What energy actually is
Energy is a measure of the capacity to cause change: to move something, heat something, light something or alter its structure. It is not a substance you can hold. It is a number that physicists assign to a situation, and the useful thing about that number is that it follows strict bookkeeping rules.
The standard unit is the joule. Lifting a small apple by roughly a meter takes about one joule, which gives a feel for the scale. A household appliance running for an hour uses millions of joules, which is why utilities usually quote energy in kilowatt-hours instead.
Kinetic energy: the energy of motion
Anything that moves has kinetic energy. For an object of mass m moving at speed v, the amount is one half times m times v squared. Two things follow from that formula. Heavier objects carry more energy at the same speed, and speed matters even more, because doubling the speed quadruples the kinetic energy. That squared relationship is part of why high-speed impacts are so much more damaging than slow ones.
Kinetic energy comes in several everyday varieties: a rolling ball, wind, flowing water, and the sound of a voice, which is the motion of air molecules.
Potential energy: stored for later
Potential energy is energy stored because of position or arrangement. Several kinds are common.
- Gravitational potential energy depends on height. A book on a high shelf has more than the same book on the floor, because it could fall farther.
- Elastic potential energy is stored in a stretched spring, a drawn bow or a compressed rubber band.
- Chemical potential energy is stored in the bonds between atoms, in fuels, food and batteries.
- Nuclear potential energy is stored in the tightly bound cores of atoms.
Potential energy is only “potential” until something lets it convert into another form.
Thermal energy and heat
Thermal energy is the total random kinetic and potential energy of the atoms and molecules inside a substance. Temperature is related to the average motion of those particles, while thermal energy depends on how many particles there are as well. A bathtub of warm water holds far more thermal energy than a cup of boiling water, even though the cup is hotter.
Heat is the name for thermal energy moving from a warmer place to a cooler one. It travels by conduction (direct contact), convection (moving fluid) or radiation (electromagnetic waves, which need no medium at all).
Other forms worth knowing
- Electrical energy is carried by moving charges in a circuit and is easy to transport and convert, which is why it is so central to modern life.
- Radiant energy travels as electromagnetic waves, from radio to visible light to X-rays. Sunlight is the main example for our planet.
- Sound energy is the organized vibration of a medium such as air.
- Magnetic energy is stored in magnetic fields and is closely linked to electrical energy.
Textbooks sort these differently, and some physicists prefer to say there are really just two basic categories, kinetic and potential, with everything else being a particular arrangement of those at a small scale. Both ways of describing it are valid. The labels are tools for thinking, not separate substances.
The conservation of energy
The central rule is that in an isolated system the total energy stays constant. Energy can change form, and it can move from one object to another, but it is not created from nothing or destroyed. This is one of the best-tested principles in all of science.
Here is a hypothetical worked example. Imagine a 2 kg ball held 3 meters above the ground, and ignore air resistance. Using g of about 9.8 m/s² for gravity, its gravitational potential energy is 2 × 9.8 × 3 = 58.8 joules. Let go, and that stored energy is steadily converted into motion. Just before landing, the potential energy is nearly zero and the kinetic energy is 58.8 joules. Setting one half × 2 × v² equal to 58.8 gives v² = 58.8, so the speed is about 7.7 m/s. At the halfway point of the fall, the ball has about 29.4 joules of each kind, and the sum is still 58.8 joules throughout.
When the ball hits the ground, it seems that the energy vanishes. It does not. It becomes a small amount of sound, a slight warming of the ball and floor, and tiny deformations. The energy has spread out into random molecular motion, where it is no longer easy to use.
Why useful energy is always being lost
Conservation says the total is fixed, but it does not say all of it stays useful. Whenever energy is converted, some portion usually ends up as low-grade thermal energy that spreads into the surroundings. A car engine, a light bulb and a human body all give off heat as a side effect of doing their jobs. This is why no machine can be perfectly efficient, and why a perpetual motion machine that keeps running forever without an energy source is not believed to be possible.
The reason is rooted in thermodynamics. Energy tends to spread out from concentrated, organized forms into dispersed, disordered ones. Concentrated energy is what we can put to work.
Energy in the world around us
Almost all the energy that drives life and weather on Earth begins with sunlight, which in turn comes from solar fusion in the Sun’s core. Plants capture some of that radiant energy and store it as chemical energy. Animals eat the plants and release it. Fossil fuels are ancient stored sunlight. Wind and flowing water are driven by sunlight heating the planet unevenly. Some sunlight is absorbed and re-emitted as heat, and the way the atmosphere holds some of it back is the greenhouse effect that keeps the planet habitable.
Humans mostly use energy by converting it. A battery turns chemical energy into electrical energy, and lithium-ion batteries are a familiar example of storing it in a compact form. A motor converts electrical energy into motion, as in electric vehicles. A solar panel converts radiant energy into electrical energy. Modern debates about the energy transition are, at their core, about which sources to convert from and how to store and move energy efficiently.
How energy connects to the forces of nature
Forces and energy are two views of the same physics. When a force acts over a distance, it does work, which transfers energy. The four fundamental forces of nature (gravity, electromagnetism, and the strong and weak nuclear forces) each have an associated form of potential energy. Gravity gives us height-based energy, electromagnetism underlies chemical and electrical energy, and the strong force binds atomic nuclei.
Thinking in terms of energy often makes a problem easier than tracking every force, because you can compare the “before” and “after” totals without following each step in between.
A mental checklist
When you meet an energy question, it helps to ask three things. Where does the energy start, in what form? Where does it end up? And how much has been dispersed as heat along the way? Answering those three questions explains a surprising amount of everyday physics, from a roller coaster to a cup of coffee cooling on a desk.