XPost: sci.physics.relativity
Aether Regained wrote:
https://www.feynmanlectures.caltech.edu/img/FLP_II/f26-06/f26-06_tc_big.svgz
Compare Figure FLP-II-26-6 with the rail gun configuration.
As in the figure, in the rail gun the currents in the rail
and in the slider (cylinder) are orthogonal, so action and
reaction of the electromagnetic forces will not balanced.
Fig. 26–6. The forces between two moving charges are not always
equal and opposite. It appears that "action" is not equal to
"reaction."
Let's see what the forces in a railgun are:
^
| Fr
----|----------------------------------------|-------
| Armature <- I | Power supply
Fa <-| (cylinder) I -> | -> Fp
----|----------------------------------------|-------
| Fr
V
If your editor clutters up the figure, see it here:
http://paulba.no/temp/Railgun.pdf
The with of the rail is W.
The distance between the armature and power supply is L.
The rail, armature and power supply make a rectangle.
The current I is flowing around this rectangle.
That will make a magnetic flux through the rectangle,
perpendicular to the current.
The flux density depend on the geometry of the rail,
but it will be in the order B ≈ 4e-7⋅I/W T
The force on the conductors in the rail will be:
Fr = I⋅L⋅B
This force is perpendicular to the current and the B field.
Since the currents in the two parts of the rail are in opposite
direction, these forces will act in opposite direction, and will
be each other's reaction force. The rail must obviously be so solid
that it can withstand these forces without breaking.
NOTE: NO FORCES ON THE RAIL ARE ACTING PARALLEL TO THE RAIL!
The force on the armature is Fa = I⋅W⋅B, acting perpendicular to
the current and the B-field. To the left on the figure.
This is the force that accelerates the armature.
The force on the power supply, or rather on the conductors in
the power supply carrying the current, is Fp = I⋅W⋅B.
Since the current in the power supply and the armature
are in opposite direction, so are the forces.
Fa and Fp are equal in magnitude and in opposite direction.
Fa and Fp are each other's reaction force.
Since the power supply is fixed on the rail the reaction
force are acting on the rail through the power supply.
--
Paul
https://paulba.no/
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