Length-contraction-magnetic-force between arbitrary currents

Discussion in 'Physics & Math' started by PengKuan, May 12, 2017.

  1. PengKuan Registered Member

    Tangential magnetic force experiment with circular coil

    If magnetic force is to respect Newton’s third law, there should be a recoil force on the vertical current which is Ft. This force is tangent to the current I1 and called tangential magnetic force. Some physicists claim that tangential magnetic force exists, this claim is supported by some experiments such as the rail gun recoil force shown by Peter Graneau and Ampère's hairpin experiment, see Lars Johansson’s paper. But these experiments did not convince the main stream physicists and tangential magnetic force is rejected. I have carried out an experiment to show tangential magnetic force acting on a circular coil.
    Please read the article at

    PDF Tangential magnetic force experiment with circular coil http://pengkuanem.blogspot.com/2017/06/tangential-magnetic-force-experiment.html
    or Word with video https://www.academia.edu/33353400/T...rce_experiment_with_circular_coil_with_video_
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  3. Q-reeus Valued Senior Member

    I remember discussing Graneau et. al.'s claims re Amperian forces back sometime in the 80's. It was being taken seriously by one researcher at a local defense establishment looking into rail-gun design. Apart from the obvious conflict with SR implied by Ampere's formulation, I pointed out any purported longitudinal forces could be made vanishingly small by applying a simple scaling argument. All agree that for separate current circuits, the correct inter-circuit forces result always agrees with the standard magnetic Lorentz force law. Therefore any 'longitudinal forces' must be found between current elements within any given circuit. But any given circuit can be sub-divided up into a bundle of an arbitrarily large number of arbitrarily fine filamentary circuits. Apply the supposed Amperian longitudinal force expressions between current elements within each such filamentary circuit, and it soon becomes evident the longitudinal force densities i.e. stresses vanish in the limit of infinitesimal filament radii. Consequently, for the undivided circuit also. If you doubt that, do your own sums!

    The impulses observed in that YouTube vid with suspended coil will for sure follow from a thorough application of the magnetic Lorentz force law. As for the example of unequal action and reaction between a transverse current element and an infinitely long current, the standard reply is it neglects momentum changes occurring in the fields. You can't just have an isolated, electrically neutral current element. It forms part of a larger circuit. Even so, the active part of any such an element is just the moving electrons, and they generate an electric and magnetic field, which together with the magnetic field of the infinitely long current, formally at least will contain precisely the 'missing' momentum change you get by looking at only the magnetic forces between the currents.

    There are situations where no such simple resolution is evident, but I'm not going there here!
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  5. danshawen Valued Senior Member

    Ha. Think about a permanent magnet, or for that matter, a magnetar. It has a magnetic field in the rest frame. That should not even be possible, should it? Okay, so I should watch the other video.

    Why doesn't a magnetar simply break up itself up into billions of smaller magnets and spontaneously flip half of them to the opposite polarity / orientation to cancel out its own magnetic field? Atomic bonds are not strong enough to prevent this, really, are they? Or is it full of magnetic monopoles or something? Pretty sure that won't be explained in the video.
    Last edited: Jun 7, 2017
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  7. PengKuan Registered Member


    Why is Ampere's formulation in conflict with SR?

    The force that lies on the radial vector is not necessarily instantaneous. Gravitational force is not instantaneous.

    The splitting of a circuit into filamentary circuits does not change the total force on a part of the circuit. Let A and B be 2 part of the same circuit. We split them into A1, A2… An and B1, B2…Bn. The total force on B1+ B2+…+Bn stays the same.

    Lorentz force is perpendicular to current. So, the torque on the coil due to Lorentz force is zero.
  8. Q-reeus Valued Senior Member

    Check any textbook on EM that includes SR. Lorentz force follows directly from a relativistic treatment of charges in relative motion. Plenty of online resources too, e.g.:
    And? This permits Amperes 'longitudinal forces' how exactly?
    Strawman argument. I nowhere suggest otherwise, and it misses my argument entirely. You seem to be like another poster who endlessly argues but never gets down to doing the sums to prove it one way or the other. My claim is correct. Do the sums!!
    A correct analysis taking all the circuit into account will for sure be consistent with Lorentz forces only.
    exchemist likes this.
  9. exchemist Valued Senior Member

    Nice link, I thought.

    Please Register or Log in to view the hidden image!

    Q-reeus likes this.
  10. PengKuan Registered Member

    Continuous rotation of a circular coil experiment
    There is a long standing debate about whether tangential magnetic force exists. In «Tangential magnetic force experiment with circular coil» I discussed this force and presented an experiment that showed the action of this force. But, as the rotation of the coil in that experiment was limited to a small angle, it does not show that tangential force exists all over the coil. So, I have carried out the present experiment that shows continuous rotation of the coil to make clear that tangential force has the same value around the coil

    Please read the article at

    PDF Continuous rotation of a circular coil experiment http://pengkuanem.blogspot.com/2017/06/continuous-rotation-of-circular-coil.html
    or Word with video https://www.academia.edu/33604205/Continuous_rotation_of_a_circular_coil_experiment


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