# Uncertainty of position in an infinite potential well

1. Apr 26, 2015

### Ananthan9470

The ground state energy of a particle trapped in an infinite potential well of width a is given by (ħ2π2)/2ma2. So the momentum is given by (2mE)1/2 = ħπ/a. Since this is a precise value, doesn't that mean that we know momentum with 100% certainty? And if that is the case shouldn't the uncertainty in position be infinite?

2. Apr 26, 2015

### Simon Bridge

Your relation for the momentum is incorrect.
You can find the momentum in QM by applying the momentum operator - just like you find the energy by applying the energy operator. So what happes when you apply the momentum operator to the ground-state energy eigenfunction for a particle in a box?

Note: The momentum wavefunction is the fourier transform of the position wavefunction.

Last edited: Apr 26, 2015
3. Apr 26, 2015

### stevendaryl

Staff Emeritus
But if all you know is $E$, then the momentum is $\pm \sqrt{2mE}$. So the expectation value of $p$ is 0 (it's just as likely to be pointing in one direction as another). A rough estimate of the uncertainty in a quantity is the standard deviation, which would tell us:

$\delta p = \sqrt{\langle p^2\rangle - \langle p \rangle^2}$ where $\langle X \rangle$ means the expectation value, or average value. So for this problem:

$\langle p \rangle = 0$
$\langle p^2 \rangle = 2mE$

So
$\delta p = \sqrt{2mE} = \frac{\hbar \pi}{a}$

So the uncertainty of $p$ is not zero.

Last edited by a moderator: Apr 26, 2015
4. Apr 26, 2015