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Expectation value of X and Y component of angular momentum

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xinoki
#1
Dec5-10, 12:03 PM
P: 6
1. The problem statement, all variables and given/known data
Show:
<Jx>=<Jy>=0

2. Relevant equations

Jx=1/2(J++J-)
Jy=1/2(J+-J-)

3. The attempt at a solution

<jm l Jx l jm> = < jm l 1/2 J+ l jm> + < jm l j- l jm >
= < jm l h/2 sqrt [(j-m)(j+m+1)] + h/2sqrt[(j+m)(j+m+1) l jm >

i am not sure how to apply the next step
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Matterwave
#2
Dec5-10, 01:59 PM
Sci Advisor
Matterwave's Avatar
P: 2,822
Note that |j,m> is NOT an eigenstate of either J+ or J-. In fact, J+|j,m>=k|j,m+1>, etc.

Additionally note that [tex]<j,m|j,n>=\delta_{m,n}[/tex]


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