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Veritasium -- How Special Relativity Makes Magnets Work.

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TimFox:
Read again what you highlighted in red.
You objected to a particular statement "[9]" that the wire is electrically neutral (no net charge).

aetherist:

--- Quote from: TimFox on March 31, 2022, 09:49:30 pm ---A very important feature of magnetic forces in engineering is that very strong forces can be produced by running reasonable currents through conductors, especially when forming the wires into coils so that the same current contributes to the field over and over.  In normal constructions, it is hard to obtain very large electrostatic forces from charged metal objects.
Under normal conditions, matter in bulk is very, very close to electrically neutral, which means that the number of protons and electrons in the piece of bulk material are very, very, very close to equal.
In setting up his calculation, Dr Purcell starts out with a metal cylinder (wire) that is electrostatically neutral, to see what happens when a current runs through it.
What possible objection could there be to starting from that initial condition?
--- End quote ---
(0) Yes of course its ok to start with neutral wire. Me myself i reckon that all metals have a concentration of free-ish electrons on the surfaces, koz electrons repel electrons. And a greater concentration at sharper areas (eg external corners & edges). The centre of the metal would have a positive charge. This must result in the absorption of electrons from the environment, & the additional electrons would result in there being less (positive) charge in the center. (1) So, metal objects would tend to have a slight natural negative charge (in the near field)(& in the far field).

(2) When a switch in a circuit is closed i suppose that initially some surface electrons would move away from the contact. A weak & brief current. Old (electron) electricity would say that electrons injected into a wire would push & compress the electron sea in the wire, & (3) the wire would get a negative charge. STR would say (should say) that (4) the now drifting electrons get closer together due to length contraction, a double dose of negative charge.
But Purcell & Co ignore (3) & (4).     
(0) is not the issue.
And its ok to ignore (1) & (2) (these are very weak effects).

I think that (1) & (2) might be (at least partly) responsible for the mmf in the Rowland-X. I might look into that next week.

aetherist:

--- Quote from: TimFox on March 31, 2022, 10:48:39 pm ---Read again what you highlighted in red.
You objected to a particular statement "[9]" that the wire is electrically neutral (no net charge).
--- End quote ---
Yes, but [9] refers to the wire being neutral after the switch is closed, ie after the electric current is turned on, ie after the drifting starts.

TimFox:
That's not what your quoted text states.
It's an initial condition for his discussion of what happens in the wire.

aetherist:

--- Quote from: TimFox on March 31, 2022, 11:12:36 pm ---That's not what your quoted text states.
It's an initial condition for his discussion of what happens in the wire.
--- End quote ---
If u look at Purcell's 3 pages attached to my reply #20, u will see his Fig 5.22 (a) &(b) & (c).
He duznt have a (d) showing the stationary protons & stationary electrons in a stationary wire in the stationary lab frame. No, Purcell is too clever for that. That would emphasise the extent of his fudge/push/lie/nonsense.
Purcell's initial condition is Fig 5.22(a), with the electrons drifting.

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