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Hellow everyone,

I got this question that has been teasing me for a while now :

I see how Lorentz force can act on a "free" moving (in a magnetic + electric field) charge, however, it is difficult for me to understand for instance, how can a steel bar carrying an electric current and placed in a magnetic field, is moved by the magnetic field.

!!! the idea of considering the steel bar as a bunch of moving charges that can be pushed or attracted by the magnetic force, didn't seem reliable because electrons are jumping from one ATOM to another, hence what is not moving are the atoms, and they make up the whole mess the steel bar.

So how does the steel bar move, do the electrons drug the atoms with them in the direction of magnetic force (and not the direction of the electric current), or is the magnetic force exerted on the atoms.

I hope I was clear in my description if I wasn't, here a simplification :

  1. Steel bar placed in a magnetic field and carrying an electric current (moving electrons)
  2. The magnetic field will act on the electrons to deviate their trajectory.
  3. The whole bar moves, why? shouldn't only the electrons be moving. Do they drag the atoms with them?

Thank you in advance

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A current is indeed subject to force when in a magnetic field. The example of an iron bar is confusing (the iron would magnetize, and exhibit torque or other force effects), so let's just call it a conductor.

In any current-carrying conductor in a magnetic field, the current carriers are nearly-free electrons inside the conductor. Those outer shell (conduction) electrons are free to hop from atom to atom. But even if you apply force to those electrons, they are NOT free to leave the metal's outer surface. The (net positive) charge of the atomic nucleus (plus its inner electrons) attracts (binds) the "free" conduction electrons.

shouldn't only the electrons be moving. Do they drag the atoms with them?

Yes, in a sense the electrons do drag the atoms with them (or the atoms drag the electrons-it's Newton's 'equal and opposite' reaction). Atoms don't spontaneously disassemble.

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    $\begingroup$ Thank you for your answer it really helped me. However, I still have a little question on the "electron-Atom" dragging force. For example, if the electrons are going in the -x direction and the magnetic field is in the +y direction than the force would be in the +z direction, wouldn't the electrons change their trajectory (+x direction), (shouldn't the electron velocity now have a z component, due the magnetic force) Thank you in advance $\endgroup$
    – Edwardo Newagte
    Commented Jan 8, 2018 at 21:34
  • $\begingroup$ @EdwardoNewagte: Yes! That describes the well-known Hall effect. The effect in a pure conductor (a metal) is difficult to discern, but not in semiconductors. A variety of sensors and switches use it. The initial piled-up electrons at the (+z) edge creates a small voltage, and their electric field 'corrects' the average trajectory back to straight in the -x direction. $\endgroup$
    – Whit3rd
    Commented Jan 8, 2018 at 23:37

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