# Homework Help: Quantum mechanics problems

1. Feb 8, 2014

### pierce15

1. The problem statement, all variables and given/known data

http://www-thphys.physics.ox.ac.uk/people/JamesBinney/qb.pdf[/PLAIN] [Broken]

page 42 in pdf (34 in the book)

problems 2.3, 2.5, 2.8, 2.9 (there are more but I'll start with these)

2. Relevant equations

I'll just include these for particular problems

3. The attempt at a solution

2.3

a. $\langle \psi | Q | \psi \rangle$ is the expected value of Q in the quantum state $| \psi \rangle$; $| \langle q_n | \psi \rangle | ^2$ - probability of $q_n$ occurring in state $| \psi \rangle$

b. the first operator is the identity operator, the second is the "observable" operator (bad phrasing?)

c. $u_n (x) = \langle x | q_n \rangle$, so $\langle q_n | \psi \rangle = u_n^*(\psi)$; I'm not really sure where to go from there

2.5

a. $\langle x \rangle = \langle \psi | \hat{x} | \psi \rangle$

b. $\langle x^2 \rangle = \langle \psi | \hat{x}^2 | \psi \rangle$

c. $\langle p_x \rangle = \langle \psi | \hat{p} | \psi \rangle$

I won't bother writing down the last one because I'm pretty sure those are wrong anyway

2.9

Ehrenfest's theorem:

$$i \hbar \frac{d}{dt} \langle \psi | Q | \psi \rangle = \langle \psi | [ Q, H ] | \psi \rangle + i \hbar \langle \psi | \frac{dQ}{dt} | \psi \rangle$$

I'm not sure where to go from there.

Any help would be greatly appreciated.

Last edited by a moderator: May 6, 2017
2. Feb 8, 2014

### Staff: Mentor

I would suggest devoting one thread per problem rather than trying to discuss all of them at once.

You also need to show more work. We can't do your homework here but we can't help get you out of a bind so do a little more research and show more work.