Re: Charged spinning disks

On May 17, 2:16 am, Benj <bjac...@xxxxxxxxxxx> wrote:
On May 16, 8:23 pm, RP <no_mail_no_s...@xxxxxxxxx> wrote:

That view is incorrect as well.  In ordinary conductors there are two
currents, one of positive charge and one of negative charge.

How did the disk get changed to an "ordinary conductor"? We are
talking about an isolated charged spinning disk. This disk is
presumably an insulator which causes the charges to spin with it. If
there are both resinous and vitreous charges in the material of the
disk they cancel at all times and move together. Hence the disk is
equivalent to isolated circulating charge and my statements stand.

Of course since you note that a B field from a conductor can't be
transformed away, that means that B fields from currents in wires are
"real" while B fields from circulating charges are not. This is an
interesting result!

[snip irrelevant answer to some other question]

Yes, the " I " in Ampere's Law refers to two currents rather than to
one and similarly for I primed. Suppose you have two parallel
conductors with equal currents in the same direction. The conductors
are identical. In this case Ampere's law has II' being proportional
to the net force per unit length of the conductors. But if you assume
the frame of reference of the electron currents, then they are no
longer currents but simply static charges at rest. The force between
the conductors continues unchanged however. Thus there must still be
II' even in this frame of reference and moreover II' cannot be
refering to the drifting electrons. In this new frame II' refers to
the drift of positive charges.

So, in the case of parallel monopolar beams Ampere's law is invalid.
You should be able to see that for the basic electromagnetic
equations, I must be defined as the average rate of flow of electrons
wrt the protons in the same conductor. For a monopolar beam of
electrons a current of this sort can thus not be defined. We can
extend that to bodies with fixed static charges on them that are in
motion. Ampere's law does not apply to them either. In fact, Ampere's
law cannot even apply to a current carrying conductor on which an
excess charge exists, the conductor must be charge neutral otherwise
Ampere's law is again invalid.






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