- #1
phosgene
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Homework Statement
Consider a particle in an infinite square well described initially by a wave that is a superposition of the ground and first excited states of the well
[itex]ψ(x,0) = C[ψ_{1}(x) + ψ_{2}(x)][/itex]
Show that the superposition is not a stationary state, but that the average energy in this state is the arithmetic mean [itex](E_{1} + E_{2})/2[/itex] of the ground and first excited state energies [itex]E_{1}[/itex] and [itex]E_{2}[/itex].
Homework Equations
Time-independant Schroedinger equation (potential set to zero):
[itex]Eψ(x) = -\frac{\hbar^{2}∂^{2}}{2m∂x^{2}}[/itex]
The Attempt at a Solution
Assuming that the well is from x=0 to x=L, my wavefunction is [itex]ψ(x) = \sqrt{\frac{1}{L}}(sin(\frac{\pi x}{L}) + sin(\frac{2\pi x}{L}))[/itex]
Then using the Schroedinger equation
[itex]-\frac{\hbar^{2}∂^{2}}{2m∂x^{2}}\sqrt{\frac{1}{L}}(sin(\frac{\pi x}{L}) + sin(\frac{2\pi x}{L}))=Eψ(x)[/itex]
[itex]=\frac{\hbar^{2}}{2m}\sqrt{\frac{1}{L}}\frac{\pi ^{2}}{L^{2}}(sin(\frac{\pi x}{L}) + 4sin(\frac{2\pi x}{L}))[/itex]
But now I'm stuck because of the 4 in front of [itex]4sin(\frac{2\pi x}{L})[/itex]