# Ehrenfest's Theorem

yukcream
Can anyone tell me what is the Ehrenfest's Theorm in quantum mechanic, I don't understand how it can provide an example for correspondence principle?

Homework Helper
Are you familiar with the equation of motion:

$$\frac{d}{dt}\langle Q \rangle = \frac{i}{\hbar}\langle [H,Q] \rangle + \langle \frac{\partial Q}{\partial t}\rangle$$

where Q is an observable?
Try putting Q=p (momentum) and Q=r (position).

DaTario
Now I am a bit confused. What's the name of that theorem which forbids static electric and magnetic charges to produce stable equillibrium condition in 3D ?

Homework Helper
Gold Member
DaTario said:
Now I am a bit confused. What's the name of that theorem which forbids static electric and magnetic charges to produce stable equillibrium condition in 3D ?
Are you looking for Earnshaw's Theorem?

DaTario
hehehehe...thank you a lot... I am getting old, my son...

yukcream
Galileo said:
Are you familiar with the equation of motion:

$$\frac{d}{dt}\langle Q \rangle = \frac{i}{\hbar}\langle [H,Q] \rangle + \langle \frac{\partial Q}{\partial t}\rangle$$

where Q is an observable?
Try putting Q=p (momentum) and Q=r (position).

It is just the result of the theorem! Any mathematical divide inside?

Staff Emeritus