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Homework Help: Rail Gun physics, need assistance

  1. Jan 1, 2005 #1
    I am working on a project for graduation and I am writing a paper on rail guns, but I am having trrouble understanding the basic principle behind it, Loretnz Force. I've tried to look online for a good site that explains it in laymens terms, but I cant seem to find one.

    I know that current flows one way on one rail and int he opposite direction, connected by an armature. And a force is created pushing the armature forward.

    Can someone explain why this force occurs?

    I thought I read somewhere that the "opposite reaction" of the force pushing the projectile is a force pushing the rails apart. Is this true? And if so does that mean that it doesnt have recoil backwards like conventional guns and the "recoil" is sidewards?

    Any other information about the physics of railguns would be very much appreciated. And please respond as if you where explaining it to a highschool senior that is in the middle of his AP Physics BC course that has only done mechanical physics so far.
  2. jcsd
  3. Jan 1, 2005 #2


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    Rail gun physics

    The currents in the parallel conductor introduce a magnetic field, [itex]\vec{B}[/itex]. The same current [itex]\vec{I}[/itex]passes through the armature, which is perpendicular to the parallel conductors.

    The force [itex]\vec{F}[/itex] is then given by [itex]\vec{F} = k \vec{I}\,\times\,\vec{B}[/tex], where k is a proportionality factor.

    This is the Lorentz force on a charge q moving with speed v is given by [itex]\vec{F} = q \vec{v}\,\times\,\vec{B}[/itex]

    See also -


    http://www.physics.northwestern.edu/classes/2001Fall/Phyx135-2/19/railgun.htm [Broken]

    Most college textbooks on Introductory Physics would provide the basic physics behind this.

    Wires with currents moving in same directions are attracted to one another, and opposite currents repel. A strong reinforcement is then needed. With high currents and therefore high magnetic fields, the magnetic pressure may cause warping of the railgun.

    The railgun does experience a reaction force from the accelerating projectile.
    Last edited by a moderator: May 1, 2017
  4. Jan 1, 2005 #3
    I read both sites before here it states:

    The current flowing through the field creates a Lorentz force on the inductor tending to push the coils apart. If one portion of the coil is free to move, this portion will slide in a direction which maximizes the size of the current loop.
    In a railgun, the inductor is a single-turn coil consisting of the power source, rails, and armature. The armature is free to slide along the rails, so the Lorentz force causes it to slide along the rails, away from the power source.

    It says the loretnz force pushes the coils apart, but what are the coils? They say that the rail gun is a single turn coil meaning one coil so what coilS are being pushed apart?
  5. Jan 1, 2005 #4


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    As you mentioned-
    I corrected my original response. Parallel currents attract and anti-parallel currents repel - so in a railgun the rails or parallel conductors (with opposite currents) are pushed apart.

    I have seen actual ballistic railguns in which the rails consisted of two curved conductors that were inlaid in a reinforced cylindrical insulator. The projectile was a specially designed round geometry, looking something like an armor-piercing shell. The current then arcs across the gaps between the projectile and the parallel rails. If design properly, the projectile does not touch the rails.
  6. Jan 1, 2005 #5
    Thank you very much for the help.

    One last clarification, so because the lorentz force is pushing the rails apart and the rails are held in place, the force is applied to the only part of the "coil" that can move, the armature, is this right?
  7. Jan 1, 2005 #6


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    Yes - the armature is free to move and is accelerated by the magnetic force imposed on the current flowing through the armature.

    The structure supporting the rails will experience a force equal and opposite that of the projectile, and it will experience the force of the conductors (rails) being pushed against it.

    With potentially high currents and magnetic pressures, this can be very dangerous if the structural support of the rails is not properly designed, i.e. it would be like a pipe bursting.
  8. Jan 1, 2005 #7
    Ah ok, thank you very much. lol im sorry but one more thing, if the equal and opposite force is applied to the structure then is there a backward force like conventional guns? if so where does that force come from if the force from the projectile is exerted sidewads?
  9. Jan 2, 2005 #8
    *bumpidy bump bump*

    Just the last question, planning to use it in the paper
  10. Jan 3, 2005 #9
    Recoil does not depend on the mechanism used to accelerate the projectile.
    Whenever you accelerate something, there is a force in the direction of acceleration. This is the force the gun exterts on the projectile. There is an equal and opposite force acting on the gun, which accelerates the gun. If you mount your gun on a concrete block, this block will be a part of the gun and therefore accelerated with it.

    The forces which act perpendicular to the force acting on the projectile do not contribute to the reaction force. They cancel each other out because they are equal on both rails. So the reaction force must be acting on the whole gun.

    So yes, rail guns do have recoil just as any other gun.
  11. Jan 3, 2005 #10
    Oh ok, so even like a sling shot has backward recoil, ok... Thanks
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