I am wondering if the following device produces a forward thrust.

enter image description here

Setup explanation

The North (red) and South (blue) poles of a permanent magnet are fixed to the cart as well as the copper spokes (brown) on the insulating disc (with orange). The disc rotates in the constant magnetic field $\vec{B}$. Only half of the disc rotates in the field. Note the position of the field is vertical. and the disc is horizontal. This is similar to the setup of the unipolar generator in Faraday experiment.

Reasons for the question

In this situation I think that the Lorenz force, $\vec{L}$ pushes the electrons in the conduction layer of the copper towards the edge of the disk creating a potential difference from the middle of the disk. In this situation, the electrons are accumulating in the copper spoke at the end of the spoke farthest to the disk center. Once enough of them are accumulated there, since there is now where for them to go, the potential eventually grows large enough such the repel forces become greater than the Lorenz force, hence no electron is moving anymore in the copper spoke, which is in the magnetic field.

The question:

What happens to the electric charge accumulated at the tip of the copper spoke while the spoke rotates in the contant perpendicular magnetic field? I think that Lorenz force continues to act on each electron, although is no longer able to move them due to electrostatic equilibrium. Is the cart then going to move forward?

Looking at the electron as to a physical ball and at the copper spoke as to a tube and at the Lorenz force as to the centrifugal force, in this situation it seems that there is a centrifugal force only in the half of the disk.

Device 2

On the same lines of though, the following device is also considered: enter image description here

  • $\begingroup$ Why the downvote without written critique? $\endgroup$
    – C Marius
    Nov 10, 2023 at 14:20

1 Answer 1


You are correct that charge would get pushed from the edge of the disk to the middle because this is a Faraday Generator. To create propulsion you need to then make sure that the electrons at the center of the disk get directed backwards.

The tricky part is where the charge goes next. If you try to redirect the charge back to the disk to make a closed circuit you will also cancel out the propulsion wherever you have the charge turn around. If you don't have a closed circuit then charge will build up somewhere and your current will stop flowing and you'll stop getting propulsion very quickly.

You could maybe think up some arrangement where the cart and disk slide along a fixed wire that is connected to ground, but I haven't yet thought of an arrangement that 1) allows the disk to continuously replenish its charge, 2) forces that the charge leaving the disk to move in only one direction so you get propulsion in one direction, and 3) doesn't start generating propulsion of its own (you basically start making the cart into the sliding part of a rail-gun)

  • $\begingroup$ Thanks for your attention. Your suggestion is interesting. Reading the comment I have realized that my drawing can be simplified to focus on the main phenomenon I do not understand. I have also realized that since electrons have negative charge, the direction of the Lorenz force was wrong. My intention is to move the electrons towards the rim of the disk. The electrons move in copper spokes. I want them to accumulate at the end of each spoke. Since the Lorenz force still pushing on them, will this push the metal rod? What is the "reaction" mechanism? $\endgroup$
    – C Marius
    Nov 10, 2023 at 23:30
  • $\begingroup$ I see, I will consider this later, but my first guess is no, you will not get net propulsion because the electrons have to "reset" each go around. $\endgroup$ Nov 11, 2023 at 4:59
  • $\begingroup$ I was actually thinking about introducing some "diodes" in the copper bars to keep the charge on the rim end the whole time. That is, to have the disc in electrostatic equilibrium the whole operation time. It'll then be like spinning batteries. At the moment I do not which is the reaction force to the Lorenz force pushing on the "spokes tip" electrons ... So what force pushes back on the magnets/magnetic field ? ... Because the filed in in just one half, that half should always experience an outwards force on its rim $\endgroup$
    – C Marius
    Nov 11, 2023 at 8:42
  • $\begingroup$ But let's forget about the diodes for the moment $\endgroup$
    – C Marius
    Nov 11, 2023 at 10:16
  • 1
    $\begingroup$ I think you are right for the first diagram. To create a net force on the disk you need a charge imbalance on the disk. But that also means there's a rotating current, which essentially turns the disk into a magnet which attracts the permanent magnet backwards. If the disk and the magnet are both attached to the cart these two forces will cancel each other out and the cart won't feel any net external force. $\endgroup$ Nov 13, 2023 at 16:49

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