A hard word to define
Energy is a surprisingly slippery idea. A common working definition is that energy is the capacity to cause change, such as making something move, heat up or glow. Energy itself is not a substance you can hold. It is a quantity that physicists calculate for a system, and its usefulness comes from what happens to it: it can be moved from place to place and changed from one form to another, but the total stays the same.
Energy is measured in joules. Lifting a small apple by about one meter takes roughly one joule, because an apple weighs about one newton, a unit described in the discussion of the laws of motion.
Common forms of energy
Energy appears in several forms, which are best seen as different ways of storing or carrying it.
- Kinetic energy is the energy of motion. A moving object has kinetic energy equal to half its mass times its speed squared. A 1,000-kilogram car moving at 10 meters per second has ½ × 1,000 × 10² = 50,000 joules.
- Gravitational potential energy is stored by an object’s position in a gravitational field. Lifting a 2-kilogram book 3 meters raises its potential energy by about 2 × 9.8 × 3 = 58.8 joules.
- Elastic potential energy is stored in stretched or compressed objects, such as a spring or a drawn bow.
- Thermal energy is the energy of the random motion of atoms and molecules. It is closely related to temperature, though as explained in the article on heat and absolute zero, the two are not the same.
- Chemical energy is stored in the bonds between atoms. Food, fuel and batteries all store energy this way, a topic touched on in chemical reactions and battery storage.
- Electrical energy is carried by moving charges in a circuit.
- Radiant energy is carried by electromagnetic waves, including visible light.
- Nuclear energy is stored in atomic nuclei.
Strictly, physicists often group these into just a few basic types, such as kinetic energy, potential energy and radiation, but the everyday labels are useful.
Energy changes form
Almost everything that happens involves energy changing from one form to another. A roller coaster is a clear example. As the cars are pulled up the first hill, the motor adds gravitational potential energy. When they roll down, that potential energy converts into kinetic energy and the cars speed up. At the next hill the process reverses, and the cars slow as they climb.
Other examples include:
- A plant converts radiant energy from sunlight into chemical energy, described in plant photosynthesis.
- Your body converts the chemical energy in food into movement and warmth, starting in the digestive system.
- A light bulb converts electrical energy into light and heat.
The conservation of energy
The principle of energy conservation says that in a closed system, the total amount of energy does not change. Energy can be transferred or transformed, but it cannot be created or destroyed. It is one of the most thoroughly tested ideas in physics.
At first glance this seems wrong. A bouncing ball does not bounce as high each time and eventually stops. A car coasting to a stop appears to lose its energy. But the energy has not vanished. Friction, air resistance and the deformation of the ball convert the visible motion into thermal energy, a small warming of the ball, the air and the ground, and a little sound. If you could add all of that up, the total would match what you started with.
This is why people say energy is “lost” or “wasted” in everyday speech. What they usually mean is that it has turned into a form, typically low-level heat spread out in the surroundings, that is hard to use again. The total is conserved, but its usefulness is not.
Why perpetual motion fails
The principle also explains why machines claiming to run forever and produce free energy cannot work. Any real machine has friction and other losses, so without an energy supply it eventually stops. Any device that seemed to put out more energy than it takes in would break the conservation law, which has held up in every careful test so far. The related idea that heat tends to spread out and that no engine can be perfectly efficient is covered by the laws of thermodynamics.
Where the idea comes from
Physicists can connect conservation of energy to a deeper principle. A result by the mathematician Emmy Noether showed that, in theories like those of physics, each conservation law follows from a symmetry. For energy, the symmetry is that the laws of physics do not change over time. This is a technical result, but it hints at why the rule holds so widely.
Energy conservation was extended by Albert Einstein’s work, which showed that mass is itself a form of energy. In nuclear reactions, as covered in nuclear fission, a small amount of mass is converted to a large amount of energy, and the total of mass and energy together is what is conserved.
Why it matters
Conservation of energy is a powerful tool. It lets scientists and engineers work out the speed of a falling object, the output of an engine or the fate of a collision without tracking every detail. It also underlies discussions of energy sources, where the real question is never creating energy, but converting stored forms into useful ones as efficiently as possible.