Operators and order

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  • #1
Lee
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In my question I have to find what the commutation of a electrons kinetic and potentials energys are, in 3 Dimensions. I have started by finding the kinetic operator T and the potential energy from coloumbs law. I have then applied commutation brackets and I'm at the stage where I'm solving the commutation bracket for the x-direction. (and then apply symmetry for my 2 other axis) My question is, as we have to retain order when dealing with operators, how do I 'deal' with my

[tex]
\newcommand{\pd}[3]{ \frac{ \partial^{#3}{#1} }{ \partial {#2}^{#3} } }

xi \hbar \pd {} {x} {}
[/tex]

I presume I can't just differentate the x as I need to preserver order, does this just sit like this till I can 'deal' with it?
 

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  • #2
nrqed
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In my question I have to find what the commutation of a electrons kinetic and potentials energys are, in 3 Dimensions. I have started by finding the kinetic operator T and the potential energy from coloumbs law. I have then applied commutation brackets and I'm at the stage where I'm solving the commutation bracket for the x-direction. (and then apply symmetry for my 2 other axis) My question is, as we have to retain order when dealing with operators, how do I 'deal' with my

[tex]
\newcommand{\pd}[3]{ \frac{ \partial^{#3}{#1} }{ \partial {#2}^{#3} } }

xi \hbar \pd {} {x} {}
[/tex]

I presume I can't just differentate the x as I need to preserver order, does this just sit like this till I can 'deal' with it?

In calculating commutators of differntial operators, it is convenient to apply th commutator on a "test function", which is is just som arbitrary function of x, y and z that must be removed at the very end of the calculation.

So if you have two operators A and B (which are differential operators) and you want to compute their commutator, just consider
[tex] [A,B] f(x,y,z) = AB f(x,y,z) - BA f(x,y,z) [/tex]
Apply all the derivatives and at the very end, remove the test function.
 
  • #3
Hurkyl
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You can apply a commutation relation if you wanted to reverse the order.

But remember that "x" is an operator, not a function. So

[tex]
\frac{\partial}{\partial x} x \neq 1
[/tex]

Instead, it's supposed to be the operator

[tex]
\psi(x, y, z, t) \rightarrow \frac{\partial (x \psi(x, y, z, t))}{\partial x}
[/tex]
 

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