added prior knowledge

docs/3_drude_model.md

@@ -18,6 +18,7 @@ _(based on chapter 3 of the book)_
     - Write down the equation describing the Lorentz force
     - Formulate Newton's equation of motion for a particle subject to a force, including a damping term
     - Discuss the concepts of voltage, current, resistivity, and conductivity
+    - Describe how the number of phonons scales with temperature $T$ (to explain why resistivity depends on $T$)
 
 !!! summary "Learning goals"