CoursesCourse 5
Energy, entropy, and order
Energy is never created or destroyed, yet the world runs down: heat flows from hot to cold, and order decays into disorder. This course explains the two great laws of heat, the deep symmetry behind the first, and how living things and organizations build order against the second.
Start lesson 1Lessons
- 1 Energy is never lostA cannon that heated up, a paddle wheel that warmed water, and a guinea pig in a box of ice showed that heat is energy, and that energy is never created or destroyed.
- 2 Why energy is conservedBehind conservation lies symmetry, and behind both lies a principle of economy: nature takes the path of least action.
- 3 Why heat flows downhillHeat flows from hot to cold and never back on its own. Entropy explains why, from steam engines to shuffled coins.
- 4 Making orderIf disorder always grows, how does anything organized exist? Living things, the earth, and even companies stay ordered by staying open.
You’ll be able to
- Explain how experiments with cannon, paddle wheels, and living animals showed that heat is a form of energy, and state the law of conservation of energy.
- Connect conservation laws to symmetry through Noether’s theorem, and explain the principle of least action with the lifeguard’s problem.
- Define entropy in two ways, as energy that can no longer do work and as a count of microscopic arrangements, and explain why it increases.
- Describe how open systems, from living cells to the earth, build order by exporting entropy, and judge when entropy talk outside physics is a useful metaphor.
About this course
This course, 能量守恒与熵增 (Conservation of Energy and Increasing Entropy), takes up the third pillar of classical physics, the science of heat. It continues from Course 4, Newton’s clockwork universe.
Alongside the main argument and examples, we add background for readers outside China, the views of other scientists where they differ, and hands-on activities. We say clearly which ideas are settled physics and which are interpretation or metaphor.