A few years ago I went to a museum, where there was a board with 2 bar magnets, on a pole each so they could rotate. If you rotated them so the lined up with the same poles (N) facing each other, they'd repel spinning until the south poles lined up, then they'd repel and spin so the north poles lined up and so. This was without any extra help, once you lined them up they would just keep spinning by their selves. How exactly is this possible that the 2 magnets kept producing kinetic energy?
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$\begingroup$ Did they continue there movement indefinitely or stopped? $\endgroup$– SMUsamaShahDec 16, 2010 at 10:31
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$\begingroup$ What means "on a pole"? Something like compass needles on points? Was there a base made from wood or plastics carrying the magnets? Or was the arrangement fixed on a table? Such gadgets typically contain some coils and electronics and a battery to avoid conflict with first law. $\endgroup$– GeorgDec 5, 2011 at 10:42
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1$\begingroup$ So anyway energy was stored in the magnets [by placing them in the desired position] and as a result another form of energy was formed [k.E]. So can we call this a renewable energy? Can it be a good source of energy? $\endgroup$– user14342Oct 25, 2012 at 0:21
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1 Answer
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They are not producing energy; their movement is a result of the work you've put during setting same poles to face each other. They will eventually stop, since this energy is dissipated by friction on their fixings and at some point their momentum will decrease to the point it will be not enough to push the system through energy pick when the same poles are nearby. After this, they will relax to their energy optimum, so probably into configuration when different poles are facing each other.
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$\begingroup$ I didn't start the magnets spinning, I just held them still so the North poles were exactly opposite each other and then let go, so hardly any energy was put in, and yet the magnets were still spinning by the time I got bored of watching them. You said they do not produce energy, yet you then said this energy will be dissipated through the fixings. $\endgroup$ Nov 2, 2010 at 21:40
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3$\begingroup$ @Jonathan: But you PUSHED the magnets together from their optimal positions. The magnets simply converted some of their potential energy into kinetic and eventually to internal energy through friction. $\endgroup$– VorticoNov 2, 2010 at 21:53
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$\begingroup$ @Vortico yes, but if I just pushed 2 non magnetic bars (of the same density and size) together in exactly the same way (same time and same energy) they wouldn't spin? $\endgroup$ Nov 2, 2010 at 21:58
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1$\begingroup$ @Jonathan - No, because the magnets produce a force on each other. By positioning the magnets from S-N S-N to S-N N-S you have done work on the system by over coming the magnetic field and put the system into a state of high potential energy. That potential energy is then converted to kinetic energy in the movement you saw and will eventually be dissipated by friction just like Vortico said. $\endgroup$– DavorakNov 2, 2010 at 22:08
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2$\begingroup$ I guess that does make sense after thinking about it but I would never have thought the energy I put into setting them = the spinni :) $\endgroup$ Nov 2, 2010 at 22:35