🚀 go-pugleaf

"Bilge" <dubious@radioactivex.lebesque-al.net> wrote in message
news:slrnbvraov.dan.dubious@radioactivex.lebesque-al.net...
> Harry:
>
>  >>
>  >>   Newton's third law fails only when an object is moving at a velocity
>  >> which is much larger than the propagation of a signal through the
object
>  >> and large enough for the simultaneity of different points on the
object to
>  >> matter. Then, newton's third law doesn't really fail so much as it
needs
>  >> to be analyzed more carefully to account for the difference in
>  >> simultaneity.
>  >
>  >I must read the allegations again! It is not just about high velocity,
but
>  >force balance with the Lorentz force, for example when a charged object
>  >moves slowly perpendicular to a wire with a DC current. Perhaps I missed
out
>  >on a textbook that carefully analyses this simple looking case.
>
>   The motion of a charge next to a wire can be found in a fairly
> straight-forward way just by finding the B-field from the wire and
> and using the lorentz force law. (Working in the frame in which the
> field from the wire is purely magnetic).

True. So now we have:

�
�  very long wire
�
�     <- -  o+    charged object , v<<c
�
�
^
I

Thus: I can see that the object is pushed downward, and, if I'm not
mistaken, that the wire is pushed counter clock wise.
In order for the forces to balance I  miss an upward longitudinal force on
the wire.

>  >>   I'm not really sure what you mean by ampere's law needing newton's
third
>  >> law. Anpere's law doesn't contain any forces. You need the lorentz
force
>  >> law for that. Neither ampere's law nor any of the other maxwell
equations
>  >> even describe charges, moving or stationary.
>  >
>  >Sorry, perhaps Ampere had several laws?!
>  >I meant the Ampere force law, which is very roughly:
>  >
>  >F = -i_m * i_n *(dm . dn / r^2) * (2 cos phi - 3 cos a cos b)
>
>   I didn't realize ampere had a "force law" other than what could
> be derived from the usual equations. In any case, that expression
> is written in a particular frame (which is okay). But, notice that
> because it's derived under the assumption of infinitely long wires,
> it's rather unphysical (at least from a relativistic standpoint),
> since the current can't ever begin flowing or stop flowing, but
> had to have existed and continue to exist for all time.

In fact, no such assumption was made, it describes the force between two
individual charge elements.

>  >See for example
>
>http://ieeexplore.ieee.org/iel1/27/6502/00256790.pdf?isNumber=6502&arnumber
=
>  >256790&prod=JNL&arSt=701&ared=713&arAuthor=Graneau%2C+N.
>
>  That doesn't work for me. I get a login page which tells me I have to
> register for a web account. As matter of general principle, I do not
> access web sites that require providing personal information of any
> kind.

Perhaps it's a paying website, sorry. My university has subscription on a
number of them...
It's IEEE trans. on Plasma Science, vol.21, no.6, dec.1993, pp. 701-713.
N. Graneau, "An electrodynamic vacuum arc ion acceleration mechanism based
on Ampere's force law".

Harald