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I know it's not possible to build a perpetual motion machine, but I still got this concept running in my mind.
If you take a (longer)copper coil, and insert an axis through it(right-angled) with a magnet fixed on it,
it would generate electricity when you spin the axis. The axis and magnet rotate, thus alteration of the flux, thus electricity.

Of course normally there is a soft-iron core inserted in the coil to achieve better efficiency,
but it also brakes the movement. If you don't use a core and insert the axis+magnet into the coil,
you'd still generate a certain amount of electricity. The magnet is only braked by the friction of the axis and the air, but this friction is almost insignificant.

So if you use good bearings to reduce this friction to a minimum, and you take a coil with as much as possible windings, and the strongest magnet you can find, you could in theory build a perpetuum mobile right? I'm almost sure I'm not right but I'm asking anyway. Just curious.
Any comments or responses are welcome.

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  • $\begingroup$ I don't understand what are your input. It's seem that you're proposing a symple dynamo? $\endgroup$ – sailx Jul 13 '14 at 12:59
  • $\begingroup$ yes kind of, but without a soft-iron core so the magnet is only braked by friction of the axis and the air. But you could reduce this to a minimum, while making the current as big as possible by using a very strong magnet and a coil with as many as possible windings. $\endgroup$ – Bart Jul 13 '14 at 13:03
  • $\begingroup$ you will have loss of energy : Joule effect RI^2 and loss of the magnetic flux. So to keep the movement you need an additional energy input. $\endgroup$ – sailx Jul 13 '14 at 13:10
  • $\begingroup$ I've tried the principle in a very simple way by using an ignition coil from a car and a the strongest magnet i could find. rotating the axis at a speed of app.800 rpm gave 200mA. I don't know how much energy is needed to rotate the axis if it has good bearings. $\endgroup$ – Bart Jul 13 '14 at 13:15
  • $\begingroup$ But did you put a load on it? 200mA in a very conductive wire is not difficult. Now attach a 1000ohm resistor and watch the magnet slow down. $\endgroup$ – BowlOfRed Aug 16 '14 at 22:49
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It is incorrect to state that

The magnet is only braked by the friction of the axis and the air.

It is not clear to me exactly what you're proposing, but whenever you deliver electrical energy to some external system you will slow down the rotation of the coil. This is usually through an inductive torque caused on the rotating coil by the currents that run on the rest of the dynamo; air resistance and friction go on top of that.

If I understood correctly, you are describing a situation like the following:

enter image description here

Assume, for the sake of simplicity, that the rotor coil made of superconducting wire so that it carries a current without the need of external intervention. As the rotor turns, you get an oscillating magnetic flux through the stator coils, and this gives you 'electricity': to be precise, this gives a potential difference between the ends of the stator.

If you leave it like that, then this device will indeed, in principle, run forever. This is contingent on the fact that there be no additional friction or residual electrical resistance, but the essential thing is that the potential difference across the stator coil terminals is not being used to deliver power to any external system.

If you do connect this to some external load, though, then you will get a current flowing through the stator coil as well. Furthermore, this will be oscillating together with the rotor, which means you get a changing current, hence a changing magnetic field, and thence electric fields. This will have one of two effects: it can damp down the current in the rotor coil, or it can slow down the rotation. Which one happens depends on what external constraints you're imposing, but the key point is that both tend to stop the functioning of the machine.

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If you are able to reduce friction to a minimum, but not to zero, may be able to run your machine for a long time, but not perpetually. Any energy loss must eventually be offset by the introduction of extra energy into the system.

If you're really interested in running forever, there's no such thing as an insignificant energy loss.

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  • $\begingroup$ no the friction cannot be disabled, but you could add more and more windings in the coil. and make the magnet stonger and stronger. This way you could get an output which is sufficient to equalize the friction was my idea. $\endgroup$ – Bart Jul 13 '14 at 12:51
  • $\begingroup$ @Bart Without a supply from outside, where do you think the power needed to counteract friction is drawn from? Has to come from somewhere, since you cannot generate energy out of nothing. $\endgroup$ – Nephente Jul 13 '14 at 13:52
  • $\begingroup$ @Bart Unless you are changing the size of the coil or the strength of the magnets as your machine runs, any dissipation will eventually consume all of its energy. Eventually. $\endgroup$ – rob Jul 13 '14 at 14:03
  • $\begingroup$ @Bart wrote "This way you could get an output which is sufficient to equalize the friction was my idea." Bart, if this reasoning were correct, then it follows that adding even more windings and/or an even stronger magnet (than that required to overcome the friction) would create an inexhaustible supply of energy. $\endgroup$ – Alfred Centauri Jul 13 '14 at 14:14
  • $\begingroup$ @AlfredCentauri yes indeed the sentece suggests that. double the amount of windings and magnetfield causes a greater current but it doesn't increase the friction right? I know this is bullshit and all but in theory somethings can be different, whatever $\endgroup$ – Bart Jul 13 '14 at 16:26

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