Why can't weak core-less electromagnets be used in conjunction with strong permanent magnets to perpetually harvest energy?

Magnets will attract ferrous materials but some materials like copper or silicon are not effectively magnetic unless a current is passed through them, if that is the case then why can't we arrange a core-less electromagnet made of copper in conjunction with strong permanent magnets to create a system in which one could easily harvest energy? Once a weak current is passed through the core-less electromagnet it could become attracted to a strong permanent magnet due to a relatively weak magnetic field surrounding it, energy would be harvested by means of compression between the electromagnet and the strong permanent magnet, the electromagnet would be turned off and another permanent magnet would drag the electromagnet away by attracting an attached permeable material, then the electromagnet could be switched back on and the cycle would reset.

The simple device below portrays my question in better light

A= Weaker permanent magnet

D= Ferrous material

C= Core-less copper electromagnet (Copper circuit)

B= Very strong permanent magnet

(A and D are both out of the effective magnetic range of C and B)


D is instantly attracted to A

enter image description here

A very weak current in passed through C creating a weak but existent magnetic field, it's instantly attracted to B the much stronger permanent magnet, dragging D from A, since C's current/magnetic field can be extremely weak as long as B's magnetic field is extremely strong, couldn't one perpetually harvest energy from this arrangement?

I had another picture but I don't have enough reputation to post it

I'm not overly familiar with the formal physical laws that will most likely prevent a system like this from working so a practical answer will be highly appreciated, I hope these images will help explain my question.

Thank you.

  • $\begingroup$ Interestingly, we do have use for a similar circuit in generators, but we don't use it to generate free energy (there's no free lunch). In modern generators, we have very large powerful electromagnets, because using permanent magnets would be prohibitively expensive. We waste a little of the generator's production to keep this electromagnet operating (wasted by turning into heat). However, when you want to start the generator up, there's no power to start the electromagnet! Our solution: a small permanent magnet that permits generating just enough power to feed the $\endgroup$
    – Cort Ammon
    Dec 1, 2016 at 3:52
  • $\begingroup$ electromagnet, so that it can "bootstrap" itself. It's no perpetual motion machine. There's waste heat all through the process, but it turns out to be a more cost effective way of generating power than trying to use all permanent magnets! $\endgroup$
    – Cort Ammon
    Dec 1, 2016 at 3:53

1 Answer 1


The "formal law" that prevents this is called conservation of energy. The problem is that it costs energy to generate a current in the electromagnet - and more importantly, according to Lenz's law you will need to put in additional energy to maintain the flow of current when the coil moves towards the permanent magnet because the changing flux in the coil (the fact that it "feels" a stronger magnetic field as it gets closer) induces a "back EMF" (reverse electromotive force). Even if you had a cool with zero resistance that effect would still be there - and it turns out it exactly cancels any energy you could "harvest".

There is no free lunch - not even in physics.

  • $\begingroup$ So is it possible to exactly cancel the force so that we could use the electromagnet to pull D from A, the compression between C and B would "break-even" the cost and we could harvest the compression of D and A? $\endgroup$ Jul 13, 2016 at 11:35
  • $\begingroup$ What do you mean by "harvest the compression"? If work is done in one part of the system it will affect the forces (and need to do electrical work) in another part. This is how electrical motors work - when you put a bigger load on them they require more current to operate. $\endgroup$
    – Floris
    Jul 13, 2016 at 11:49
  • $\begingroup$ Is it possible to "break-even" (with the current) with a device like this and if so how hard is it, also besides Lenz's law are their any other laws that prevent this from creating energy or are the only problems the energy cost of creating the magnetic field and the additional energy cost due to the "back EMF?" Appreciate the help btw $\endgroup$ Jul 13, 2016 at 13:18
  • $\begingroup$ David, people try to create all kinds of devices where they think the conservation of energy is violated, but they never show a single calculation. For a physicists it is not necessary to explicitly show a calculation for every device, as we know that energy is conserved by the forces involved in theses devices. Those are theorems, that is, almost as good as explicit calculations for specific devices. $\endgroup$
    – user65081
    Jul 13, 2016 at 14:51
  • $\begingroup$ You asked "are there any other laws that prevent this". The problem is that the ensemble of physical laws all come back to a few basic principles, including conservation of energy. For example, the laws of thermodynamics are in some sense a statement of conservation of energy. They are sometimes simplified as: "(1) you cannot win, you can only break even; (2) you can only break even at absolute zero; (3) you cannot reach absolute zero". The short answer remains: physics says this cannot be done. Unless you give a detailed calculation that appears to show otherwise, there's not much to add. $\endgroup$
    – Floris
    Jul 13, 2016 at 15:04

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