---
title: 'Interpret the macroscopic by the microscopic.'
---

To build :
Talk about :
- the atomic hyposthesis and the Bohr atom.
- extremely large numbers to go from macroscopic to microscopic, and the number of Avogadro.
- which require probabilistic methods, birth of the static physics with Botzmann.
- the notion of entropy, a hyper useful addition to the notion of energy.
- statistical physics / thermodynamics
- Law of entropy: only physical equation in which the arrow of time appears.