Quantum Physics For Dummies, Revised Edition
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In quantum physics, when working with a box potential, you can make things simpler by assuming that the box is actually a cube. In other words, L = Lx = Ly = Lz. When the box is a cube, the equation for the energy becomes

image0.png

So, for example, the energy of the ground state, where nx = ny = nz = 1, is given by the following, where E111 is the ground state:

image1.png

Note that there's some degeneracy in the energies; for example, note that

image2.png

So E211 = E121 = E112, which means that the first excited state is threefold degenerate, matching the threefold equivalence in dimensions.

In general, when you have symmetry built into the physical layout (as you do when L = Lx = Ly = Lz), you have degeneracy.

The wave function for a cubic potential is also easier to manage than the wave function for a general box potential (where the sides aren't of the same length). Here's the wave function for a cubic potential:

image3.png

So, for example, here's the wave function for the ground state

image4.png

About This Article

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Steven Holzner is an award-winning author of technical and science books (like Physics For Dummies and Differential Equations For Dummies). He graduated from MIT and did his PhD in physics at Cornell University, where he was on the teaching faculty for 10 years. He’s also been on the faculty of MIT. Steve also teaches corporate groups around the country.

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