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I am not a physicist, but I know the first law of thermodynamic (conversation of energy), and I have connected this with an idea that came to my mind some time ago to create perpetual motion machine which impossible according to the first law of thermodynamic.

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This spinning wheel is made of ferromagnetic material ,So a force will be applied on the wheel causing a torque ,If we put this in place with no air ,and these are permanent magnets so they are going to apply a force permanently,and the atoms in the ferromagnetic material will not be aligned because the permanent are in opposite directions so how would this wheel stop acceleration ?

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  • $\begingroup$ I've removed a number of comments that were attempting to answer the question and/or responses to them. Please keep in mind that comments should be used for suggesting improvements and requesting clarification on the question, not for answering. $\endgroup$ – David Z Jul 12 '20 at 10:27
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You are incorrect that there is a torque on the wheel. While the field from the permanent magnet may pull a bit on some sections of the wheel, the total pull averages to exactly zero.

Yes, some of the bits of the wheel want to move clockwise to be closer to the magnet, but some other bits of the wheel want to move counterclockwise to be closer. We don't get to only consider the bits directly in front of the pole of the magnet. The entire magnetic field must be considered. When summed (assuming the wheel is homogenous), the torque will be found to be zero.

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  • $\begingroup$ What if we change the shape of the wheel not to be a circle ,like a water wheel for example or to change the angle of the magnets ? $\endgroup$ – Mohammad Alshareef Jul 11 '20 at 22:31
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    $\begingroup$ Won't help. The wheel will rotate to occupy the state of minimum potential energy (just like a rock rolls downhill). If that state is unique, it will roll there and not move away (like a rock at the bottom of a valley). If the state is not unique, then the force field won't create a torque (just like gravity can't cause a ferris wheel to rotate). $\endgroup$ – BowlOfRed Jul 11 '20 at 22:34
  • $\begingroup$ what if we applied a magnetic field that takes a circle shape in the spin what would happen ? $\endgroup$ – Mohammad Alshareef Jul 11 '20 at 22:39
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    $\begingroup$ Shape doesn't matter. Do you know (at least in principle) how to calculate the potential energy of an object in a field? Imagine how turning the wheel slightly changes that energy. If it is releasing potential energy, it will move in that direction. You would need to find a magnetic field where the energy gets lower as the wheel rotates. For a homogenous wheel, that doesn't happen. For an asymmetric wheel, there will be a minimum energy state that you can't get away from. $\endgroup$ – BowlOfRed Jul 11 '20 at 22:44
  • $\begingroup$ Thank you so much ,I admitted from the first statement that I am an physicist. $\endgroup$ – Mohammad Alshareef Jul 11 '20 at 22:47
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Though user BowlOfRed has provided an excellent answer clarifying the flaw in your argument, here's my take at trying to provide a visual reasoning as to why this idea might not work.

First of all, the direction of the forces in your illustration which comprise the couple aren't in the correct direction. Here I'm assuming your "very good" disk has no memory of magnetization whatsoever and it's a simple piece of ferromagnetic material. The best you could do was to invert the black arrows in your diagram. Still, you'd be missing something and it'd fetch you $0$ net torque as the user mentions ;)

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Here, if we see carefully, we can see the two pairs of torques (marked in violet) which are mutually cancelling each other.

Hence, the idea might not work very well.

For analysis in greater detail, I think we must also consider the interactions due to the magnets but that will complicate the matter further and moreover isn't of much use here.

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  • $\begingroup$ am not OP, but why are there 2 torques(it seems to defy my intuition)? Why do you recommend inverting the black arrows? $\endgroup$ – Viradeus Jul 12 '20 at 20:47
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This idea has popped up ever since magnets were first discovered and reappears at regular intervals since then. It is the so-called magnetic motor and you'll find lots of mostly worthless writings about it on the web.

The proof I recall about why it is in principle impossible to make one of these relies on the fact that the divergence of the magnetic field is zero. This means it is impossible to arrange a set of permanent magnets in such a way as to allow one magnet to "sneak up" on another and then suddenly be repelled or attracted to it, and then to "let go" so the next pair of magnets in the array can repeat the process and spin up a bunch of "free energy" for you.

This fact has had absolutely no influence on all the inventors and tinkerers and con men who have claimed to do the impossible and design a free energy motor based on permanent magnets. They will probably persist until the end of time.

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