# Why is there no help: momentum expectation value 2D particle in a box

1. Nov 28, 2013

### Duave

Is there anyone out there that knows how to define the p operator for a 2-d box. Please can you give a full answer, and not only a hint. I think that no one on this planet knows what it is. I have looked all over the internet. If there is no answer. Why don't people just say it? I think nobody knows. There is absolutely no information on the square of the magnitude of the momentum for a 2-D particle in box. Is this a unexplored portion of quantum physics? Am I embarking on a new concept that Schrodinger never considered? It seems so. Please feel free to respond to my discovery. An answer would be nice. Thank you.

2. Nov 28, 2013

### ZapperZ

Staff Emeritus
First of all, try to dial back on the silliness. Just because you can't find it on the web doesn't mean it doesn't exist. This is a standard topic in many QM classes, and certainly a topic covered in solid state physics. So I hate to burst your bubble, but you haven't found anything new.

Secondly, look at this:

http://users.ece.gatech.edu/~alan/E...tures/King_Notes_Density_of_States_2D1D0D.pdf

Finally, you sound as if you have asked this before. Did you just violated our rules and came back as a sock puppet?

Zz.

3. Nov 28, 2013

### bp_psy

For the 2D infinite square well the Schrodinger equation is separable therefore you can get the expectation values for each individual momentum component. The total momentum expectation will be the sum of the momentum components. So there are no new physics here.

The momentum operator is defined as usual only that one of the 3 components is missing.
http://en.wikipedia.org/wiki/Momentum_operator

4. Nov 28, 2013

### Duave

@ZapperZ and bp_psy:

Thank you very much for the response. In order to start working on my problem I need to know the general form of the momentum average for a 2D particle in a box. I am a chemistry major taking a physics course. I can not understand the links. I am really in need of an example of the general form of a 2D particle in a box for the momentum average. Can you give me a response to my question in a form like this please:

<p^2> =∫(2/a) sin(n*pi*x/a)(ih[d/dx](2/a) sin(n*pi*y/a)dxdy

I am not saying that what i wrote is a true statement for the momentum average for the a 2D particle in a box. I just need the answer in this kind of form from you. A link does not help me because, honestly, I ran across the links that you both gave me when I was initially trying to find the information that I am asking about. The links are not helping me. Can you please give me a direct mathematical answer? Thank you again for your response.

5. Nov 28, 2013

### ZapperZ

Staff Emeritus
This is a HW/Coursework-type question and must be done in the HW/Coursework forum. You must provide the entire problem and follow the requirement of the template in that forum..

Zz.