Re: MIT's Walter Lewin's twice surprises the EE professors! (fun)
- From: "William R. Frensley" <frensley@xxxxxxxxxxxx>
- Date: Wed, 10 Jan 2007 22:07:12 +0000 (UTC)
Cyberkatru wrote:
MIT physicist/astronomer is quite fun to what lecture. In lectures 20There is most certainly a widespead misconception here, but unfortunately
and 16 of his online basic E&M lectures he makes some interesting
points. ...
http://ocw.mit.edu/NR/rdonlyres/Physics/8-02Electricity-and-MagnetismSpring2002/C29636DB-AB80-49E0-BFAB-BCE00159F00E/0/lecsup41.pdf
So do we agree with him on this bit of basic physics 101?
this work does not find the real problem.
The fundamental misconception is the identification between the electrostatic
potential of Maxwell's theory and the circuit-theory concept of voltage, which
for reasons explained below should always be called "node voltage."
We all understand that the Maxwell quantity is defined as limit of U(r)/Q
as Q approaches zero, and U(r) is the potential energy of an infinitesmal test
charge Q. The circuit-theory voltage is actually (-) the electrochemical
potential for mobile electrons within a chunk of matter that contains enough
electrons to be considered an electron reservior, and that is in a local
quasi-equilibrium. This chunk of (almost always metallic) matter is represented
in circuit space as a "node" in the circuit graph.
The minus sign in the above definition of voltage is a result of the negative
charge on the electron. (Benjamin Franklin had a even chance, but unfortunately
he made the wrong choice.) The electrochemical potential for electrons is
known in semiconductor device theory as the (quasi)Fermi level, not to be
confused with other meanings of "Fermi energy" in solid-state physics. This is
the quantity that voltmeters measure, as pointed out by William Shockley
(Electrons and Holes in Semicnoductors, 1950, p. 305). To understand this,
let's assume that the voltmeter is a classic Wheatstone Bridge, consisting of
an ammeter connected between the unknown voltage node and an adjustable voltage
standard (which can be just a battery connected to a potentiometer). The measurement procedure is to adjust the voltage standard until the current
through the ammeter reads zero. The condition for zero (electron) current is
that the electrochemical potentials for electrons on the two sides of the meter
must be equal.
When we understand node voltage to be this statistical quantity, all the
Krichoff's Law paradoxes disappear. The potential energy of the electrons in
the node can be raised or lowered by the electrostatic potential phi, or by
dA/dt: the voltage is still a scalar quantity. Its gradient is NOT in general
equal to the electric field E.
Why has the confusion between these quantities persisted for so long? The first
problem is that they are measured in the same units (Volts). The second reason
is that -> within a given metallic material <- the voltage and the electrostatic
potential are coupled by an additive constant, the work function. Connecting
two metals with different work functions produces a structure with a constant
voltage, but with a discontinuity in the electrostatic potential, which is
known as a "contact potential."
The additive linkage between potential and voltage is obviously broken in
vacuum, and also in most semiconductor device structures. A very little known
fact is that a difference in the spatial behavior of the potential and of
the voltage is a necessary condition for the operation of active (i.e. gain-
producing) electron devices.
- Bill Frensley
.
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