# Homework Help: Angular Momentum Eigenfunctions for Bead on a Wire

1. Dec 21, 2011

### thelonious

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

A bead of mass m on a circular ring has the wave function Acos$\stackrel{2}{}$θ.
Find expectation value, eigenfunctions & eigenvalues.

2. Relevant equations

The differential operator for the angular momentum is L = $\hbar$/i ($\partial$/$\partial$θ).

3. The attempt at a solution

I have found that the expectation value is zero, and now I would like to solve for eigenvalues and eigenfunctions. I understand that the eigenfunctions must have the form Ce-iθ/$\hbar$ ... but I don't know where to go from here. How can I begin to solve for these eigenfunctions?

Last edited: Dec 21, 2011
2. Dec 21, 2011

### vela

Staff Emeritus
How did you come up with that form for the eigenfunction? It's not quite right.

3. Dec 21, 2011

### thelonious

It is my understanding that since the eigenfunction will have the form L|λ> = α|λ> (where |λ> denotes the eigenvector and α its eigenvalue), and the operator L contains a partial with respect to θ, the eigenfunction must include e. Is this incorrect? I do not know how to solve for the exact eigenfunctions, but I expect that they will have this form.

4. Dec 21, 2011

### vela

Staff Emeritus
It's close, but the function should depend on the eigenvalue a.

You want to solve the differential equation
$$\frac{\hbar}{i}\frac{\partial \psi}{\partial \theta} = a\psi$$to find the eigenfunctions. The boundary conditions will then tell you what the eigenvalues are.

5. Jan 5, 2012

### thelonious

For the eigenfunctions, I have found:

L$\Psi$=2A$\hbar$icos$\phi$sin$\phi$

The boundary conditions:

$\Psi$($\phi$,0) = $\Psi$($\phi+2\pi$,0)

Can I find the eigenvalues with these equations?

6. Jan 5, 2012

### vela

Staff Emeritus
Forget about the given wave function for right now. You want to solve the differential equation and apply the boundary conditions to find the eigenvalues and eigenfunctions.

Once you have those, then you want to write the given wave function in terms of those eigenfunctions.