Energy, power and the units that measure them: What energy is

1.1 Energy is the capacity to change things, and it comes in a handful of forms

The textbook definition says energy is the capacity to do work, and work is a force pushing something through a distance. That is correct but abstract. A more useful starting point is to list the forms energy takes, because every real system mixes several of them.

Kinetic energy is the energy of motion: wind, a moving train, the spinning rotor of a turbine. Potential energy is energy stored by position or strain: water held behind a dam, a raised weight, a compressed spring. Thermal energy is the jiggling of atoms and molecules that we feel as heat. Chemical energy is held in the bonds between atoms, in food, wood, coal, oil and gas, and is released when those bonds rearrange during digestion or combustion. Electromagnetic energy covers light, radio waves and the electricity in a wire. Nuclear energy is bound up in the cores of atoms and is released when heavy nuclei split or light ones fuse.

These forms are interchangeable, which is what makes energy so useful and so universal. Smil calls it the only universal currency: stars, plate tectonics, plant growth and every economy all run by converting one form into another. No process escapes that rule.

Two cautions follow. First, energy is a property of a system, not a thing you can hold. You can hold a lump of coal, but its energy only shows itself when the coal reacts with oxygen. Second, the forms differ greatly in how easily they can be turned into others. Electricity and motion convert readily into almost anything. Warm air in a room converts into very little. That difference in quality is the subject of the next module on the laws of energy.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'What energy is'
• Jaffe and Taylor, The Physics of Energy (2018) · via Smil Mastery Course, M1 'What energy is'

1.2 Every process is a chain of conversions

Once you know the forms, you can follow energy through almost anything. Take a bowl of rice. Deep in the Sun, hydrogen nuclei fuse into helium and release nuclear energy. That energy travels to Earth as sunlight, an electromagnetic flow. In the rice leaf, photosynthesis captures a small fraction of that light, typically around 1% or less over a growing season, and stores it as chemical energy in starch. You eat the rice, your cells break the starch down, and the released chemical energy powers your muscles, which do mechanical work, and keeps your body warm, which is thermal energy. In the end almost all of it leaves you as heat.

The same habit of tracing works for modern systems. A car burns petrol, turning chemical energy into heat in the cylinders, heat into the motion of pistons, motion into the turning of wheels, and finally, through friction with the road and air and the brakes, back into heat. An electric car shortens the chain: electricity stored chemically in a battery drives a motor directly.

Smil's whole method rests on this kind of tracing. Whether he is studying a cell, a farm, a city or a national economy, he asks where the energy comes from, through which converters it passes, how much is lost at each step and where it ends up. The answers explain why some technologies spread and others stall, and why the material world costs what it does.

It also explains a phrase that sounds odd at first: we never really consume energy. We convert it. What disappears with use is not the quantity but the usefulness, because every chain finishes as diffuse, low-temperature heat.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'What energy is'
• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M1 'What energy is'

1.3 Stores and flows: savings versus income

A second distinction runs through every energy debate: stores versus flows. Coal, crude oil, natural gas and uranium are stores. The fossil fuels are ancient sunlight, captured by plants and plankton and concentrated by heat and pressure over tens to hundreds of millions of years. Uranium is older still, forged in exploding stars before the Earth existed. Stores can be dug up and used at whatever rate we choose, until they run out or become too costly to extract.

Sunlight, wind, flowing rivers, waves and tides are flows. They arrive continuously, but only at the rate nature sets, and they must be captured as they arrive or lost. Biomass sits in between: a forest is a store, but it refills only as fast as trees grow.

The household analogy is useful. Fossil fuels are like a large inheritance in a savings account: generous, easy to draw on, but finite. Renewable flows are like a salary: they never run out, but you cannot spend faster than they are paid, and some days pay more than others. The industrial world was built by drawing down savings. Moving towards flows means learning to live on income, which requires capturing energy over large areas and storing it for the days the income does not arrive.

The idea of energy itself is surprisingly young. Aristotle used the word energeia, but the modern concept, one quantity that appears in many forms and is always conserved, came together only in the 1840s and 1850s through the work of Mayer, Joule, Helmholtz, Clausius and William Thomson, later Lord Kelvin. Steam engines were already changing the world before anyone could say precisely what they were converting.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'What energy is'
• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M1 'What energy is'

See it in the table

Check your understanding

A solar panel charges a battery that later runs a fan. Which sequence of forms is right?

Which of these is a flow rather than a store?

Why do energy analysts say we convert energy rather than consume it?