What is Energy?
Energy is one of the most important ideas in all of science. It affects everything around us — from the food we eat, to the light in our classrooms, to the movement of cars and rockets.
Energy is the ability to do work or cause a change. It cannot be created or destroyed, only transferred or transformed from one form to another.
We experience energy every single day:
- A bowl of cereal gives you chemical energy to run and think
- The Sun sends light energy to warm the Earth
- A phone battery stores electrical energy to power your screen
- A moving football has kinetic energy
Think of energy like money in a bank account. You can move money between accounts (transfer energy), change it into different currencies (transform energy), but the total amount of money in the whole system stays the same — it doesn't disappear!
Because energy causes change, scientists need a reliable way to measure it. That way, we can compare different energy sources, understand how much fuel a car needs, or figure out how much food an athlete should eat.
The Unit of Energy: The Joule
To measure anything in science, we need a unit — a standard amount that everyone agrees on. For energy, that unit is the joule.
The joule (symbol: J) is the SI (International System) unit used to measure energy. It is named after the English physicist James Prescott Joule, who did important experiments on heat and energy in the 1840s.
Here are some everyday examples to give you a feel for how big a joule is:
| Everyday Event | Approximate Energy |
|---|---|
| Lifting a small apple 1 metre upward | ~1 J |
| One heartbeat | ~1 J |
| A single press-up | ~300 J |
| A 100 W lightbulb on for 1 second | 100 J |
| A chocolate bar | ~1,000,000 J |
Because a joule is quite small, scientists often use kilojoules (kJ) for larger amounts of energy. 1 kJ = 1,000 J. Food energy is usually shown in kilojoules on nutrition labels.
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