Merge branch 'master' into local

src/2_debye_model.md

@@ -32,9 +32,9 @@ _(based on chapter 2.2 of the book)_
     Before the start of this lecture, you should be able to:
 
     - Recall how atoms are modeled in the Einstein model
-    - Describe a procedure for calculating the heat capacity of a solid
+    - Derive the heat capacity of a solid in the Einstein model 
     - Describe how the frequency of a sound wave depends on the wavenumber $k=2\pi/\lambda$, where $\lambda$ is the wavelength.
-    - Formulate volume integrals using spherical coordinates
+    - Express a volume integral in spherical coordinates
     
 !!! summary "Learning goals"