What energy is

One joule lifts a 100 g apple about 1 m; a 1,000 kg car at 90 km/h carries about 310,000 of them.

Energy is what lets things change

A rolling ball, a stretched rubber band, a warm cup of tea and a charged phone battery have something in common. Each can make something else happen: push, lift, heat or light something up. Physicists give that ability one name, energy, and measure it in one unit, the joule (J). The joule is named after James Prescott Joule, who showed in the 1840s that mechanical work and heat are two faces of the same quantity.

A joule is not much. OpenStax gives a handy picture: one joule would lift a small 100 g apple about one metre. Check it with the formula you will meet in unit 4: 0.1 kg × 9.8 m/s² × 1 m = 0.98 J, close to one. Food energy is counted in larger units. One food Calorie (kcal) is 4,186 J, and a recommended day of food for an adult is about 1.2 × 10⁷ J, twelve million joules.

Many forms, one quantity

Energy comes in many forms, and the names describe where it sits. Kinetic energy belongs to anything that moves. Gravitational potential energy is stored when something is lifted against gravity, and elastic potential energy when a spring or a bow is bent. Chemical energy is held in fuels, food and batteries. Thermal energy is the random jiggling of atoms and molecules, which we feel as temperature. Electrical energy moves through wires, radiant energy travels as light, and nuclear energy is released when atomic nuclei split or fuse.

Kinetic and potential energy together are called mechanical energy. They are the two forms you can see at a glance on a swing or a roller coaster, and they are the ones this course works with most. The other forms appear when energy leaves the mechanical world, most often as heat.

Energy is never made or lost

The most useful fact about energy is that the total never changes. OpenStax states the law of conservation of energy this way: total energy is constant in any process; it may change form or move from one system to another, but the total remains the same. When a car brakes, its kinetic energy does not vanish. It becomes thermal energy in the brake discs, which is why they get hot. When you charge a phone, electrical energy becomes chemical energy in the battery, and later light and heat from the screen.

If energy is conserved, why do we talk about using it up? Because the form matters. Energy that has spread out as warmth in the air is still there, but it is hard to collect and use again. A power plant that burns coal turns only about 40 percent of the fuel's chemical energy into electricity; the rest leaves as heat. Unit 5 measures this with a number called efficiency.

How big is a joule? A scale of energies

The table puts a few everyday amounts side by side, using values from OpenStax Table 7.1. The range is enormous. A mosquito in flight has about a millionth of a joule of kinetic energy; a gallon of gasoline holds about a hundred million joules. Scientific notation keeps these numbers readable: 1.2 × 10⁸ means 1.2 followed by eight places, 120,000,000.

You can check the car line yourself. At 90 km/h the speed is 25 m/s, and kinetic energy is ½ × 1,000 × 25² = 312,500 J, which rounds to 3.1 × 10⁵ J. Unit 3 explains where that formula comes from.

Energy can also move without changing form. A hot pan warms the handle by heat transfer, a battery pushes energy down a wire to a lamp, and sunlight carries energy across space from the Sun to Earth. Physicists keep two words apart. A transformation changes the form of energy, such as chemical into kinetic. A transfer moves energy from one object or system to another, either by work (a force acting through a distance) or by heat (flow from hot to cold). The next unit is about the first of these, work.