- #1
Domnu
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Problem
One thousand neutrons are in a one-dimensional box, with walls at x = 0, x = a. At t = 0, the state of each particle is
[tex]\psi(x, 0) = Ax(x-a)[/tex]
a) Normalize [tex]\psi[/tex] and find the value of the constant [tex]A[/tex].
b) How many particles are in the interval [tex](0, a/2)[/tex] at [tex]t=0[/tex]?
c)How many particles have energy [tex]E_5[/tex] at [tex]t=0[/tex]?
d)What is [tex]\langle E \rangle[/tex] at [tex]t=0[/tex]?
Solutions
a) This is quite straightforward... just set [tex]\int_0^a \psi^2 dx = 1[/tex] and solve for [tex]A[/tex], which yields [tex]A = \sqrt{30/a^5}[/tex].
b)Just find [tex]\int_{0}^{a/2} \psi(x, 0)[/tex], which is 0.5. Now, 0.5 * 1000 = 500 neutrons.
c) This is the part where I have problems... could someone help out? Exactly what eigenstates am I supposed to break [tex]\psi[/tex] up into? Is it a one-dimensional particle in a box scenario? If so, should I express the function [tex]\psi[/tex] in terms of
[tex]\sqrt{\frac{2}{a}} \sin \frac{n \pi x}{a}[/tex]
? I already tried to express [tex]\psi[/tex] in terms of the momentum eigenstates,
[tex]\phi_n = \frac{1}{\sqrt{2\pi}} e^{ikx}[/tex]
This didn't work out too well (without TI-89)... how are we supposed to find the number of particles in [tex]E_5[/tex] exactly? I know that the energy levels of the neutrons are discrete, but how are we supposed to model this without an integral? If it's an infinite summation, could someone tell me how to do this?
d) This part is pretty simple... we have to just do
[tex]\int_{0}^{a} \psi^* \hat{E}\psi dx = \int_{0}^{a} Ax(x-a) \cdot -\frac{\hbar^2}{2m} \frac{\partial^2 \psi}{\partial x^2} dx = \int_{0}^{a} Ax(x-a) \cdot 2A dx = -\frac{30}{a^5} \cdot \frac{\hbar^2}{2m} \cdot -\frac{a^3}{3} = \frac{5\hbar^2}{ma^2}[/tex]
I'm just having a bit of an issue with part c... could someone help out?
One thousand neutrons are in a one-dimensional box, with walls at x = 0, x = a. At t = 0, the state of each particle is
[tex]\psi(x, 0) = Ax(x-a)[/tex]
a) Normalize [tex]\psi[/tex] and find the value of the constant [tex]A[/tex].
b) How many particles are in the interval [tex](0, a/2)[/tex] at [tex]t=0[/tex]?
c)How many particles have energy [tex]E_5[/tex] at [tex]t=0[/tex]?
d)What is [tex]\langle E \rangle[/tex] at [tex]t=0[/tex]?
Solutions
a) This is quite straightforward... just set [tex]\int_0^a \psi^2 dx = 1[/tex] and solve for [tex]A[/tex], which yields [tex]A = \sqrt{30/a^5}[/tex].
b)Just find [tex]\int_{0}^{a/2} \psi(x, 0)[/tex], which is 0.5. Now, 0.5 * 1000 = 500 neutrons.
c) This is the part where I have problems... could someone help out? Exactly what eigenstates am I supposed to break [tex]\psi[/tex] up into? Is it a one-dimensional particle in a box scenario? If so, should I express the function [tex]\psi[/tex] in terms of
[tex]\sqrt{\frac{2}{a}} \sin \frac{n \pi x}{a}[/tex]
? I already tried to express [tex]\psi[/tex] in terms of the momentum eigenstates,
[tex]\phi_n = \frac{1}{\sqrt{2\pi}} e^{ikx}[/tex]
This didn't work out too well (without TI-89)... how are we supposed to find the number of particles in [tex]E_5[/tex] exactly? I know that the energy levels of the neutrons are discrete, but how are we supposed to model this without an integral? If it's an infinite summation, could someone tell me how to do this?
d) This part is pretty simple... we have to just do
[tex]\int_{0}^{a} \psi^* \hat{E}\psi dx = \int_{0}^{a} Ax(x-a) \cdot -\frac{\hbar^2}{2m} \frac{\partial^2 \psi}{\partial x^2} dx = \int_{0}^{a} Ax(x-a) \cdot 2A dx = -\frac{30}{a^5} \cdot \frac{\hbar^2}{2m} \cdot -\frac{a^3}{3} = \frac{5\hbar^2}{ma^2}[/tex]
I'm just having a bit of an issue with part c... could someone help out?