- #1
fluidistic
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Homework Statement
In a QM problem I must calculate the expectation value of the kinetic energy, namely ##\langle \Psi _c ,\frac{ \hat { \vec p } ^2}{2m} \Psi _c \rangle##. Where ##\Psi _c=ce^{-\alpha x^2}##.
Homework Equations
##\int _{-\infty}^\infty \exp \{ -a x^2 +bx \} dx = \sqrt{\frac{\pi}{a}}\exp \{ -\frac{b^2}{4a} \}##.
The Attempt at a Solution
##\langle \Psi _c ,\frac{ \hat { \vec p } ^2}{2m} \Psi _c \rangle = \int _{-\infty}^\infty c e^{-\alpha x^2} \frac{\hbar ^2}{2m} \frac{d^2}{dx^2}(ce^{-\alpha x^2})dx####=...=-\frac{\hbar ^2}{m}\alpha c^2 \underbrace{\int _{-\infty}^\infty e^{-2\alpha x^2}dx}_{=I}+\frac{2\hbar ^2}{m} \alpha c^2 \underbrace{\int _{\infty}^\infty x^2 e^{-2\alpha x^2}dx}_{=J}##.
After some algebra, I reached that ##I=\sqrt{\frac{\pi}{2\alpha}}## while ##J=\frac{\sqrt \pi}{2^{5/2}\alpha ^{3/2}}##.
From which I reached that ##\langle \Psi _c ,\frac{ \hat { \vec p } ^2}{2m} \Psi _c \rangle = \frac{\hbar ^2 c^2 \pi ^{1/2}}{2^{3/2}m} \left ( \frac{1}{\alpha ^{1/2}} -\alpha ^{1/2} \right )##.
But I'm having a hard time to swallow the fact that this number is negative if alpha is greater than 1. What do you think? I made some mistake somewhere?! Right?