# When calculating the momentum expectation value

1. Jan 4, 2014

### rwooduk

when calculating the momentum expectation value the term i(h-bar)d/dx goes inbetween the complex PSI and the 'normal' PSI, so do you differentiate the normal PSI and then multiply by the complex PSI? or do you differentiate the product of the two PSI's i.e. the modulus of PSI?

thanks for any direction on this.

2. Jan 4, 2014

### CompuChip

Yes. The expectation value of $\hat A$ is $\int \Psi^* \hat A \Psi \, dx$ where any differential operators in $\hat A$ works to the right.

3. Jan 4, 2014

### vanhees71

Another hint:
$$\hat{p}=-\mathrm{i} \frac{\mathrm{d}}{\mathrm{d} x}.$$
Note the sign!

4. Jan 4, 2014

### rwooduk

Brilliant! Many Thanks CC!

hm, not sure what you're getting at, the negative sign just comes to the front of the integral, im probably missing something. but thanks also!

5. Jan 4, 2014

### vanhees71

I just wanted to make sure that you get that sign right, because you got it wrong in your original posting!