Energy and Heat

Energy is one of the most used words in science and one of the hardest to define. It is not a substance; you cannot bottle it. It is better thought of as a quantity, a number that can be calculated for any system and that has one extraordinary property: when you add it all up, it never changes. Energy can move from place to place and change from one form to another, but the total is always the same. That rule, the conservation of energy, is the most reliable law in physics.

The forms

A moving object has kinetic energy, more if it is heavier or faster. A rock on a cliff has potential energy, stored by its position; let it fall and the potential becomes kinetic, and when it hits the ground the kinetic becomes heat, sound and a dent. A stretched spring, a charged battery, a tank of petrol, a slice of bread all hold chemical or mechanical potential energy, released when bonds break or tension is let go. Light carries energy; so does an electric current. And mass itself is a form of energy, as Einstein's E = mc² states: a tiny amount of matter corresponds to a vast amount of energy, which is why the Sun can shine for ten billion years by converting four million tonnes of matter to light every second.

Every event in nature is energy changing form. A plant turns light into sugar; you turn sugar into motion and warmth; a power station turns coal into heat, heat into steam, steam into spinning, spinning into electricity, and your lamp turns that back into light and heat. At every step the total is conserved.

Heat and temperature

Heat is energy in its most disordered form: the random jiggling of atoms and molecules. The molecules of the air around you are flying about at hundreds of metres a second, colliding billions of times a second, and the faster they move, the hotter the air. Temperature is a measure of that average motion. Heat is the energy transferred when a hot thing touches a cold one and the fast molecules knock the slow ones into moving faster.

Cool something and its molecules slow down. There is a limit: at minus 273.15 degrees Celsius, absolute zero, motion is as slow as the laws of physics allow. Nothing can be colder, and nothing has ever quite got there, though laboratories have come within a billionth of a degree. There is no upper limit to temperature; the centre of the Sun is 15 million degrees, and the first instant of the Big Bang hotter than anything since.

The one-way street

The conservation of energy says what can happen. A second law says which way it goes. Heat flows from hot to cold, never the reverse by itself. A dropped cup shatters; the pieces never leap back together. A hot coffee cools to room temperature; a cool one never warms itself. In each case energy is conserved, but something else changes: the energy spreads out, becomes more disordered, and that disorder, which physicists call entropy, always increases in the end.

This is why no engine can turn heat entirely into work. A steam engine, a car engine or a power station takes in heat at a high temperature, does some work, and must dump the rest as waste heat at a lower temperature; the most efficient power stations convert about 60 percent of their fuel's energy to electricity and the rest warms the river or the air. Perpetual motion machines are impossible not because energy runs out but because every real process loses some of it to disorder. The same law explains why time seems to run forward: the past had lower entropy than the future, and the arrow of time is the direction in which disorder grows.

Energy and life

Living things seem to defy this, building order out of chaos, but they do it by consuming energy from outside and exporting disorder. Plants take in high-quality energy from sunlight and radiate low-quality heat. Animals eat the plants and radiate heat in turn; a resting person gives off about a hundred watts, the same as an old light bulb. Life is a way of using the flow of energy from the hot Sun to cold space to build and maintain temporary pockets of order. Switch off the Sun and it would all run down.

Why it matters

Energy is the currency of everything that happens. Understanding how it is conserved and how it degrades gave us engines, refrigerators, power stations and an account of why the universe has a past and a future. It also frames the central problem of our time. Humanity uses about 18 terawatts of power, nearly all of it from fuels that release carbon; the Sun delivers ten thousand times that to the planet's surface every moment. The physics permits a solution. The engineering is the hard part.