# The laws of energy: conservation, entropy and efficiency Two laws govern every energy system. The first says energy is never created or destroyed; the second says that every conversion degrades its quality until it ends as low-grade heat. Together they set the limits on efficiency, explain why power stations waste most of their fuel, why heat pumps seem to beat 100% and why statisticians argue about how to count primary energy. Source page: https://www.materialinsights.org/learn/energy/laws-of-energy/ [Understanding Energy](https://www.materialinsights.org/learn/energy/) / Module 2 ## The laws of energy: conservation, entropy and efficiency Two laws govern every energy system. The first says energy is never created or destroyed; the second says that every conversion degrades its quality until it ends as low-grade heat. Together they set the limits on efficiency, explain why power stations waste most of their fuel, why heat pumps seem to beat 100% and why statisticians argue about how to count primary energy. 5 parts · 15 sections · about 45 minutes · 0 of 5 parts read ### By the end you will be able to - State the first and second laws of thermodynamics in plain terms and apply them to real machines - Calculate the efficiency of a conversion chain and explain how losses compound - Use the Carnot formula to explain why power stations, engines and geothermal plants have the efficiencies they do - Describe how heat moves by conduction, convection and radiation, and what that means for buildings and cooling - Explain exergy and why a joule of electricity is worth more than a joule of warm water - Distinguish primary, final and useful energy, and explain how the substitution and physical energy content methods count electricity from wind, solar and nuclear ### Contents 1. [Part 1 The first law: energy is conserved](https://www.materialinsights.org/learn/energy/laws-of-energy/p1/) The first law of thermodynamics is the strictest rule in energy accounting: in a closed system the total amount of energy never changes. It cannot be made from nothing and it cannot vanish. What does change is how much of it ends up doing what you wanted. That share is efficiency, and it varies from a few per cent to almost a hundred. - Energy is never created or destroyed, only converted - Efficiency is useful output divided by input, and it spans a huge range - In a chain, losses multiply, and cheaper useful energy invites more use 2. [Part 2 The second law: entropy and the quality of energy](https://www.materialinsights.org/learn/energy/laws-of-energy/p2/) The first law says the quantity of energy is fixed. The second law says its quality only goes one way: down. Heat flows from hot to cold, concentrated energy spreads out, and no conversion can be reversed for free. That single direction explains why every machine ends by warming its surroundings, and why living things and cities need a constant inflow of high-quality energy. - Heat flows downhill, and disorder only grows - Every conversion ends as low-grade heat - Life and cities are dissipative structures 3. [Part 3 The limits to efficiency: Carnot and beyond](https://www.materialinsights.org/learn/energy/laws-of-energy/p3/) If every heat engine must reject some heat, how much must it reject? In 1824 the French engineer Sadi Carnot answered with a formula that depends only on two temperatures. It explains why power stations rarely beat 45%, why combined-cycle gas plants reach 60%, why geothermal and ocean thermal plants are so inefficient, and why heat pumps can deliver more heat than the electricity they use. - The Carnot limit caps every heat engine - Why engineers chase ever higher temperatures - Heat pumps run the engine backwards 4. [Part 4 Heat and how it moves](https://www.materialinsights.org/learn/energy/laws-of-energy/p4/) Most of the energy used in homes, offices and factories is used to make heat, keep it in or get rid of it. Heat moves by conduction, convection and radiation, and materials differ greatly in how much they store and how well they block its flow. Those few principles explain insulation, glazing, cooling and why water is so central to every energy system. - Conduction, convection and radiation - Water stores heat well and absorbs huge amounts when it evaporates - Buildings leak heat through their weakest parts 5. [Part 5 Exergy and the three stages of energy accounting](https://www.materialinsights.org/learn/energy/laws-of-energy/p5/) The second law implies that a joule is not always a joule. Exergy puts a number on that: the share of energy that can still do work. It changes how efficiency looks, and it underlies a running argument among statisticians about how to count primary energy from wind, solar and nuclear. That argument is why the world's 2024 energy total is quoted as 592 EJ rather than the roughly 635 EJ the older convention gives. - Exergy measures how much work energy can still do - Primary, final and useful energy are three different totals - How statisticians count electricity from wind, sun and atoms 6. [Wrap-up Key terms, try it in the table, final quiz, sources](https://www.materialinsights.org/learn/energy/laws-of-energy/review/) [Start: Part 1, The first law: energy is conserved →](https://www.materialinsights.org/learn/energy/laws-of-energy/p1/) ## Other sections - [The first law: energy is conserved](https://www.materialinsights.org/learn/energy/laws-of-energy/p1/index.md) - [The second law: entropy and the quality of energy](https://www.materialinsights.org/learn/energy/laws-of-energy/p2/index.md) - [The limits to efficiency: Carnot and beyond](https://www.materialinsights.org/learn/energy/laws-of-energy/p3/index.md) - [Heat and how it moves](https://www.materialinsights.org/learn/energy/laws-of-energy/p4/index.md) - [Exergy and the three stages of energy accounting](https://www.materialinsights.org/learn/energy/laws-of-energy/p5/index.md) - [Key terms, quiz and sources](https://www.materialinsights.org/learn/energy/laws-of-energy/review/index.md)