Exercises head is <h2> now (4_sommerfeld_model.md)

src/4_sommerfeld_model.md

@@ -208,7 +208,7 @@ $g(E) \propto E$ ⇒ total energy is $T \times T^2$ ⇒ $C_V \propto T^2$.
 
 $g(E) \propto E^2$ ⇒ total energy is $T \times T^3$ ⇒ $C_V \propto T^3$.
 
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+## Exercises
 #### Exercise 1: potassium
 The Sommerfeld model provides a good description of free electrons in alkali metals such as potassium, which has a Fermi energy of 2.12 eV (data from Ashcroft, N. W. and Mermin, N. D., Solid State Physics, Saunders, 1976.).