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American Journal of Physics

The particle in a box is not simple

Frank E. Cummings

Published Feb 1, 1977• 32 citations
Publisher

Abstract

Naive use of the position representation wave functions for a particle in either a finite or infinite square well predicts properties of the momentum distribution in disagreement with those calculated in the momentum representation. The discrepancy is removed by explicit consideration of the effects of the discontinuity in the potential at the walls on position space calculations.

DOI
10.1119/1.10981

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Quantum Mechanics and Pressure Balance in the Presence of Flux and Momentum Discontinuity

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One may consider constant fluxes which exist for all x points in space. These are associated with a constant velocity and a momentum. Consider next a discontinuity which does not change energy. For example, one may have energy E, momentum p and a constant potential V1 which suddenly changes to V2 at x=xo with E remaining constant. This leads to a momentum discontinuity as well as a flux discontinuity, but no work is done. As a result, there exists a pressure balance (related to an incident flux, a reflected one and the new p2 flux for x>xo), i.e. there is no discontinuity in pressure. A single constant flux also has no discontinuity in pressure either. We suggest there should exist a continuous function of momentum and x which allows for/ensures this pressure continuity. The momentum discontinuity may be at any xo, but the pressure balance is the same.As a result, one would expect this function and its first derivative to be continuous at xo, but a combination of the two should eliminate the function (of p and xo) leaving only a pressure balance equation. In other words, two equations need to combine to create a single classical pressure balance equation, suggesting that there may exist a square root flux type probability function. We suggest that pressure balance at momentum discontinuity points (with E remaining constant) may be the only consideration necessary to ultimately obtain the quantum free particle exp(ipx). An advantage of this pressure-only viewpoint is that it accounts for weighted averages of exp(ipx) having the same form, namely exp(ipave(rms) x) and exp(-ipave(rms) x) in a potential with infinite walls. Pressure considerations may be applied to a single particle exp(ipx) or an ensemble, yielding the same form if the same conditions apply. We argue that both a square well potential and infinite well potential are examples of momentum discontinuities, except in this case one has discontinuities in ensemble values (i.e. average values).

•Aug 19, 2023•0 citations
Naive use of the position representation wave functions for a particle in either a finite or infinite square well predicts properties of the momentum distribution in disagreement with those calculated in the momentum representation. The discrepancy is removed by explicit consideration of the effects of the discontinuity in the potential at the walls on position space calculations.
In this paper we have solved analytically the Schrodinger equation for a microscopic particle in a one-dimensional box with two infinite walls, which the potential function inside it, has a linear form. Based on the solutions of this special quantum mechanical system, we have shown that as the quantum number approaches infinity the expectation values of microscopic particle position and square of the linear momentum are equal to the classical time average of particle position and its square of the linear momentum, respectively.
One may consider constant fluxes which exist for all x points in space. These are associated with a constant velocity and a momentum. Consider next a discontinuity which does not change energy. For example, one may have energy E, momentum p and a constant potential V1 which suddenly changes to V2 at x=xo with E remaining constant. This leads to a momentum discontinuity as well as a flux discontinuity, but no work is done. As a result, there exists a pressure balance (related to an incident flux, a reflected one and the new p2 flux for x>xo), i.e. there is no discontinuity in pressure. A single constant flux also has no discontinuity in pressure either. We suggest there should exist a continuous function of momentum and x which allows for/ensures this pressure continuity. The momentum discontinuity may be at any xo, but the pressure balance is the same.As a result, one would expect this function and its first derivative to be continuous at xo, but a combination of the two should eliminate the function (of p and xo) leaving only a pressure balance equation. In other words, two equations need to combine to create a single classical pressure balance equation, suggesting that there may exist a square root flux type probability function. We suggest that pressure balance at momentum discontinuity points (with E remaining constant) may be the only consideration necessary to ultimately obtain the quantum free particle exp(ipx). An advantage of this pressure-only viewpoint is that it accounts for weighted averages of exp(ipx) having the same form, namely exp(ipave(rms) x) and exp(-ipave(rms) x) in a potential with infinite walls. Pressure considerations may be applied to a single particle exp(ipx) or an ensemble, yielding the same form if the same conditions apply. We argue that both a square well potential and infinite well potential are examples of momentum discontinuities, except in this case one has discontinuities in ensemble values (i.e. average values).