Recent content by Tipi

Thanks for your answer. Could you explain a little were the first equation come from and how you go to the second? Thanks, TP [edit] OK. I get the first one, I didnt saw its evidence on first look. And for the second one, you used H_\text{crit} =30\sqrt (\rho (m_p/m^3))?

Ok. That was not the answer I was looking for but it's interesting. So, how could we determine the positive energy of matter, for instance? Thanks, TP [edit] Enhanced title...

How to determine the total energy of matter in the universe Hi all, while looking at a table of order of magnitudes of energies in nature, I just realize that I really don't know how to determine the total energy contained in the universe (near 10 to the 69). Can someone get this number...

It depends on what you want. Do you want to see, on your graph, the weight that is lost each second, or the weight that have been lost since the beginning of the run? TP

Taylor serie around ka = 0 : \cos ka = \frac{\cos 0}{0!} - \frac{ka\sin(0)}{1!} - \frac{(ka)^2\cos(0)}{2!} + ... Note that you misreproduced my statement, it's a minus before the second order term. For this system, yes, because the maximum is at k = 0.

Hi all, I've calculated the exciton binding energies for different semiconductors using the Bohr model. It works remarkably well for direct gap semiconductors, but it is not good for indirect gap semiconductors (in Si and Ge, there is an underestimation by a factor of 3, approximatively)...

First of all, I would like to thank you all for your help in understanding the interpretation of the Fermi-Dirac distribution at the Fermi-Level. I think everything have been said and the issue is well outlined in the last posts. To conclude this thread, I would like to quote Kiréev* who give a...

In the band theory of metals, two electron in the same energy level (orbital) don't interact with each other (at all). It's the Fermi gaz... This model cannot explain the transition from conducting to insulating when we increase the nearest neighbor spacing (by heating for example). The Hubbard...

I've made the calculation in the solution of a final exam. Look at the first question in this file : http://www.forumphysique.com/levesque/Site/Matière_Condensée_files/A05-PHY6505-final-SOL.pdf It's in french but, as usual, the maths are... in math. In the document, Case 1 is the limit where...

Hi weejee, I am looking for a physical argument that could help me to understand that fact. Suppose a simple problem : two (free) electrons in a box of length L at absolute zero. Then, the possible k are k_n=\frac{n\pi}{L}, where L is finite. Since we can put two electrons per state...

Even in the ground state?

Hi Pete, I agree. I will look in this direction. I think this make sense and throw away the need for an interpretation of the Fermi level. But in fact, you can derive de Fermi-Dirac distribution function for discrete spectrum. The normalization stated above is than made by a sum instead of...

Kittel, Thermal Physics 7th edition, p. 156 :

Hi Pete, And do you have any hint on why one would like to define it as such? I've seen this definition somewhere, but there was no a priori (nor a posteriori) justification. Thanks in advance. A colleague told me that the one-half at E=mu is only a thermal average. Maybe this is the key...

I'm not sure, but his post make me think he wonder about something like the difference between Hubbard model and general band structure theory. Could you clarify your question, and give a source of the affirmation you are giving here? It will help us to answer... TP