Energy Definition: The Power to Make Things Happen

The energy definition physicists use

Physicists describe energy as the capacity of a system to do work. In this sense, work has a precise meaning: a force moving something through a distance. Whenever you push a door, boil water or switch on a lamp, energy is being transferred from one place or form to another.

The most important rule is the conservation of energy: in a closed system, energy is never created or destroyed. It only changes form. The chemical energy in petrol becomes the motion of a car, plus heat in the engine and sound. Some of it always ends up as heat that spreads out and is hard to use again, an idea linked to entropy.

Main forms of energy

Plants turn radiant energy into chemical energy during photosynthesis, and your body then releases that chemical energy when you eat.

Units and a simple worked example

The SI unit of energy is the joule. Other common units are the kilowatt-hour on electricity bills (one kilowatt-hour equals 3.6 million joules) and the food calorie on nutrition labels (one food calorie is about 4,184 joules).

Try two quick calculations:

1. Potential energy of a 2 kg bag lifted 1.5 metres: mass times gravity times height, so 2 times 9.8 times 1.5 = about 29 joules.
2. Kinetic energy of a 1,000 kg car moving at 10 metres per second: half of mass times speed squared, so 0.5 times 1,000 times 100 = 50,000 joules.

Doubling the car's speed makes its kinetic energy four times larger, which is one reason fast crashes do so much more damage.

Energy in everyday speech

1. Personal vigour. "After the holiday she came back full of energy." Here energy means strength and the will to be active. The opposite is fatigue.
2. Effort and attention. "Put your energy into the parts you can change."
3. Power supply. "Energy prices rose this winter." This covers electricity, gas and fuel, and the industry that supplies them.
4. Mood or atmosphere. "The crowd had amazing energy." A casual, figurative use for the feeling a place or group gives off.

In some spiritual and wellness traditions, energy is also used for an inner life force. That use is a matter of belief rather than physics, and the two should not be confused.

Example sentences

• Switching to LED bulbs cut our household energy use noticeably.
• The toddler's energy lasted far longer than his parents' patience.
• Wind turbines turn the kinetic energy of moving air into electricity.
• I don't have the energy to argue about it tonight.
• The team's energy dropped after the second goal against them.

Where the word came from

Energy comes from Greek energeia, activity or operation, built from en (in, at) and ergon (work). Aristotle used energeia for actuality, as opposed to mere potential. English picked it up in Tudor times by way of French and Late Latin, first meaning force or vigour of expression in speech or writing. The scientific sense came much later: the English scientist Thomas Young used energy in its modern physical sense in lectures published in 1807. The same Greek root gives the unit name erg and the word ergonomics.

Energy, power and force: not the same

These three get mixed up often. Energy is the total stock of work available, counted in joules. Power is how fast energy is used or delivered, measured in watts (one watt is one joule per second). Force is a push or pull, measured in newtons. A 60-watt bulb left on for an hour uses 60 watt-hours of energy. More science words are listed in our science and ideas hub, including matter and inertia.