# QM- A bit of manipulation of expectation values.

1. Apr 18, 2009

### trv

1. The problem statement, all variables and given/known data
The variance of an observable Qhat in a state with wavefunction psi is,

(delta Qhat)2=<(Qhat-<Qhat>)2>

Show that this can be written as,

(delta Qhat)2=<Qhat2>-<Qhat>2

2. Relevant equations

As above.

3. The attempt at a solution

(delta Qhat)2=<Qhat2-Qhat<Qhat>-<Qhat>Qhat+<Qhat>2>
L.H.S=<Qhat2-2Qhat<Qhat>+<Qhat>2>

2. Apr 18, 2009

### maverick_starstrider

Right, you've got it

$\langle \hat{Q}^2 -2\hat{Q}\langle \hat{Q} \rangle +\langle \hat{Q} \rangle \rangle$

Now recognize that $\langle X \rangle$ is just a number (the expectation value) and thus the expectation value of an expectation value is just the expectation value (that's a mouth full) i.e. $\langle \langle \hat{Q} \rangle \rangle = \langle \hat{Q} \rangle$ and so on. Therefore you get:

$\langle \hat{Q}^2 \rangle - 2 \langle \hat{Q} \rangle \langle \hat{Q} \rangle + \langle \hat{Q} \rangle ^2$

and I assume you can take it from there

3. Apr 18, 2009

### trv

Yep that helps. Thanks Maverick.