# Why do we need the position operator?

• I
As I understand it, |Ψ|2 gives us the probability density of the wavefunction, Ψ. And when we integrate it, we get the probability of finding the particle at whichever location we desire, as set by the limits of the integration. But when we use the position operator, we have integrand Ψ*xΨ dx, which gives us the probability of finding the particle at a given place.

It seems to me as though, both of the above operations are doing the exact same thing. Then why do we have a position operator?

## Answers and Replies

BvU
Science Advisor
Homework Helper
which gives us the probability of finding the particle at a given place
No. It gives an expectation value.

One is a probability density, the other is an expectation value. Different things ! Just like a Gauss distribution and the corresponding average are different things.

• Phys12
DarMM
Science Advisor
Gold Member
The latter integrand will give you the mean position.

• dextercioby
No. It gives an expectation value.

One is a probability density, the other is an expectation value. Different things ! Just like a Gauss distribution and the corresponding average are different things.
Oh...I see, because it's the *expectation value*, got it. Thank you!