What Is Energy?

Energy is one of the most important ideas in physics. It is the ability to do work: to make something happen. Without energy, nothing in the universe would change. The Sun would not shine. Cars would not move. Plants would not grow. You could not even think (your brain uses energy with every thought). Energy comes in many forms, like motion, heat, light, electricity and chemical bonds, and it can change from one form to another. The amount of energy in the universe stays the same, but it never stops moving and changing.

  • What it isAbility to do workTo move or change things
  • UnitJoule (J)Or kilojoule, megajoule
  • Cannot be createdOr destroyedJust transferred between forms
  • From the Sun174,000 terawatts hit EarthPowers nearly all life
  • Daily food energyAround 8,400 kJ per adult2,000 calories
  • Average power stationAround 1 gigawattEnough for a city

The many forms of energy

Energy comes in many different types:

  • Kinetic energy: the energy of motion. A moving car, a flowing river, a thrown ball.
  • Potential energy: stored energy, ready to be released. A book on a shelf (held up by gravity), a stretched rubber band (held by elastic forces).
  • Chemical energy: stored in the bonds between atoms in molecules. Food, fuel, batteries.
  • Electrical energy: carried by flowing electric charge.
  • Thermal energy (heat): from the motion of particles in matter.
  • Light (radiant) energy: carried by photons of light.
  • Sound energy: carried by sound waves.
  • Nuclear energy: locked inside the nucleus of every atom, released in fusion or fission.
  • Magnetic energy: stored in magnetic fields.

Energy changes form

One of the most useful things about energy is that it can change from one form to another. Some examples from your everyday life:

  • A solar panel turns LIGHT energy into ELECTRICAL energy.
  • A torch turns ELECTRICAL energy back into LIGHT energy.
  • A car engine turns CHEMICAL energy (in petrol) into KINETIC energy (motion) and HEAT.
  • A radio turns ELECTRICAL energy into SOUND energy.
  • Photosynthesis turns LIGHT energy into CHEMICAL energy (in plant sugars).
  • Your muscles turn the CHEMICAL energy in food into KINETIC energy (movement) and HEAT.
  • A waterfall turns POTENTIAL energy (water at height) into KINETIC energy (water flowing down).
  • A hydroelectric plant catches the KINETIC energy of falling water and turns it into ELECTRICAL energy.
Fact Energy is measured in joules (J), named after the English physicist James Prescott Joule. One joule is roughly the energy needed to lift a small apple a metre into the air. A typical kitchen kettle uses about 100,000 joules (100 kJ) of energy per minute when boiling. A typical adult eats about 8 million joules of food energy in a day.

The big rule: conservation of energy

One of the most important rules in physics is the law of conservation of energy: in an isolated system, the total amount of energy stays the same. Energy is never created and never destroyed; it only changes form.

Some examples:

  • A pendulum swinging: kinetic and potential energy keep swapping, but the total stays the same (minus tiny losses to air friction).
  • A car braking: kinetic energy of motion turns into heat in the brake pads.
  • A torch battery dying: chemical energy becomes electrical, then light and heat. Eventually all the chemicals have reacted and no more energy can come out.

See conservation of energy for more.

Where energy comes from on Earth

Almost all the energy on Earth comes from one source: the Sun. Sunlight powers:

  • Plant photosynthesis (which gives us food and oxygen).
  • The water cycle (evaporation that makes rain).
  • Weather and winds (warming uneven parts of Earth differently).
  • Ocean currents.
  • Fossil fuels (formed from ancient plants and animals that absorbed sunlight long ago).

A few other energy sources do not come from the Sun:

  • Nuclear energy: stored in atoms ever since the universe began.
  • Geothermal energy: heat from Earths interior, partly from radioactive decay of rocks.
  • Tidal energy: from the Moons gravity.
Did you know? Einsteins famous equation E = mc^2 says that mass and energy are actually two forms of the same thing, and that a small amount of mass can be turned into a huge amount of energy. This is the principle behind nuclear power stations and nuclear weapons. A single gram of matter, fully converted, contains as much energy as 22,000 tonnes of TNT.

How much energy do we use?

The whole human race uses about 180,000 terawatt-hours of energy per year (180 trillion kWh). About 80 per cent of this still comes from burning fossil fuels (oil, gas, coal), causing climate change. The rest comes from nuclear and renewable sources like wind, solar, hydro and biomass.

The Sun delivers about 174 thousand terawatts to Earths surface, far more than humanity uses. If we could capture even a tiny fraction with solar panels, we would have all the energy we need with no greenhouse emissions.

Try this Look around your room and count the different forms of energy in action. The light bulb is light and heat. The radio (if on) is sound and electrical. Anything moving has kinetic energy. Books on a shelf have potential energy. A laptop battery has chemical energy. A magnet on the fridge has magnetic energy. Even your warm body has thermal energy. Energy is literally everywhere, every moment, in dozens of forms.
Deeper dive: the history of the energy concept

The modern concept of energy is surprisingly recent. For most of human history, scientists used many separate ideas (heat, motion, light, electricity, magnetism, chemical reactions) without realising they were all forms of the same thing.

One of the first big steps came from English physicist James Prescott Joule in the 1840s. Joule (a brewer by trade) carried out painstaking experiments showing that heat and motion are related. In a famous experiment, he stirred water with paddle wheels driven by falling weights and showed that the water warmed up by a precise, predictable amount. The energy of the falling weights became the heat energy in the water.

Around the same time, the German physicist Hermann von Helmholtz proposed the general conservation of energy principle in 1847: energy is neither created nor destroyed, only changes form. This was a revolutionary idea. Before Helmholtz, scientists believed in "vital forces" in living things, and treated heat as a separate "fluid" (caloric). His insight unified physics, chemistry and biology under one elegant principle.

By the end of the 1800s, scientists had identified most of the major forms of energy. James Clerk Maxwell unified electricity, magnetism and light into one theory of electromagnetism (and showed they all carry energy). At the start of the 20th century, Einstein added the most surprising twist with E = mc^2: mass itself is a form of energy.

Today, the conservation of energy is one of the most well-tested principles in all of physics. Every experiment ever performed (in chemistry, biology, engineering, particle physics) is consistent with it. If you ever read a story claiming a perpetual motion machine, you can be certain it does not work: the universe simply does not allow energy to be created from nothing.

For more, see kinetic energy and conservation of energy.