# Energy

Every second of every day, the Sun pumps energy toward Earth, firing our planet with light and life. Though you can't always see it, energy is everywhere you look. It's locked in the atoms bouncing inside things, and it keeps the heart that pumps blood through your veins beating steadily. It shoots comets through space and makes trees reach for the sky. Energy is the secret power behind everything in our world.

### What is Energy

Things happen because forces push and pull, and whenever forces are at work, energy is needed to power them. Mass (the ordinary matter around us) is another kind of energy. Tiny amounts of mass can be converted into huge amounts of energy.

Measuring energy

We measure energy in units called joules (J), named after English physicist James Joule (1818-89), who investigated energy forms. When you lift an apple weighing 100 g up in the air to a height of 1 m, you use one joule of energy to work against the force of gravity.

Converting energy

There's a fixed amount of energy in the Universe. When we think we’re using energy, we’re really just converting it into a different form. The total energy before something happens is exactly equal to the total energy afterward.

### Types of energy

Energy can exist in many different forms, most of which can be converted into other forms. When you burn a lump of coal, you change the chemical energy locked inside the coal into heat. If you do this in a power station, you can convert the heat energy into electricity.

Once energy is in electric form, it's easy to change it into movement, light, heat, sound, or virtually any other kind of energy.

Light energy

Light travels at high speed and in straight lines. Like radio waves and X-rays, it is a type of electromagnetic energy.

Potential energy

This is stored energy. Climb something, and you store potential energy to jump, roll, or dive back down.

Nuclear energy

Atoms are bound together by energy, which they release when they split apart in nuclear reactions.

Kinetic energy

Moving things have kinetic energy. The heavier and faster they are, the more kinetic energy they have.

Electrical energy

Electricity is energy carried by charged particles called electrons moving through wires.

Heat energy

Hot things have more energy than cold ones, because the atoms inside them move around more quickly.

Chemical energy

Food, fuel, and batteries store energy within the chemical compounds they're made of.

Sound energy

When objects vibrate, they send energy waves traveling through the air, which we hear as sounds.

Dark energy

Most of the Universe’s energy is in the form of mysterious dark energy. No one really knows what it is.

### Energy waves

When energy moves through materials such as air and water, it travels as waves. We can see waves on water, but we can’t see light waves or seismic waves carrying earthquakes underground.

Different types of waves

1. Transverse Wave: Ocean waves are transverse. As energy races over the surface, the water moves up and down while traveling forward.
2. Longitudinal Wave: As you bang a drum, the air is squeezed (compressed) and stretched (rarefied) as the sound waves race toward your ears.

Measuring waves

Energy waves have an up-and-down pattern of crests and troughs. We can measure waves in three ways. The height of a wave is its amplitude. The distance from one wave crest to the next is the wavelength. The number of crests passing in one second is the frequency.