What is energy?

Kick a ball, and it flies across the playground. Switch on a lamp, and the room lights up. Eat your breakfast, and you have the strength to run and play. Behind all of this is one and the same thing: energy.

What is energy?

Energy is what makes things happen. Whenever something starts to move, warms up, lights up or makes a sound, energy is at work.

Energy is not a thing you can hold, like a stone or a cup of water. But you can always see what it does. Things move, get warm, shine or make a noise.

The many forms of energy

Energy can hide in many different forms. Here are the most important ones.

Form of energy	Where you can find it
Movement	a rolling ball, the wind, you riding a bike
Stored in height	a book on a high shelf, you at the top of a slide
Stored in stretching	a stretched rubber band, the spring in a wind-up toy
Stored in food and fuel	bread, apples, wood, petrol, batteries
Heat	a warm radiator, hot soup
Light	sunshine, a torch
Sound	a drum, a voice, a barking dog
Electrical	electricity flowing through wires

Everything that moves has movement energy. The heavier and faster it is, the more energy it has. A truck rolling down a street has far more than a marble rolling at the same speed.

Stored energy is energy that waits until it is needed. Lift a ball up high, and it gets stored energy, ready to turn into movement when you let go. A stretched rubber band stores energy too. Release it, and it snaps back.

Food and fuel are full of stored energy as well. It stays locked inside until your body, a fire or an engine sets it free. Heat and light are energy too, and so is the electricity that flows through wires as electric current.

Energy changes form

Energy loves to change from one form into another. Look around, and you will see it happening everywhere:

• A lamp turns electrical energy into light, and a little heat.
• A toaster turns electrical energy into heat.
• On a slide, your height turns into speed.
• A car engine turns the energy stored in fuel into movement, and a lot of heat.
• A drum turns the movement of your hands into sound.

However it changes, energy never disappears. It only moves from place to place or turns into another form. This is one of the most important rules in all of science. Read more in why energy never disappears.

A long journey: from the Sun to your legs

Where does the energy in your body come from? Let’s follow it on its long journey.

The Sun sends light down to Earth. Plants catch that light with their leaves. They use its energy to make sugar, and the energy stays stored inside the plant.

Then you eat the plant, as bread, fruit or vegetables. Or you eat food from an animal that ate plants. Your body sets the stored energy free, and your muscles use it to run and jump.

Running makes you warm. In the end, the energy spreads out into the air around you as heat.

Most of the energy we use on Earth began its journey in the Sun. That includes the food we eat and the wood we burn. Even the wind blows because the Sun warms some places more than others.

How do we measure energy?

Scientists measure energy in joules (say “jools”). They write J for short. The name honours James Prescott Joule, an English scientist who studied heat and energy about 180 years ago.

One joule is a small amount. It is about the energy you need to lift a small apple from the floor up to a door handle. Because a joule is so small, we often count in kilojoules (kJ). One kilojoule is 1,000 joules.

Look at a food packet. It shows how much energy the food contains, in kilojoules (kJ) and in kilocalories (kcal). People often shorten kilocalories to “calories”. One kilocalorie is about 4,200 joules.

A banana holds about 400 kJ, or about 100 kcal. That is as much energy as lifting a small apple up to a door handle 400,000 times!

Go deeper: Movement energy in numbers for grown-ups and the extra curious

The energy of a moving object is called kinetic energy:

$$E = \frac{1}{2} \cdot m \cdot v^2$$

Here $E$ is the energy in joules (J), $m$ is the mass in kilograms (kg) and $v$ is the speed in metres per second (m/s).

A child with a mass of 30 kg running at 4 m/s (about 14 km/h) has $\frac{1}{2} \cdot 30 \cdot 4^2 = 240$ joules of movement energy.

Because the speed is squared, twice the speed means four times the energy. That is why a fast car is so much more dangerous than a slow one: it has far more energy to get rid of in a crash.

Energy stored by height (gravitational potential energy) is $E = m \cdot g \cdot h$. Here $h$ is the height in metres and $g \approx 9.8\ \text{m/s}^2$ tells how strongly the Earth pulls. A 0.1 kg apple lifted by 1 m gets $0.1 \cdot 9.8 \cdot 1 \approx 1$ joule: the apple from the text.

Go deeper: Energy and power are not the same for grown-ups and the extra curious

Energy tells how much. Power tells how fast: how much energy is moved or changed every second. Power is measured in watts (W), and one watt is one joule per second:

$$1\ \text{W} = 1\ \frac{\text{J}}{\text{s}}$$

A 10 W LED lamp uses 10 joules every second. A 2,000 W kettle uses 2,000 joules every second, 200 times as fast.

Electricity bills count energy in kilowatt-hours (kWh). One kilowatt-hour is 1,000 watts for one hour: $1000 \cdot 3600 = 3\,600\,000$ joules.

Go deeper: Where the picture of energy forms breaks down for grown-ups and the extra curious

• Energy is not a substance like water. It is a quantity that can be calculated for any situation, and nature keeps its total the same.
• The list of forms is a handy way to sort energy, but several forms are the same thing in disguise. Heat is the movement energy of tiny jiggling particles, and sound is air moving back and forth. Energy in food and fuel is stored in the way atoms are joined together.
• Most forms belong to two big families: kinetic energy (energy of movement) and potential energy (stored energy). Light is special: it carries energy on its own, even through empty space.
• The Sun’s own energy comes from its centre, where hydrogen is squeezed together into helium. This is called nuclear fusion.

Check yourself

Where did the energy in your breakfast cereal come from in the first place?

From the factory that made it From the Sun From the cereal box

Show the answer

From the Sun — Plants caught sunlight and stored its energy in sugar. Cereal is made from those plants, so its energy began its journey in the Sun.