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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?

  Submitted by Jack Lim    
Rating: 3.5000 (2 votes)
Solution: (Hide)
Newton's Third Law of Motion: Every action has an equal and opposite reaction.

When objects are launched into space, they receive an initial velocity; however, producing acceleration is a problem. The generator COULD beam energy back to Earth, but the problem is that it's theoretically impossible to rout part of the energy output to keep the coil rotating. Acceleration is required to counteract the magnetic forces that would eventually cause the generator to stop rotating, if unchecked.

How are spacecraft propelled in space?

Rockets.

Nobody has ever used a propeller as there is nothing to push. In order to produce motion in space, ironically, the only way to to push a mass in the opposite direction. This would create a reaction force that produces the acceleration required to keep the magnets rotating.

But once you throw mass away like that, it can never return without expenditure of energy to bring the mass back. In space rockets, they carry enough fuel to expend through the entire trip. However, if we are going to be talking about the magnet rotating for a long time (protons have a half-life of 10^35 years if I remember correctly), it's not possible to carry enough mass onboard.

Creation of mass is impossible, and even if it were not, E=mc^2, so the energy required to create that bit of mass is ridiculously high. Propelling masses to the generator to supply it with "fuel" to throw back would involve an energy input.

Under these circumstances, a space generator will end up expending more energy than it could produce. So the big hole in the argument is the simple statement "this can be easily be counteracted by routing a bit of energy from an external circuit to the magnet" as this is actually nowhere as easy as it sounds.

So the law of conservation of energy holds!

Comments: ( You must be logged in to post comments.)
  Subject Author Date
re(2): thermodynamics (& other musings)matt2006-12-10 00:04:50
re: thermodynamics (& other musings)Kenny M2006-09-29 16:07:33
Solutionthermodynamicsmatt2006-08-06 12:50:21
One more thing...Kenny M2006-04-18 18:29:46
Much simpler answer (or - engineers triumph again)Kenny M2006-04-17 19:39:16
SolutionNo SubjectCharlie2006-04-17 09:23:37
re(4): E = mc^2Hugo2006-04-17 08:29:14
re(3): E = mc^2Eric2006-04-17 06:44:01
re(2): E = mc^2Dej Mar2006-04-17 06:36:15
re: E = mc^2Hugo2006-04-17 05:43:26
QuestionE = mc^2Dej Mar2006-04-17 01:59:53
just a matter of inductanceLeming2006-04-16 18:01:30
I have trouble with...Hugo2006-04-16 14:22:41
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