- #1
Domnu
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Problem
Consider a free particle moving in one dimension. The state functions for this particle are all elements of [tex]L^2[/tex]. Show that the expectation of the momentum [tex]\langle p_x \rangle[/tex] vanishes in any state that is purely real. Does this property hold for [tex]\langle H \rangle[/tex]? Does it hold for [tex]\langle H \rangle[/tex]?
Solution
For [tex]\langle p_x \rangle [/tex], we have
[tex] \langle p_x \rangle = \int_{-\infty}^{\infty} \phi^* \hat{p_x} \phi dx[/tex]
[tex] = - i \hbar \int_{-\infty}^{\infty} (Ae^{ikx} + Be^{-ikx})(-Aik \cdot e^{-ikx} + Bik \cdot e^{ikx}) dx[/tex]
[tex] = -\hbar k \int_{-\infty}^{\infty} [A^2 - B^2][/tex],
since we need to have [tex] kx = n\pi[/tex] to satisfy the condition that the wavefunction must be real. But, the above integral diverges to infinity (assuming that [tex] A \neq B[/tex]).
I'll post the second and third parts a bit later, but have I correctly shown that the expectation of the momentum vanishes?
Consider a free particle moving in one dimension. The state functions for this particle are all elements of [tex]L^2[/tex]. Show that the expectation of the momentum [tex]\langle p_x \rangle[/tex] vanishes in any state that is purely real. Does this property hold for [tex]\langle H \rangle[/tex]? Does it hold for [tex]\langle H \rangle[/tex]?
Solution
For [tex]\langle p_x \rangle [/tex], we have
[tex] \langle p_x \rangle = \int_{-\infty}^{\infty} \phi^* \hat{p_x} \phi dx[/tex]
[tex] = - i \hbar \int_{-\infty}^{\infty} (Ae^{ikx} + Be^{-ikx})(-Aik \cdot e^{-ikx} + Bik \cdot e^{ikx}) dx[/tex]
[tex] = -\hbar k \int_{-\infty}^{\infty} [A^2 - B^2][/tex],
since we need to have [tex] kx = n\pi[/tex] to satisfy the condition that the wavefunction must be real. But, the above integral diverges to infinity (assuming that [tex] A \neq B[/tex]).
I'll post the second and third parts a bit later, but have I correctly shown that the expectation of the momentum vanishes?