Is frictionless vacuum possible?

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Would a piece of metal suspended between two magnets in a vacuum-sealed jar be in a situation of frictionless vacuum? If yes, could such a situation be exploited to build a perpetual motion machine? If no, would the friction be enough to result in an appreciable loss of energy if the piece of metal was to be set in motion?
 

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  • #2
Drakkith
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Let's be clear. A piece of metal set in motion in a perfect vacuum could be considered to be in perpetual motion, however it is not a machine. A machine is something used to accomplish a task, something you get work out of. A perpetual motion machine is not possible and is against PF rules for discussion, so just be careful.
 
  • #3
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Sorry! Bad terminology usage. I didn't mean any offense to the law of conservation of energy. Could you call that a perpetual motion apparatus? Is that possible to realize?
 
  • #4
Drakkith
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I am unsure if your specific example would work, however it seems plausible to me.
 
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"...metal suspended between two magnets...in motion...."

WAG here, but eddy currents might come into play and eventually damp the system.
 
  • #6
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A spacecraft in the vacuum of space could also be considered in perpetual motion (even though even the vacuum of space is not perfect). No need to invoke magnets, it'll keep going almost forever. However, once you try to get some work out of it you WILL slow it down, one way or another.
 
  • #7
russ_watters
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Could you call that a perpetual motion apparatus? Is that possible to realize?
It all depends on how picky you are. The earth has been orbiting the sun for some 4 or 5 billion years and will orbit for several billion more. It won't orbit forever, nor will your magnetically levitated object spin forever. So you'll have to decide if that's close enough for you.
 
  • #8
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Drakkith: thanks then.

Greg23: I'm not trying to get work out of it. Sorry if my wording was wrong.

Lsos: except I can't use a spacecraft as a ornament for my desk (the intended use for my apparatus)

russ_watters: if I could put it on my desk and die of old age before it stops spinning I would be much more than satisfied. So I guess it could work!

There one question left though. How can you set in motion something sealed in a jar?
 
  • #9
phinds
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There one question left though. How can you set in motion something sealed in a jar?
Start it spinning before you close the jar and pump out the air
 
  • #10
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I understand now what you're trying to do. Someone correct me if I'm wrong, but isn't it impossible to keep a piece of metal stabilized suspended in air with only permanent magnets?
 
  • #11
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"Greg23: I'm not trying to get work out of it. Sorry if my wording was wrong."

You wording seems fine.

But, again a WAG, it seems that any metal object moving in a magnetic field will have eddy currents generated inside the metal. There is a YouTube video showing a magnet falling in a copper tube that is slowed by eddy currents. In your case the magnetic field is fixed and the metal is moving. Same result. Small, but potentially fatal.
 
  • #12
f95toli
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but isn't it impossible to keep a piece of metal stabilized suspended in air with only permanent magnets?
True. This is one reason for why superconducting flywheels are quite attractive (superconductors are diamagnetic, so they CAN be used to create stable configurations). This is essentially what the OP was describing. A flywheel can essentially be used as a battery, and has -in certain circumstances- several advantages over a chemical battery for energy storage.

However, no flywheel is perfect, and eventually it will come to a stop.
 
  • #13
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I'm almost sure you can keep a piece of metal suspended for a short while if its spinning... There was some kind of toy doing it.
 
  • #14
f95toli
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I'm almost sure you can keep a piece of metal suspended for a short while if its spinning... There was some kind of toy doing it.
Yes, a spinning top can be made stable (with some difficulty). But the nice thing about superconductors is that you don't need anything to hold the wheel BEFORE it starts spinning.
 
  • #15
Hardest part may be to keep the metal in the magnetic field while you pump out the air. It will certainly try to escape through your opening, affecting your ability to continue pumping and to continue rotation.
 
  • #16
Drakkith
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Hardest part may be to keep the metal in the magnetic field while you pump out the air. It will certainly try to escape through your opening, affecting your ability to continue pumping and to continue rotation.
Unlikely. Most vacuum pumps don't suck air out very quickly. And since your don't have a FLOW of air through the chamber, unless the object is extremely close to the opening, there won't be any wind.
 

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