
#1
Mar2310, 04:18 AM

P: 8

Hello, this is just a general question, how is <x^2> evaluated, if
<x> = triple integral of psi*(r,t).x.psi(r,t).dr (this is the expectation value of position of wavepacket) Is it possible to square a triple integral? Is <x^2> the same as <x>^2 ? I'm only wondering how the squared works in this situation, I would understand how to use <x> if the square wasn't there. Thank you! 



#2
Mar2310, 09:37 AM

P: 676

You cannot square the integral. The way it is written is (in 1 dimension):
[tex]\left<x^2\right> = \int \psi(x)^{\dagger}x^2\psi(x) dx[/tex] It will be different in most cases from <x>^2. For example, [tex]\int x^2 dx = \tfrac13 x^3 \neq \left(\int x dx\right)^2 = \tfrac14 x^4[/tex] So you are unable to take the square outside of the integral. 



#3
Mar2310, 11:23 PM

P: 8

Got it, thank you very much!



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