---
tags:
  - science/physics
  - laws
  - thermodynamics
  - entropy
created: 2026-05-16
breadcrumb: "Physics > Laws of physics > Laws of thermodynamics"
---

# Laws of thermodynamics

> **Mechanism:** Energy and entropy in macroscopic systems

## Items

- **Zeroth law** — If A and B are each in thermal equilibrium with C, then A and B are in equilibrium. Defines temperature.
- **First law** — Energy is conserved: ΔU = Q − W. Heat in minus work out equals change in internal energy.
- **Second law** — Entropy of an isolated system never decreases. Defines an arrow of time.
- **Third law** — Entropy of a perfect crystal at absolute zero is zero. Absolute zero is unreachable in a finite number of steps.

## Related

- [[conservation-laws]]
- [[famous-physics-equations]]