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

On Sat, 12 Jul 2008 06:30:50 -0700 (PDT) Benj <bjacoby@xxxxxxxxxxx> wrote:
| On Jul 12, 7:19 am, phil-news-nos...@xxxxxxxx wrote:
|
|> 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.
|
| Not really. Yes there are two ways and electric field can be produced
| in a conductor (wire). One would be the relative motion between a
| magnetic field and the conductor. That is related to the Lorentz force
| law on charges. A simple example would be the moving of a magnet in
| and out of a coil of wire. Also generators work on this principle and
| the calculation of operation this way is usually called "flux cutting"
| methods. The wires pass through the flux "cutting" it and producing
| Lorentz EMF inside the wires.
|
| The other way of producing electric fields inside wires is known as
| "flux changing". IF we have two coils of wire and we put a current
| through one of them, a pulse of voltage appears in the other. Note
| that NOTHING is moving so there is no "flux cutting". Only the flux
| through the second coil is changing because the current in the first
| coil is changing.
|
| So now there are TWO ways to produce an "emf" in a wire by means of
| magnetic flux. But flux methods of BOTH types are limited. Sometimes
| they fail altogether! A Faraday generator is a perfect example. The
| magnetic field is not changing so there is no flux changing. And since
| the disk is solid and continuous it really isn't "cutting" the flux
| either. There are other examples of geometry that show the failure of
| flux methods. Feynman notes that in such cases you go back to first
| principles (Maxwell's equations) no matter how handy flux methods are
| in calculating answers in many practical cases. In the case of mutual
| induction the Vector Magnetic Potential can be seen to play a vital
| role.

I don't see an issue with considering the Faraday generator solid disk as not
fitting the Lorentz force law. "Cutting" was an erroneous convenience that
explained some things prior to understanding this in terms of particles.

Certainly there are cases where you have to consider both laws to figure out
what is happening. But how does a transformer need the Lorentz law? How does
the Faraday disk need Maxwell's upgrade of Faraday's law?


| There have been many attempts to convert these two things into one.
| For example people have argued that as a current increases in a wire
| the magnetic field expands from the wire and thus produces Lorentz
| forces in neighboring wires. These theories to date have all failed.
| Flux theories have all pretty much failed to be fundamental. In fact
| the whole concept of "lines of flux" is rather bogus to begin with!
| Not that they are not useful, mind you.

If you want to convert both laws into one, you will have to consider space
and time as a single entity, and use a vector product that works between them.


| So yes, there is no combining these things. You are allowed to go
| "fix" the Wikipedia article you know!

This is an area I still have a lot to learn about. Unfortunately, many of the
puzzles I am pondering are not addressed in the multitudes of examples I have
found both in Wikipedia and elsewhere. Google has helped, but it doesn't seen
to have a way to do a search like "take the example described on this page and
find a page that describes a scenario which transposes the field and the motion
and flips the EMF".

For example, instead of using a solid disk in the Faraday generator, use a
spoked wheel. I did see one such example somewhere online with the claim
that it works the same as the solid disk, but only described a scenario of
stationary magnets creating a uniform field over the entire wheel. Now
change this to rotate the magnets with the wheel. I don't see why it would
be any different than the disk with rotating magnets, but I cannot be 100%
sure since apparently no one tested it (so I might have to). However, I am
more interested in additional modification of this wheel with a non-uniform
magnetic field, using magnets rotating with the wheel. So it still should
be a case of no flux change for any particle of the wheel mass itself. What
would be different is that some spokes have the field in one direction and
other spokes have the field in the other direction. So I would expect the
latter spokes to be producing an EMF in the opposite direction, continuously.

If I need to do a test of this, it would seem the best way to measure current
between alternating spokes would be to have some kind of low voltage light on
the wheel itself. That way, there are no external wire leads that could be
influenced by the flux changing going on in the extraneous field that reaches
beyond the wheel. I would expect this extraneous field to be much less since
the field lines would be between the opposing magnets instead of being limited
to being between the opposite sides of the wheel assembly. I expect this to
better confine the field to the wheel. Still, what remains of the extraneous
field could disturb the experiment.

I also suggest that instead of having what amounts to a group of loops making
up the wheel (a loop exists between each spoke pair with opposite fields), the
alternating spokes could be "wired in series" by breaking the loop near the
edge for loops in one direction, and near the axis for loops in the other
direction. This would end up being a zig-zag around the wheel, where even
spokes doing the zig have a field in one direction, and odd spokes doing the
zag have a field on the other direction. Shouldn't this series circuit have
an increased voltage with less current?

--
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| Phil Howard KA9WGN (email for humans: first name in lower case at ipal.net) |
.

Relevant Pages

  • Re: rotating magnetic field
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    (sci.physics.electromag)
  • Re: How close can you get to a Neutron star (Pulsar?) and not cook
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  • Re: Error in Wikipedia article: Faradays law of induction
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    (sci.physics.electromag)
  • Re: Explain magnetic field lines please.
    ... the diagrams showing magnetic field lines are just that -- diagrams. ... particles in that fluid for one second and drew a picture of the tracks. ... In places where the velocity was large, you would find more tracks because ... So what is flux? ...
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    ... A single wire that passes through a bead, torus, or core ... The flux lines of half the ... builds a magnetic field in the core. ...
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