# 2: The Classical Wave Equation

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- 11779

The aim of this section is to give a fairly brief review of waves in various shaped elastic media—beginning with a taut string, then going on to an elastic sheet, a drumhead, first of rectangular shape then circular, and finally considering elastic waves on a spherical surface, like a balloon. The reason we look at this material here is that these are “real waves”, hopefully not too difficult to think about, and yet mathematically they are the solutions of the same wave equation the Schrödinger wave function obeys in various contexts, so should be helpful in visualizing solutions to that equation, in particular for the hydrogen atom.

- 2.1: The One-Dimensional Wave Equation
- The mathematical description of the one-dimensional waves can be expressed as solutions to the "wave equation." It may not be surprising that not all possible waves will satisfy the wave equation for a specific system since waves solutions must satisfy both the initial conditions and the boundary conditions. This results in a subset of possible solutions. In the quantum world, this means that the boundary conditions are responsible somehow for the quantization phenomena in Chapter 1.

- 2.2: The Method of Separation of Variables
- Method of separation of variables is one of the most widely used techniques to solve partial differential equations and is based on the assumption that the solution of the equation is separable, that is, the final solution can be represented as a product of several functions, each of which is only dependent upon a single independent variable. If this assumption is incorrect, then clear violations of mathematical principles will be obvious from the analysis.

- 2.3: Oscillatory Solutions to Differential Equations
- Characterizing the spatial and temporal components of a wave requires solving homogeneous second order linear differential equations with constant coefficients. This results in oscillatory solutions (in space and time). These solutions solved via specific boundary conditions are standing waves.

- 2.4: The General Solution is a Superposition of Normal Modes
- Since the wave equation is a linear differential equations, the Principle of Superposition holds and the combination two solutions is also a solution.

- 2.5: A Vibrating Membrane
- It is pleasant to find that these waves in higher dimensions satisfy wave equations which are a very natural extension of the one we found for a string, and—very important—they also satisfy the Principle of Superposition, in other words, if waves meet, you just add the contribution from each wave. In the next two paragraphs, we go into more detail, but this Principle of Superposition is the crucial lesson.

- 2.E: The Classical Wave Equation (Exercises)
- These are homework exercises to accompany Chapter 2 of McQuarrie and Simon's "Physical Chemistry: A Molecular Approach" Textmap.