Re: Error in Wikipedia article: Faraday's law of induction


<phil-news-nospam@xxxxxxxx> wrote in message
news:g5a3vt013mm@xxxxxxxxxxxxxxxxxxxx
It seems there is an error in this Wikipedia article:

http://en.wikipedia.org/wiki/Faraday%27s_law_of_induction

Faraday's law says the induced EMF is in proportion to the rate of CHANGE
(my emphasis) of the magnetic flux. The article describes this much OK.
Later on, the article then says:

In the case of motional EMF, the idea is to move all or part of the
circuit through the magnetic field, for example, as in a homopolar
generator.

But this last case is a situation where the magnetic flux does not change.
Isn't this the aspect that Faraday saw as a paradox because it did not
obey
his law of induction? Apparently he realized that no flux change happened
when he rotated the magnets with the rotating disk in his unipolar
generator.

As I understand all this, there are TWO ways to induce electricity from a
magnetic field:

1. A conductor at a right angle to the magnetic field alignment
experiencing
a change in the magnetic field flux density (strength). [Faraday's law
of induction]

2. Motion of a conductor at right angle to the magnetic field alignment
and
simultaneously at right angle to the line of the conductor. [Fleming's
right hand rule, Lorentz force law]

The article seems to be trying to convert everything into one.

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Faraday considers the flux through a closed loop. Lorentz is a force
relationship considering, in this case, the force on an electron in a
conductor and the basic definition of potential difference.

Both situations occur in a normal motor- speed voltage due to motion and
"transformer "voltage due to the field. d(flux)/dt The flux through a path
may be a function of both time and position so it becomes d(phi with
position held constant)/dt + d(phi with time held constant )/dx times dx/dt
This accounts for both the magnetic field itself varying with time but also
the geometry varying with position which is changing with time. This appears
to be where the Wiki author is coming from-No problem.
If you look at a machines text such as Krause, you will see the two
terms -often expressed in terms of flux linkages or inductances d((Li)/dt
=L(di/dt) +i (dL/dt). In a motor the speed term generally is much larger
than the transformer voltage.

P.S. right angle motion is not required but that gives the most bang for the
buck -function of the sine of the angle. --

Don Kelly dhky@xxxxxxxxxxxx
remove the X to answer
-------------------------------


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Relevant Pages

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    ... |> Faraday's law says the induced EMF is in proportion to the rate of CHANGE ... In the case of motional EMF, the idea is to move all or part of the ... circuit through the magnetic field, for example, as in a homopolar ... |> But this last case is a situation where the magnetic flux does not change. ...
    (sci.physics.electromag)