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I'm part of a group working on a Physics II project based on electromagnetism, and my group decided to create a proof-of-concept railgun, shown here:

https://doityourselfgadgets.com/2013/10/homemade-railgun-experiment.html

Instead of creating the full approximation of a railgun (since we were short on time), we created the small one consisting of a battery and aluminum foil rails. In that article, the author uses small, round magnets on the ends of the metal rod in order to "amplify the effect." However, my group and I can't really find any definitive answers as to exactly WHY this amplifies the effect and causes the rod to move faster. Can anyone help out with some answers or thoughts? Thank you!

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    $\begingroup$ I'm not sure about "amplifying the effect with magnets" but to understand the very basic of how railgun work see [web.mit.edu/mouser/www/railgun/physics.html] $\endgroup$
    – user6760
    Commented Apr 16, 2015 at 3:26
  • $\begingroup$ There are railguns with and without external magnetic field. For small currents adding an external magnetic field increases the forces on the projectile, but for very large currents (MA), the magnetic field created by the current itself dominates and adding ferromagnetic materials becomes a problem: the railgun's magnetic field would simply destroy them (mechanically and by changing their magnetization). $\endgroup$
    – FlatterMann
    Commented Sep 25, 2022 at 4:18

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I can't see why they would. A railgun works because of the Lorentz force $\vec F = q\vec v \times \vec B$ on the charges moving in the projectile. That force is perpendicular to both direction of the current flow, $\vec v$, and the direction of the magnetic field, $\vec B$, because of the vector cross product.

If the coin magnets and the ends of the railgun's crossbar were oriented with their field axes parallel to the direction of the crossbar, so that an iron crossbar would want to act like a big bar magnet, then $\vec B$ is parallel to the current and there is zero extra Lorentz force.

If the magnets were oriented so their field axes are perpendicular to the current, then they will rotate with the bar, and any "amplification" will take place only half the time.

It's possible there's a second-order effect I haven't considered; I'm curious to see other answers.

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    $\begingroup$ I am unsure what you mean - if the magnets are positioned properly they can produce an external magnetic field that is perpendicular to the current - thus adding an additional term in the Lorentz force due to the external magnetic field. Imagine a constant magnetic field in the direction perpendicular to current flow and calculate the resulting Lorentz force. In other words, the two magnets on the sides have antiparallel 'field axes' (approximating them as dipoles). $\endgroup$
    – Akerai
    Commented Mar 12, 2021 at 19:28
  • $\begingroup$ I think I understand the issue. I just want to make it clear that all the railguns I've ever worked with (up to 13MA drivers) did not induce rotation in the armature. Naturally, it is a better design choice to have the external magnets placed separately from the armature to evade this issue, and lower the mass of the projectile. $\endgroup$
    – Akerai
    Commented Mar 12, 2021 at 19:52
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I think it's because you get an additional Lorentz force component due to a current in fixed external magnetic field. One picture is usually worth many words.

enter image description here

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