Update 1_einstein_model.md - added warm up exercises

src/1_einstein_model.md

@@ -340,6 +340,13 @@ ax.set_ylim((0, 3));
 
 ## Exercises
 
+### Quick warm-up exercises
+
+1. Sketch the Bose Einstein distribution as a function of $\omega$ for two different values of $T$
+2. Sketch the heat capacity of an Einstein solid for two different values of $T_E$
+3. What is the high-temperature heat capacity of an atom is a solid with two momentum and two spatial coordinate degrees of freedom
+4. Why is the heat capacity per atom of an ideal gas typically $3/2 k_B$ and not $3 k_B$?
+
 ### Exercise 1: Heat capacity of a classical oscillator.
 
 Let's refresh the connection of this topic to statistical physics.