# MIT's Walter Lewin's twice surprises the EE professors! (fun)

by Cyberkatru
Tags: lewin, professors, surprises, walter
P: n/a

## MIT's Walter Lewin's twice surprises the EE professors! (fun)

But Bill, he actually uses the voltmeter to make the point. He moves
the voltmeter without changing where it is connected and gets a
different reading? This is puzzling if there is a real scalar function
(node voltage) along the wire whose changes around a loop add to zero.
This demonstration is in lecture 16. He actually used the physical
voltmeter to show that the changes around the loop don't add to zero.

Another thing that bothers me with your explanation is that you said
that the node voltage and the electrostatic potential are related by a
constant. How could an additive constant make a difference here? KVL
wouldn't be affected by an additive constant would it?

You might have the right explanation but it still not clear to me. It
is almost as if you are saying that the line integral \int E \dot dl is
not the relevant quantity for circuites. But this seems inconsitent
with Jackson's book.

On Jan 10, 4:07 pm, "William R. Frensley" <frens...@utdallas.edu>
wrote:
> Cyberkatru wrote:
> > MIT physicist/astronomer is quite fun to what lecture. In lectures 20
> > and 16 of his online basic E&M lectures he makes some interesting
> > points. ...

>
>
> > So do we agree with him on this bit of basic physics 101?There is most certainly a widespead misconception here, but unfortunately

> 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

 P: n/a Chris H. Fleming wrote: > > > I don't get the last paragraph of the first page. > > "There is no electric field in this loop if the resistance of the wire > making up the loop is zero." > > Then he says. > > "(this may bother you - if so, see the next section)" > > Well I read on and it didn't stop bothering me. Remember that ideal capacitors have zero resistance in their plates and terminals, just as ideal inductors have zero resistance in their windings. If you apply a voltage across the terminals of an ideal inductor, the value of current will start changing continuously, and it would go to infinity if you wanted to wait for infinite time to pass. A perfectly conductive loop is not a short circuit, instead it is a small-value inductor. > When he is explaining > this on page 4, in paragraph 3 he sets all the currents at every point > in the loop to be the same, but in the very next paragraph he > contradicts himself with charge buildups at certain points on the loop. He's just simplifying things by ignoring the charge-buildup process. The circuit will "adjust itself" over a very brief time, creating surface charges and patterns of potential. After this "transient" has occurred, things will work as he describes. In other words, he might be describing what happens over a period of many milliseconds, while ignoring the changes which occur in a range of nanoseconds. A more complete explanation would have to include ALL changes at ALL time scales, including the currents which led to the charge buildups. Here's another cool article which goes into similar problems, similar in that most books ignore it (but it's about flashlight circuits, not about induced currents in conductor loops:) Chabay/Sherwood, A unified treatment of electrostatics and circuits http://www4.ncsu.edu/%7Erwchabay/mi/circuit.pdf ((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer beaty a chem washington edu UW Chem Dept, Bagley Hall RM74 billb a eskimo com Box 351700, Seattle, WA 98195-1700 ph425-222-5066 http://staff.washington.edu/wbeaty/
 P: n/a I don't get the last paragraph of the first page. > > "There is no electric field in this loop if the resistance of the wire > making up the loop is zero." > He is idealizing isn't he. If the wire is really a perfect conductor why would force be needed to keep them moving along. It is like no friction. Not the case for a real wire but so what? Its an adealization where all the resitance is put into the resistor.