Update 2_debye_model.md - added prior knowledge

src/2_debye_model.md

@@ -27,6 +27,15 @@ configure_plotting()
 
 _(based on chapter 2.2 of the book)_
 
+!!! success "Expected prior knowledge"
+
+    Before the start of this lecture, you should be able to:
+
+    - Describe how atoms are modeled in the Einstein model 
+    - Describe the low-temperature behaviour of the Einstein heat capacity per atom 
+    - Describe how the frequency of a sound wave depends on the wavenumber $k=2\pi/\lambda$ of the soundwave, where $\lambda$ is the wavelength.
+    - Formulate a volume integral in spherical coordinates
+    
 !!! summary "Learning goals"
 
     After this lecture you will be able to: