Will rotating magnets slow down one another when interacting?

Two discs are spinning. Both have separate magnets attached. Disc one has a diameter of $8~\mathrm{cm}$ and disc two has a diameter of $4 ~\mathrm{cm}$.

The discs rotate in opposite directions. Disc one spins anti-clockwise and disc two spins clockwise. Both interacting magnets travel at the same speed and are attracted to one another. At the points shown in the diagram below, will the separate magnets slow down or speed up one another before reaching a point in which separation is forced by the rotational path of the separate discs?

Disc 1. = Anti-clockwise

Disc 2. = Clockwise

X= Permanent magnet

Z= Electromagnet

enter image description here

In the 1st image, the magnets are coming together, travelling at the same speed. However for $45^{\circ}$ X travels Z travels $90 ^{\circ}$ due to the size of the attached discs. Z is $90^{\circ}$ from the magnets confluence point (the point at which magnets are the closest together) and X is accordingly $45^{\circ}$ away. At this point the magnets begin to attract one another.

enter image description here

In the 2nd image the magnets are now attempting to attract each other and would otherwise stop or interrupt the rotation of the discs but the electromagnet Z turns off and the attraction presumably ceases. So presuming I haven't forgotten anything, can the magnets pass one another without an issue?


2 Answers 2


Switching the electromagnet on and off at appropriate times will make the arrangement into a (not particularly efficient) electric motor.

In order to use it to spin up the disks -- or just overcome the friction in the bearings for an extended time -- you will find that the electromagnet requires slightly more electric energy deposited into it in order to get it to turn on, than it delivers back to your electric circuit when you turn it off. The missing energy has gone to accelerate the disks (and some of it also lost as heat and electromagnetic radiation).

  • $\begingroup$ I agree. This is far from being an efficient device, but it will indeed work as an electric motor, using of course at least as much electrical energy than it produces kinetic energy is the disks. $\endgroup$
    – Alfred
    Nov 1, 2019 at 16:03

In answer to your 1st question, whether the magnets attract or repel depends on how their poles are oriented with respect to each other. However, as they pass the confluence point C (ie position in 2nd image) the interaction will overall have no effect on the speed of the discs. For example, N & S poles approaching each other will attract and accelerate the discs, but as they pass the attraction will decelerate them back to their original speed.

Your final paragraph suggests an "obvious" answer. If the 2 magnets interact (attract or repel, it makes no difference) but the electromagnet (EM) is suddenly switched off, then the interaction will cease from that instant and the discs will continue whatever motion they had immediately before that happened.

Perhaps you are thinking : The EM could be switched on at the position in the 1st image. The magnets then attract as they approach, accelerating both discs. At the position in image 2 the EM is switched off, the magnets do not attract as they separate, avoiding the deceleration of the discs back to their original speed. Would this work as a method of accelerating the discs?

I think where you're going with this is to ask : Can I use this to generate free energy by switching the EM on and off at opportune times?

The answer to my 2nd question above is that this would definitely not work as a means of generating free energy. As you suggest, something else will "put a spanner in the works."

But I think it may still work as a means of accelerating both discs. It will cost you electrical energy to do so of course, and this must be at least as much as the increase in kinetic energy. There will also be losses due to heating in the wires, and sparking across the switch to dissipate the energy stored in the magnetic field. (See: https://answers.yahoo.com/question/index?qid=20100316110734AAzLsIt.) However, you might be able to avoid the latter large waste of energy by incorporating the EM in an oscillating circuit with another EM or solenoid, so that the magnetic energy is stored elsewhere when the EM is "off".

An electronic circuit will be required, to match the frequency of switching of the EM to the frequency of rotation - and gradually increase the latter. I think (but I am not sure) that the interaction will keep the discs in sync - small amounts of over/under-shoot will generate a restoring force towards synchronization.

The speed of rotation will be limited by how quickly the magnetic field collapses.

  • $\begingroup$ That about covers it but If I were to have them attract, speed up the rotation and then immediately turn off the electromagnet, it would work just like that or are their other factors to take into account such as the electromagnet retaining a slight magnetic field e.c.t (I understand that wouldn't be a big issue even if it were an issue but are their any other specific factors that will "put a spanner in the works"so to speak. $\endgroup$ Jul 17, 2016 at 17:40
  • $\begingroup$ @DavidNolan : Your question in the comment above seems to be just what I'm proposing in bold. In my opinion this is much more interesting than the trivial questions in your text. Maybe you're thinking of this as some kind of energy-generating device. The PSE community will love explaining why it won't work. If this is really your intention, you should make it the focus of your text (and type it in bold). I've changed my down-vote to an up-vote already in anticipation! $\endgroup$ Jul 17, 2016 at 17:54

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