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Some physics problems? (Posted on 2006-04-16) Difficulty: 5 of 5
This was a problem I troubled myself with when the Physics teacher taught electromagnetism.

Assuming that we can create a gigantic alternating current generator in space, rotating a gigantic circular magnet around a gigantic wire coil to induce an alternating current, will we not get infinite energy and defeat the law of conservation of energy?

As the whole set up is in space, there is no friction whatsoever between the components though the wire naturally has some resistance. The wire's induced magnetic field can oppose the rotation of the magnet, but this can be easily be counteracted by routing a bit of energy from an external circuit to the magnet. As the magnet is circular the gravitational attraction between the circular magnet and coils cancels out. The power from the wire coil will then be connected to a device which will transmit the energy to earth via an electromagnetic wave.

The only energy inputs are that to keep the ring rotating and the initial energy input to put the setup in space. The output in energy is theoretically infinite.

SOMETHING is wrong with this argument that I later discovered myself. The Physics teacher was unable to detect it and walked away quite puzzled. Can you guys find out why this would never work?

See The Solution Submitted by Jack Lim    
Rating: 3.5000 (2 votes)

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Much simpler answer (or - engineers triumph again) | Comment 9 of 13 |

Although I cannot disagree with most of the previous postings, there is a much simpler answer.  Dej Mar was close, as were others, but not to the exact point.  Also, being in space is a total red herring.

A perfect generator is one that is 100% effcient - the energy in = the energy out (no losses whatsoever).  In any generator, in any environment, there is a torque that MUST be applied to turn the rotor past the coils. The more electical energy produced, the higher the torque.  If any of the output is shunted to assist the magnetic field, that just means less output.  in other words, Kinetic enertgy in = Electrical energy out, at best, when there is theortically no friction, no heating, no windage, no losses of any kind. 

What gets most people on this type of problem is that there is a belief that you just have to spin the gnereator.  Not true.  Look up the principle of back EMF and also the force necessary to move a conductor in a magnetic field.  Conservation of energy holds!

 

Edited on April 17, 2006, 7:41 pm
  Posted by Kenny M on 2006-04-17 19:39:16

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