Re: [Phys-l] fermi energy and electrostatic potential

[John Denker <jsd@av8n.com>]

On 04/27/2008 12:41 PM, fizix29@aol.com wrote:
> I am having trouble with a "gut understanding" of why the fermi level
> is constant across a junction between two materials.? It seems to
> trump the electrostatic potential.

You need to account for the protons.  There is a tradeoff
involving electron KE, electron-electron electrostatics, 
and electron-proton electrostatics.  Total energy "trumps"
electrostatic energy.  Typically there is a higher density 
of unshielded protons (in the form of ion cores) on one side 
of the junction relative to the other.  Also a higher KE on
one side relative to the other.

The KE terms in the equation of motion are often given too 
little attention.  Keep in mind the potential energy terms
are non-directional, in the sense of being spherically
symmetrical.  All of the directionality we see in chemical
bonds (especially in organic molecules) comes from the
directionality of the _KE_ terms.
 
> I have a mental picture of electrostatic potential as the surface of
> water and if two containers filled with water are joined, perhaps by
> a tube, water will move from one container to the other until the two
> surfaces are at the same height.? Can I add a feature to this mental
> picture to include fermi energy?

Let one arm of a U-tube be inside a capacitor.  Charge up
the capacitor.  The water will rise higher in that arm,
because water is a dielectric.  Total energy "trumps"
gravitational potential energy.