## Relationship between uncertainties of position and momentum

$\Delta p \Delta x\geq \frac{h}{4\pi }$

Lindsey Chheng 1E
Posts: 110
Joined: Fri Aug 30, 2019 12:16 am

### Relationship between uncertainties of position and momentum

In his lecture, Dr. Lavelle said that if you know something's position really well, then the uncertainty in the position will go down and the uncertainty in the momentum will go up. I understand why the uncertainty in the position goes down, but can someone explain the other?

005206171
Posts: 107
Joined: Wed Sep 18, 2019 12:20 am

### Re: Relationship between uncertainties of position and momentum

Heisenberg's uncertainty principle is different from Bohr's (classical) model. Bohr's model assumes that electrons are orbitting an electron and that is not the case. Electrons are really moving in a "cloud". So if you wanna know its position, that means you have to stop the electron so you can record its position. But now that you've stopped the electron, it no longer has momentum. That's why when uncertainty in position goes down, uncertainty in momentum goes up.

I like to think of standing outside the car. If I take a picture of the car I know exactly where it is, but I know nothing about its speed and therefore momentum. If i watch the car, I know how fast its going/momentum but I can't really pin down where it's going (position).

Hope this helps!

Lindsey Chheng 1E
Posts: 110
Joined: Fri Aug 30, 2019 12:16 am

It did! Thanks!