"Neil Bates" wrote in message ...
Neil Bates wrote in message
...

snip
Side note: I think we can make a sort of homopolar generator by rotating a
thin
(not long) disk magnet, with a wire loop underneath at 90° turn. There is
nothing so incredible about homopolarity, as long as the loop is accelerating
in
some way (as for example, a charge accelerates beside a loop in proper
orientation to receive curl E, not transformed away by charge velocity.)
...
After consideration, I don't think the rotating magnet+loop would generate
induction in the loop. Rotating frames are tricky in electromagnetism. One must
calculate the fields produced by one rotating object, then use the separate
velocities of each part of the second rotating frame for transformation of those
fields. You can't just pretend it all rotates together like a unit. However, the
last example, of co-accelerating charge and loop, is probably valid.

Hi Neil,

A rotating magnet generates a E field.


- put yourself in the referentiel of the magnet : only B exist.
- use lorentz transformation to go back to the initial referentiel
(where the magnet turns)
- you get both B and E field.

This completely explains all experimental results deduced with
homopolar generators.

Rgds,
Arthur

(arthur) wrote in message . com...
"Neil Bates" wrote in message ...
Neil Bates wrote in message
...

snip


Hi Neil,

A rotating magnet generates a E field.


- put yourself in the referentiel of the magnet : only B exist.
- use lorentz transformation to go back to the initial referentiel
(where the magnet turns)
- you get both B and E field.

This completely explains all experimental results deduced with
homopolar generators.

Rgds,
Arthur

Naturally, relative motion in the *presence* of a pole means an E
field, because of the velocity-transformation of B. Hence, I realize
that a rotating magnet generates an E field, but I was specifically
referring to the situation where the poles are far away from the
region of interest (or absent, due to the dipoles running around a
complete circle.) If a very long, or circular magnet, rotated and you
were near the middle, I don't think there would be a significant E
field. There would be a significant rate of dA/dt. I was asking if
that should be thought canceled out due to effective regions of charge
density (contributors to phi) in the "current loops", even though that
is not a very literally appropriate model.

Second, what *are* the experimental results deduced with homopolar
generators? Is it true, that you can't make one without at least one
"commutator" or brush? (except, for the odd case of accelerating a
loop alongside a charge - but you'd need to shield the loop from the
raw field, or it would spoil the effect.)

Neil

(Neil) wrote in message om...

Hi Neil,

Naturally, relative motion in the *presence* of a pole means an E
field, because of the velocity-transformation of B. Hence, I realize
that a rotating magnet generates an E field, but I was specifically
referring to the situation where the poles are far away from the
region of interest (or absent, due to the dipoles running around a
complete circle.) If a very long, or circular magnet, rotated and you
were near the middle, I don't think there would be a significant E
field.

I agree with you

There would be a significant rate of dA/dt. I was asking if
that should be thought canceled out due to effective regions of charge
density (contributors to phi) in the "current loops", even though that
is not a very literally appropriate model.

?

Second, what *are* the experimental results deduced with homopolar
generators?

http://www.marmet.ca/louis/induction...y/KellyFa3.pdf

Is it true, that you can't make one without at least one
"commutator" or brush?

Perhaps we have to discuss about the exact definition of a brush but I
think that is false. You can make one just with one brush...
(examples are given in the link).

(except, for the odd case of accelerating a
loop alongside a charge - but you'd need to shield the loop from the
raw field, or it would spoil the effect.)

let's not accelerate loops yet...


A+
Arthur