ABSTRACT
Physics is in its essence an experimental science, based on making quantitative predictions for how different systems behave under a set of given conditions and checking experimentally that these predictions are fulfilled. A central tool in physics modelling is the continuity equation, which is a relation among different intensive properties based on an extensive property that may flow from one system to another. People familiar with with ordinary differential equations will know that, in order to find a unique solution, it is necessary to specify a sufficient number of boundary conditions. Before delving deep into the business of modelling the world around us using differential equations, it will be beneficial to introduce a handful of different ways of denoting partial derivatives. A fundamental tool in the treatment of linear differential equations, partial as well as ordinary, is the superposition principle.
