Example:

The two state system.

The first example is one of the simplest one can think of$.$ In a two$-$state system, there are only

two states, one with energy $0$ and the other with energy $>0.$ What is the average energy of th

system?

Example:

Isothermal atmosphere:

Estimate the number of molecules in an isothermal atmosphere as a function of height.

Example:

The Sun:

The main fusion reaction in the Sun