy (the heat), it causes the air molecules to get really excited and move quickly, which makes them spread out. Because the heated air is spread out, the fabric of the balloon stretches, and so there is more volume. This makes the air in the balloon less dense than the air around it, and makes the balloon raise up.

When the air in the balloon cools down, the molecules slow down and get pulled closer together. This causes the the volume of the air inside the balloon to be smaller, thus making the air denser, and the balloon starts to fall slowly.

Introduce the concept of heat as energy, not temperature:

When the balloon was being moved, we didn’t push it, or pull it with a crane; all we did was heat it up. This is because heat is a type of energy, just like kinetic and potential energy for the roller coaster!

Ask the class: We measure speed by seeing how quickly it moves over a distance, such as in miles per hour. We measure the potential energy in terms of an object's height and the strength of gravity. So how do we measure heat? (We use temperature).

Temperature is not the same thing as heat, however. Heat is like wind. There's no direct method of measuring the speed of wind itself, but you can measure how much it pushes on objects to figure out how strong the wind is.

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