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Magnetic motor (contradicts 2nd law of thermodynamic? )

  1. Aug 16, 2012 #1
    Hi, everyone ! I am a new member to PF but I find it helpful as people will give ideas and suggestions for me :)

    Recently,I come across something called as a magnetic motor. From the design, a few permanent magnets are placed on the edge of a circular disk. Then, as another magnet approaches the disk, the disk spins due to the attraction between the magnet. Due to continuous magnetic attraction, the disk seems able to spin forever, like a perpetual motion machine. However, I realise that the 2nd law of thermodynamics does not allow perpetual motion. So, does the magnetic motor contradict with this law? Besides, I always come across the term ' free energy' which raises my doubt whether it's truly 'free'?

    I hope that PF members can help me to clear my doubt. I am truly eager to know the physics and logic behind this contradiction.

    Thank you everyone ^^
     
  2. jcsd
  3. Aug 16, 2012 #2

    Drakkith

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    Staff: Mentor

    Welcome to PF! Unfortunately discussion of perpetual motion machines is not allowed here on PF due to the tendency for the discussions to get out of hand with crackpottery and nonsense. I will say that you have forgotten that when magnets attract each other they don't simply stop attracting after they pass by each other, so the disk has no way to spin past each magnet with more energy than someone was to give it initially. No power can be produced. No violation of the 2nd law.
     
  4. Aug 17, 2012 #3
    'so the disk has no way to spin past each magnet with more energy than someone was to give it initially'

    I don't really get the reason behind actually. Would you mind explain the sentence above? My doubt is why will the disk spin faster and faster if no energy is used to do that? Is magnet the one which provides this energy? Then,if there's a continuous attraction between the permanent magnet on the disk and the magnet near the disk, will the disk spin forever?

    Thank you for your explanation :)
     
  5. Aug 17, 2012 #4

    Drakkith

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    Staff: Mentor

    It won't spin faster. The magnets will attract each other equally as they get closer to each other and as they get further away. There is no net acceleration, the disk will simply spin down as it loses energy to friction.
     
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