What is energy?

A bulb lights up. A kettle boils. A roller coaster zooms. A plant grows. What do all of these need? Energy.

What is energy?

Energy is what makes things happen. Whenever something moves, heats up, lights up, makes a sound or changes in any way, energy is involved.

Energy is measured in joules (J). You cannot see energy itself, only what it does.

Forms of energy

Energy comes in several forms. Here are six of the most common:

• Light: energy that travels from the Sun and lets us see.
• Heat: the energy of tiny jiggling particles. Hot things have more.
• Motion: the energy of anything that is moving. Scientists call it kinetic energy.
• Stored: energy waiting to be used, like a ball held up high, a stretched spring or the chemical energy in food. Scientists call the height kind potential energy.
• Electrical: energy carried by electricity in wires.
• Sound: energy that travels through the air as vibrations.

Stored energy and motion energy

These two forms swap with each other all the time. Watch a ball roll down a hill:

• At A, the ball is at the top and not moving. All of its energy is stored, because it is high up.
• At B, some stored energy has turned into motion. The ball is rolling.
• At C, at the bottom, almost all of it is motion energy. The ball is moving fastest.

Go up a hill, and it works backwards: motion turns back into stored. Marble Machine shows this with a live energy bar.

Energy changes form, but it is never lost

This is one of the most important ideas in physics. Energy can change from one form to another, but it can never be created or destroyed. Scientists call it the conservation of energy.

Here are some energy changes:

• A battery turns chemical energy into electrical energy, and a bulb turns that into light and heat.
• A plant turns light from the Sun into stored chemical energy.
• You eat the plant, and your body turns that stored energy into motion, heat and growth.
• A roller coaster turns stored energy at the top of a hill into motion.

Where does the energy go?

If energy is never lost, why does a rolling ball stop? Because friction turns its motion energy into heat. The energy is still there, but it is spread out in the floor and the air as tiny jiggles that we cannot use again. That is the grey part of the energy bar in Marble Machine.

That is also why a bouncing ball bounces lower each time, and why a swing slows down unless you keep pushing.

Why does energy matter?

Everything we do uses energy. People work out how to get it (from the Sun, wind, water and fuels), how to store it (in batteries and dams) and how to use it well without wasting it as heat. Understanding energy helps us do all of that.