HW4_solutions_fa11

HW4_solutions_fa11 - Physics 3220 Homework 4 Solutions 1...

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Physics 3220 Homework 4 Solutions Physics 3220 HW4_solutions.1 Fall 2011 1. Visualizing a possible harmonic oscillator ground state. (5 points) In PHYS 2170 and occasionally in class, we write the Schrödinger equation to emphasize the curvature of the wave functions. You can use the curvature to argue what the shape of the wave function might be like. For example, here’s the Schrödinger Equation for the Simple Harmonic Oscillator (SHO), written to emphasize the curvature of the wave function: 2 22 0 21 2 dm E m x dx       Let’s see if we can use the curvature idea to get some idea of what the ground state wave function might look like. We emphasized three special regions: Assume that the wave function is positive valued. First consider the regions beyond the classical turning points i.e., places where the energy is BELOW the classical potential. Use what the Schrödinger Equation says about the curvature (what DOES it say?) AND the fact that we must be able to normalize the wave function (in other words how must the wave function behave as you go to infinite distance?) to explain roughly what the wave function might look like in these regions. You should draw something that is positive valued, and concave up, but that also decays toward zero as you get farther from the well. These areas show the wave function decaying to zero, so it can be successfully integrated to infinity to get a normalized result. Next, (still assume a positive wave function) consider the points right at the classical turning points. What does the Schrödinger Equation say about the curvature there? Therefore, what shape must the function have right at the turning points? The curvature at these turning points is zero. Therefore, the wave function must be locally linear, or as is the case here, have an inflection point. Third, consider the position in the middle of the well i.e., at x=0. Again, for a positive wave funtion, what does the Schrödinger Equation say about the curvature? You’d have a wave function that is positive, and with downward curvature. Now sketch a smooth symmetrical function that successfully pieces together all these regions. What’s it look like to you?

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Physics 3220 Homework 4 Solutions Physics 3220 HW4_solutions.2 Fall 2011 You should have pieced these things together to produce a symmetrical function that looks very much like a Gaussian. Ta da! 2. Guessing about a possible harmonic oscillator wave function. (25 points) In class, we used the Schrödinger equation for the SHO to argue what the shape of the wave function might be like. Here’s the Schrödinger Equation, written to emphasize the curvature of the wave function: 2 22 0 21 2 dm E m x dx       We emphasized three special regions.
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This note was uploaded on 03/03/2012 for the course PHYS 3220 at Colorado.

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HW4_solutions_fa11 - Physics 3220 Homework 4 Solutions 1...

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