Merge branch 'master' of ssh://animus.robocracy.org/srv/git/knowledge

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diff --git a/physics/quantum/fermigas b/physics/quantum/fermigas
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+===============
+Fermi Gas
+===============
+
+Derivation of the Fermi Energy
+---------------------------------
+Consider a crystal lattice with an electron gas as a 3 dimensional infinite
+square well with dimensions :m:`$l_{x}, l_{y}, l_z$`. The wavefunctions of 
+individual fermions (pretending they are non-interacting) can be seperated
+as :m:`$\psi(x,y)=\psi_{x}(x)\psi_{y}(y)\psi_{z}(z)$`. The solutions will be
+the usual ones to the Schrodinger equation:
+
+:m:`$$\frac{-\hbar^2}{2m}\frac{d^2 \psi_x}{dx}=E_x \psi_x$$`
+
+with the usual wave numbers :m:`$k_x=\frac{\sqrt{2mE_x}}{\hbar}$`, and quantum
+numbers satisfying the boundry conditions :m:`$k_x l_x = n_x \pi$`. The full
+wavefunction for each particle will be:
+
+:m:`$$\psi_{n_{x}n_{y}n_{z}}(x,y,z)=\sqrt{\frac{4}{l_{x}l_{y}}}\sin\left(\frac{n_{x}\pi}{l_{x}}x\right)\sin\left(\frac{n_{y}\pi}{l_{y}}y\right)\sin\left(\frac{n_{z}\pi}{l_{z}}z\right)$$`
+
+and the associated energies (with :m:`$E = E_x + E_y + E_z$`):
+
+:m:`$$E_{n_{x}n_{y}n_z}=\frac{\hbar^{2}\pi^{2}}{2m}\left(\frac{n_{x}^{2}}{l_{x}^{2}}+\frac{n_{y}^{2}}{l_{y}^{2}}+\frac{n_{z}^{2}}{l_{z}^{2}}\right)=\frac{\hbar^2|\vec{k}|^2}{2m}$$`
+
+where :m:`$|\vec{k}|^2$` is the magnitude of the particle's k-vector in k-space.
+This k-space can be imagined as a grid of blocks, each representing a possible
+particle state (with a double degeneracy for spin). Positions on this grid have
+coordinates :m:`$(k_{x},k_{y},k_z)$` corresponding to the positive integer 
+quantum numbers. These blocks will be filled
+from the lowest energy upwards: for large numbers of occupying particles, 
+the filling pattern can be approximated as an expanding spherical shell with 
+radius :m:`$|\vec{k_F}|^2$`. 
+
+Note that we're "over counting" the number of occupied states because the
+"sides" of the quarter sphere in k-space (where one of the associated quantum
+numbers is zero) do not represent valid states. These surfaces can be ignored
+for very large N because the surface area to volume ratio is so low, but the
+correction can be important. There will then be a second correction due to
+removing the states along the individual axes twice (once for each
+side-surface), u.s.w.
+
+The surface of this shell is called the Fermi surface and represents the most
+excited states in the gas. The radius can be derived by calculating the total
+volume enclosed: each block has volume :m:`$\frac{\pi^3}{l_x l_y
+l_z}=\frac{\pi^3}{V}$` and there are N/2 blocks occupied by N fermions, so:
+
+:m:`$$\frac{1}{8}(\frac{4\pi}{3} |k_{F}|^{3})&=&\frac{Nq}{2}(\frac{\pi^{3}}{V})\\|k_{F}|&=&\sqrt{\frac{3Nq\pi^2}{V}}^3=\sqrt{3\pi^2\rho}^3$$`
+
+:m:`$\rho$` is the "free fermion density". The corresponding energy is:
+
+:m:`$$E_{F}=\frac{\hbar^{2}}{2m}|k_{F}|^{2}=\frac{\hbar^{2}}{2m}(3\rho \pi)^{2/3}$$`
diff --git a/physics/units b/physics/units
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+======================
+Units
+======================
+
+.. contents::
+
+SI Units
+--------------------
+The SI system uses meters-kilograms-seconds. It also defines the Coulomb as 
+a unit for measuring electric charge, which introduces redundant conversions
+between mass-length-time units and the electric charge.
+
+cgs Units
+--------------------
+The cgs system uses centimeters-grams-seconds, and also defines electric charge
+in terms of the fundamental quantities of mass, length, and time. The unit of 
+charge is "esu" or electrostatic unit.
+
+Natural Units
+--------------------
+Natural units are a system of units which replace (or re-scale) the usual mass,
+length, and time bases with quantities which have "natural" (physical)
+constants associated with them. The two constants usually chosen are the speed
+of light (c) and Plank's constant (:m:`$\hbar$`); the gravitational constant
+(G) is a possibility for the third constant/unit, but energy (in
+electron-volts: eV) is often used instead because it gives more useful
+relations and because there is no accepted theory of quantum gravity to unite
+these three constants. See _`Plank Units` for more on using G as a unit.
+
+Working with natural units simplifies physical relations and equations because
+many conversion factors drop out.
+
+Given the relations between cgs units (gm, cm, sec) and natural units (c,
+:m:`$\hbar$` , eV), we can find the natural units of an arbitrary quantity
+:m:`$[Q]=[gm]^{a}[cm]^{b}[sec]^{c}=[c]^{\alpha}[\hbar]^{\beta}[eV]^{\gamma}$`:
+
+:m:`$$(\alpha,\beta,\gamma)=\left(\begin{array}{ccc} -2 & 1 & 0\\ 0 & 1 & 1\\ 1 & -1 & -1\end{array}\right)\left(\begin{array}{c} a\\ b\\ c\end{array}\right)=(-2a+b,b+c,a-b-c)$$`
+
+or in reverse:
+
+:m:`$$(a,b,c)=\left(\begin{array}{ccc} 0 & 1 & 1\\ 1 & 2 & 2\\ -1 & -1 & -2\end{array}\right)\left(\begin{array}{c} \alpha\\ \beta\\ \gamma\end{array}\right)=(\beta+\gamma,\alpha+2\beta+\gamma,-\alpha-\beta-2\gamma)$$`
+
+Plank Units
+----------------
+Plank units (defined by Plank soon after defining his constant :m:`$\hbar$`) are a version of _`Natural Units` using the gravitational constant G as the the 
+third unit (instead of the common measure of energy). When converted back into
+mass-length-time units we get three quantities which define the "Plank Scale",
+which may provide estimation of the domain where quantum gravity effects become
+important (similar to how the speed of light and Plank's constant provide 
+estimation of when special relativistic and quantum mechanical effects become
+important).
+
+