When you think about where energy comes from, you can see that most of the energy we use originally came from the sun. The sun also provides energy in the form of both light and heat. Food that gives you energy to work and play comes from plants that grow thanks to sunlight and from animals that eat those plants. The fossil fuels you use to power your vehicles and factories were once plants or animals, too. In this activity, you will learn more about the connection between thermal energy and kinetic energy.

When talking about the sun, heat, and temperature, you also need to know about thermal energy. Thermal energy is very important to people — without it, almost nothing could exist!

You know that our universe is made up of matter and that matter is made up of molecules and atoms. You also know that atoms are always moving — this movement of atoms (the kinetic energy of the atoms) creates thermal energy. The faster the molecules move, the higher the temperature.

   

You also know about how different materials can change their physical states, and you should remember that the kinetic energy of the atoms in a substance varies depending on whether the substance is in the form of a solid, a liquid, a gas, or plasma.

For example, you realize that when water is in a solid form (ice) — when its atoms are moving more slowly — it is cold. When water is in liquid form, it is warmer than water that is frozen. That means the atoms are moving quickly. When water is in the form of a gas, such as the steam escaping from a pot of boiling water, it is much hotter than an ice cube or glass of water. The molecules of water are moving very quickly in this state.

You can also see thermal energy at work when you consider the idea of thermal expansion in construction. Thermal expansion happens when heat transfers to a solid: The molecules start to move more quickly, so the material expands or gets bigger.

That is why you see bridges or railway tracks built in pieces and joined together with joints that allow room for the material to expand or contract (get larger or smaller) depending on its thermal energy. If a bridge or a set of train tracks was built in one solid piece, it would be more likely to crack or break as it expands or contracts according to the weather (cold weather results in train wrecks!). Thermal energy is related to the kinetic energy of all particles (atoms or molecules) that make up a substance.

In the activity that follows, you will look more closely at the relationship between kinetic energy and thermal energy as well as the ways that thermal energy can be transferred from one substance to another.