QM · Ch 8 · WKB bound states of V(x)=α|x|

Physics · Quantum Mechanics · Griffiths Ch 8

Problem

Use the WKB / Bohr–Sommerfeld quantization condition to estimate the bound-state energies of a particle of mass mm in the potential V(x)=α∣x∣V(x) = \alpha|x|, with α>0\alpha>0.
Intuitive explanation

The Bohr–Sommerfeld condition quantizes closed orbits by requiring the action integral ∮p dx\oint p \, dx around a complete cycle to equal integer multiples of hh. For this V-shaped potential, the particle oscillates symmetrically between two classical turning points, so you'll integrate the momentum p(x)=2m(E−α∣x∣)p(x) = \sqrt{2m(E - \alpha|x|)} across one full bounce and set it equal to (n+12)h(n + \tfrac{1}{2})h (the half-integer accounts for the phase at turning points). This turns a transcendental integral into an algebraic equation for EnE_n.

Intuitive explanation

The Bohr–Sommerfeld condition quantizes closed orbits by requiring the action integral ∮p dx\oint p \, dx around a complete cycle to equal integer multiples of hh. For this V-shaped potential, the particle oscillates symmetrically between two classical turning points, so you'll integrate the momentum p(x)=2m(E−α∣x∣)p(x) = \sqrt{2m(E - \alpha|x|)} across one full bounce and set it equal to (n+12)h(n + \tfrac{1}{2})h (the half-integer accounts for the phase at turning points). This turns a transcendental integral into an algebraic equation for EnE_n.

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