What Is Potential Energy? Stored Energy Explained Simply

Potential energy is often described as stored energy, which is useful as a first picture, but the word stored can hide an important detail. Potential energy belongs to a system of interacting objects rather than sitting inside one object by itself. For gravitational potential energy, for example, the system includes the object and Earth.

If you lift a book from the floor to a shelf, you do work against gravity. Energy is transferred into the Earth-book system, increasing its gravitational potential energy. If the book later falls, that potential energy decreases while kinetic energy increases. The energy has not appeared from nowhere. It has changed form within the system.

This transfer is the heart of potential energy. The value depends on how the parts of a system are arranged and on the reference point you choose. In school problems, the floor or another convenient level is often assigned zero gravitational potential energy so changes are easy to calculate.

Near Earth's surface, gravitational potential energy can often be calculated with the expression mass times gravitational field strength times height. Raising a heavier object by the same height gives a larger increase in gravitational potential energy. Raising the same object higher also gives a larger increase.

Elastic potential energy appears when an elastic object such as a spring or rubber band is stretched or compressed. Work done to deform it can be stored in the arrangement of its particles. When released, the object can transfer that energy into kinetic energy, sound, thermal energy, or other forms.

These examples make potential energy easier to recognise. Look for a system whose configuration can change under a force. Height matters in a gravitational system, stretch matters in an elastic system, and separation matters in electric systems. The exact formula changes, but the idea of energy connected with arrangement stays.

Try tracing energy rather than saying that it simply gets used up:

• •A raised object can lose gravitational potential energy as it falls.
• •A compressed spring can transfer elastic potential energy into motion.
• •A pendulum continually exchanges gravitational potential energy and kinetic energy.
• •Friction can transfer mechanical energy into thermal energy.
• •Energy is conserved overall even when the form that interests you decreases.

A roller coaster is a good example. The motor that pulls a car uphill transfers energy into gravitational potential energy. As the car descends, gravitational potential energy decreases while kinetic energy grows. Friction transfers some energy into thermal energy and sound, so the car will not climb back to exactly the same height without another energy input.

Questions about potential energy often become easier when you compare two positions instead of worrying about an absolute amount. Ask which arrangement has more potential energy relative to the chosen reference, then follow where the energy goes as the system changes.

Gravitational potential energy is used on a large scale in pumped storage power stations. These have two reservoirs at different heights. When electricity is plentiful and cheap, pumps push water uphill into the top reservoir. When demand suddenly rises, the water is released downhill through turbines, turning potential energy back into electricity within minutes. A well known example is Dinorwig in north Wales, built inside a mountain, which can respond very quickly when millions of people switch on appliances at the same time.

It is worth working out how much energy water at height actually holds. One cubic metre of water has a mass of about 1,000 kilograms. Raising it 100 metres stores roughly 1,000 times 9.8 times 100 joules, close to one million joules. That sounds large, but it is only about a quarter of a kilowatt hour, roughly enough to boil a full kettle once or twice. This is why pumped storage needs huge volumes of water and big height differences, and why some energy is always lost to friction in pumps and turbines.

The formula mass times gravitational field strength times height works well near the ground because gravity barely changes over a few metres or even a few kilometres. Over distances comparable to the size of a planet it fails, because gravity weakens as you move away. Physicists then choose a different reference point: zero potential energy when two objects are infinitely far apart. Since gravity pulls things together, you must add energy to separate them, so every closer arrangement has less than zero. An object near a planet has negative potential energy, which becomes less negative as it climbs away.

This choice makes a famous calculation simple. To escape Earth completely, a launched object needs enough kinetic energy to cancel its negative gravitational potential energy, with nothing left owing. Working that through gives an escape speed of about 11.2 kilometres per second from Earth's surface, ignoring air resistance. The same pattern appears inside atoms. An electron bound to a nucleus has negative total energy, and the energy needed to pull it free, called the ionisation energy, is the size of that debt. For a hydrogen atom in its lowest energy state it is about 13.6 electronvolts, a figure that matches measurements of hydrogen's spectrum.

The energy in food, petrol and batteries is also potential energy, in this case electrical potential energy linked to the arrangement of electrons and nuclei in molecules. A common misconception is that chemical bonds store energy that is released when they break. In fact, breaking any bond always takes energy in, just as pulling apart two magnets does. Energy is released only when new bonds form. A reaction gives out energy overall when the bonds in the products are, in total, stronger than those in the reactants, so the system ends up in a lower-energy arrangement, rather like a ball settling lower on a hill.

Burning methane, the main part of natural gas, shows the bookkeeping. Using typical average bond energies, breaking four carbon-hydrogen bonds and two oxygen-oxygen double bonds takes about 2,650 kilojoules for each mole of methane. Forming two carbon-oxygen double bonds in carbon dioxide and four oxygen-hydrogen bonds in water releases about 3,470 kilojoules. The difference, roughly 800 kilojoules per mole given out, is close to the value measured in experiments. The same reasoning is why biologists warn against calling the phosphate links in ATP high-energy bonds. The energy comes from ATP reacting with water to form more stable products.

Potential energy is energy linked to the arrangement of parts of a system, such as height in a gravitational system or stretch in an elastic one. Track how that arrangement changes, choose a clear reference level, and follow the energy transfers. The concept becomes much less mysterious when you treat it as part of an energy story.