Anton Akhmerov
--- a/src/4_sommerfeld_model.md
+++ b/src/4_sommerfeld_model.md
 #### Example 3: phonons in 3D at low temperatures.

 $g(E) \propto E^2$ ⇒ total energy is $T \times T^3$ ⇒ $C_V \propto T^3$.
+
+## 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.).
--- a/src/3_drude_model.md
+++ b/src/3_drude_model.md
 where $R_{\rm H}=-\frac{1}{ne}$ is the _Hall resistance_. So by measuring the Hall resistance, we can obtain $n$, the density of free electrons in a material.

 While most materials have $R_{\rm H}>0$, interestingly some materials are found to have $R_{\rm H}<0$. This would imply that the charge carriers either have a positive charge, or a negative mass. We will see later (chapter 17) how to interpret this.
+
+## Exercises
+
+### Motion of free electron due to electric field
+(a) Consider N free electrons in copper. Explain their motion under no electric field and while applying electric field in the direction from left to right side of the conductor.