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Simple Nature - Light and Matter

Simple Nature - Light and Matter

Simple Nature - Light and Matter

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f / The energy levels of a particlein a box, contrasted with those ofthe hydrogen atom.Approximate treatmentRather than leaping straight into a full mathematical treatment,we’ll start by looking for some physical insight, which will lead toan approximate argument that correctly reproduces the form of theBohr equation.A typical st<strong>and</strong>ing-wave pattern for the electron consists of acentral oscillating area surrounded by a region in which the wavefunctiontails off. As discussed in subsection 13.3.6, the oscillatingtype of pattern is typically encountered in the classically allowedregion, while the tailing off occurs in the classically forbidden regionwhere the electron has insufficient kinetic energy to penetrateaccording to classical physics. We use the symbol r for the radiusof the spherical boundary between the classically allowed <strong>and</strong> classicallyforbidden regions. Classically, r would be the distance fromthe proton at which the electron would have to stop, turn around,<strong>and</strong> head back in.If r had the same value for every st<strong>and</strong>ing-wave pattern, thenwe’d essentially be solving the particle-in-a-box problem in threedimensions, with the box being a spherical cavity. Consider theenergy levels of the particle in a box compared to those of the hydrogenatom, f. They’re qualitatively different. The energy levels ofthe particle in a box get farther <strong>and</strong> farther apart as we go higherin energy, <strong>and</strong> this feature doesn’t even depend on the details ofwhether the box is two-dimensional or three-dimensional, or its exactshape. The reason for the spreading is that the box is taken tobe completely impenetrable, so its size, r, is fixed. A wave patternwith n humps has a wavelength proportional to r/n, <strong>and</strong> therefore amomentum proportional to n, <strong>and</strong> an energy proportional to n 2 . Inthe hydrogen atom, however, the force keeping the electron boundisn’t an infinite force encountered when it bounces off of a wall, it’sthe attractive electrical force from the nucleus. If we put more energyinto the electron, it’s like throwing a ball upward with a higherenergy — it will get farther out before coming back down. Thismeans that in the hydrogen atom, we expect r to increase as we goto states of higher energy. This tends to keep the wavelengths ofthe high energy states from getting too short, reducing their kineticenergy. The closer <strong>and</strong> closer crowding of the energy levels in hydrogenalso makes sense because we know that there is a certain energythat would be enough to make the electron escape completely, <strong>and</strong>therefore the sequence of bound states cannot extend above thatenergy.When the electron is at the maximum classically allowed distancer from the proton, it has zero kinetic energy. Thus when the electronis at distance r, its energy is purely electrical:[1] E = − ke2r884 Chapter 13 Quantum Physics

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