# Energy, power and the units that measure them: What energy is Source page: https://www.materialinsights.org/learn/energy/energy-power-units/p1/ [Understanding Energy](https://www.materialinsights.org/learn/energy/) / [Module 1: Energy, power and the units that measure them](https://www.materialinsights.org/learn/energy/energy-power-units/) Part 1 of 5 ## What energy is Energy is easiest to understand not as a substance but as a capacity to make things change: to move, warm, light or rearrange matter. Physicists recognise a small number of forms, and almost everything interesting in the world, from a growing rice plant to a steel mill, is energy being passed from one form to another. Learning to see those conversion chains is the first habit of energy analysis. ### 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. Process flowchart From rain on a hillside to electricity Inputs **Water held behind a dam**1 tonne falling 100 m holds about 1 MJ of potential energy Process steps 1 **Penstock** potential energy → kinetic energy of fast-moving water 2 **Turbine** kinetic energy → rotation of a shaft 3 **Generator** rotation → electricity, about 95-98% efficient Outputs **Electricity**about 0.9 MJ (0.25 kWh) **Heat and noise**about 0.1 MJ Potential energy = mass x gravity x height: 1,000 kg x 9.8 m/s2 x 100 m is about 0.98 MJ. Source: Smil, Energy: A Beginner's Guide (2006), via Smil Mastery Course, M1 'What energy is' Worked example #### A dam in four conversions A hydroelectric dam is a clean example of a conversion chain. Rain falling on high ground carries gravitational potential energy, itself a gift of the Sun, which evaporated the water in the first place. Behind the dam that potential energy is stored. When gates open, the falling water speeds up, so potential becomes kinetic energy. The stream pushes turbine blades, so kinetic becomes rotational energy in a shaft. A generator turns that rotation into electricity. Each step loses a little, mostly as heat and noise, but large hydro plants turn about 90% of the water's potential energy into electricity, one of the highest figures of any power source. - 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. Process flowchart From the Sun to a day's work Inputs **Sunlight on a rice field**nuclear fusion in the Sun, delivered as light Process steps 1 **Photosynthesis** light → chemical energy in starch, around 1% or less of incident sunlight over a season 2 **Digestion and metabolism** starch → chemical energy cells can use 3 **Muscles** chemical energy → mechanical work, roughly 20-25% efficient Outputs **Useful work**a fraction of the food energy **Body heat**most of the food energy Source: Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008), via Smil Mastery Course, M1 'What energy is' Common misreading #### "Using up" energy People speak of consuming energy, and statisticians record energy consumption, but nothing is destroyed. When you drive to work, every joule of chemical energy in the petrol still exists afterwards, spread out as slightly warmer air, road and tyres. What has gone is its ability to do anything useful. Keeping this in mind prevents a common confusion later: efficiency is about how much of the input reaches the form you wanted before it degrades into waste heat, not about energy vanishing. - 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. In the data #### How old is a store? Most of the world's coal formed from swamp forests of the Carboniferous period, roughly 300 to 360 million years ago, although large deposits in India and China are younger. Oil and gas typically come from marine plankton buried tens to a couple of hundred million years ago. A barrel of oil therefore represents sunlight gathered over a very long time and a large area. That concentration is the reason fossil fuels are so cheap to use per unit of energy, and also why they cannot be replenished on any human timescale. - 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 [H₂O Water](https://www.materialinsights.org/commodities/water/)[Oil Crude oil](https://www.materialinsights.org/energy/crude_oil/)[Gas Natural gas](https://www.materialinsights.org/energy/natural_gas/)[U Uranium](https://www.materialinsights.org/energy/uranium/)[He Helium](https://www.materialinsights.org/commodities/helium/)[Ri Rice](https://www.materialinsights.org/commodities/rice/)[H2 Hydrogen](https://www.materialinsights.org/chemicals/hydrogen/)[O2 Oxygen](https://www.materialinsights.org/chemicals/oxygen/) ### Check your understanding A solar panel charges a battery that later runs a fan. Which sequence of forms is right? The panel turns light into electricity, the battery stores it chemically, discharging turns it back into electricity and the fan motor turns that into motion. Which of these is a flow rather than a store? Wind arrives continuously at a rate nature sets and must be captured as it comes. Ore, gas and coal are stores built up over geological time. Why do energy analysts say we convert energy rather than consume it? The quantity of energy is conserved. What use removes is its ability to do work, as it ends up as diffuse low-temperature heat. [← Module contents](https://www.materialinsights.org/learn/energy/energy-power-units/)[Mark read · Part 2: Units and orders of magnitude →](https://www.materialinsights.org/learn/energy/energy-power-units/p2/) ## Other sections - [Module overview](https://www.materialinsights.org/learn/energy/energy-power-units/index.md) - [Units and orders of magnitude](https://www.materialinsights.org/learn/energy/energy-power-units/p2/index.md) - [Energy versus power](https://www.materialinsights.org/learn/energy/energy-power-units/p3/index.md) - [Energy in matter: fuels and energy density](https://www.materialinsights.org/learn/energy/energy-power-units/p4/index.md) - [Electricity and the nucleus](https://www.materialinsights.org/learn/energy/energy-power-units/p5/index.md) - [Key terms, quiz and sources](https://www.materialinsights.org/learn/energy/energy-power-units/review/index.md)