# Homework Help: Infinite Square Well

1. Oct 16, 2014

### jaydnul

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

The wording of the question is throwing me off. It is a standard inf. pot. well problem and we are given the initial position of the particle to be in the left fourth of the box,

$\Psi(x,0)=\sqrt{\frac{4}{a}}$

We are asked to a) write the expansion of the wave function in terms of energy eigenfunctions, b) explicitly compute the expansion coefficients, and c) give an expression for psi at later times.

2. Relevant equations

$E_n=\frac{n^2\pi^2\hbar^2}{2ma^2}$

3. The attempt at a solution

I got b) and c) (I can show my work if necessary)

b) $c_n=\frac{4\sqrt{2}}{n\pi}$ for n= odd
$c_n=0$ for n= even

c) some long expression that I don't want to latex f I don't have to, but will if needed (on my phone :) )

But for a), I would think E_n would be the same as any inf. pot. well problem, wouldn't it? So
$E_n=\frac{n^2\pi^2\hbar^2}{2ma^2}$. Is this right?

2. Oct 16, 2014

### jaydnul

Like usual, writing it out on PF helped me solve the problem. At t=0, $k_n=\frac{4n\pi}{a}$ which allows the calculation of energy.

Thanks anyways

3. Oct 16, 2014

### tacosforlife

What did you get for part a) ?? I am stuck and I am confused what the first step should be. Thanks.

4. Oct 17, 2014

### Simon Bridge

Welcome to PF;
You will need to work out what $\psi$ (the initial position wavefunction) is from the description - show us what you got along with your reasoning.

For part (a) start with: $$\psi = \sum_n c_n\psi_n : \hat H\psi_n=E_n\psi_n$$ ... you should have notes for what each $\psi_n$ will be so you can look them up.

Last edited: Oct 17, 2014