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idea about perpetual motion and want to know why wont work

Posted: Wed Oct 29, 2008 4:45 pm UTC
by urmommawasfun
okay so a couple nights ago i had this idea for perpetual motion. I do not believe this will work now i am searching for a good explination on why it wont work. Iv ask alll the science and math teachers and none know why it wont work can you tell me????

Re: idea about perpetual motion and want to know why wont work

Posted: Wed Oct 29, 2008 4:47 pm UTC
by Azrael001
I assume that the inside is a vacuum? I don't think that magnets keep full strength permanently, so the whole thing may lose power over time. I could be wrong though.

Re: idea about perpetual motion and want to know why wont work

Posted: Wed Oct 29, 2008 4:59 pm UTC
by seladore
I can't see what would make it turn in the first place. There is no reason for the clockwise moments to be different from the anti-clockwise moments. Plus the fact that your outer, fixed magnets will have positive polarity between them, which also imparts a force on the inner ring.

To actually get the thing to spin, I think you would have to alternatively power the magnets on opposing sides, and the energy needed to do this would be greater than the energy obtained from the torque.

Re: idea about perpetual motion and want to know why wont work

Posted: Wed Oct 29, 2008 5:01 pm UTC
by simdude
Well I THINK the reason it doesn't work is because you have to remember that those outside magnets aren't monopoles and DO have a positive side. The machine will eventually find some sort of equilibrium where it's being equally attracted and repelled.

Bascially youtube perpetual motion device and you'll get all sort of magnet based ones like this and I think I've seen one or two actually built and though a video would never show it slowing to an equilibrium I remember seeing one where it needed the initial spin and just based on the fact it needed an initial spin says that some sort of equilibrium exists.

Re: idea about perpetual motion and want to know why wont work

Posted: Wed Oct 29, 2008 5:07 pm UTC
by seladore
Exactly... from a standing start, the thing is already in equilibrium, as the anti-clockwise moments are equal to the clockwise moments.

As soon as you draw the missing 'plus' signs on, it becomes easier to see.

Re: idea about perpetual motion and want to know why wont work

Posted: Wed Oct 29, 2008 5:10 pm UTC
by Bluggo
Even if the magnets keep full strength, this wouldn't work - the structure will reach an equilibrium position, in which the all the forces cancel out one another, and then stop.

It would take some calculations to find out the equilibrium position (I suspect that it will be one with the inner magnets close to the outer ones, and the repulsion of their - side being balanced with the attraction of the + side, but I am not completely sure of that), but the existence of such a position can be derived easily from basic electrodynamics.

The field generated by magnetes is a so-called conservative field, that is, it can be described in terms of potential (something similar to the potential gravitational energy mgh), and since this potential is a continuous function over a compact space it has a minimum.

The inner wheel will move until it reaches one of these minimum positions (of course, you have to describe the potential in terms of the degrees of freedom and of the magnetic setup of the inner wheel, but these are just computational details) and then it will stop moving - probably, after oscillating for a while because of inertia.

If you assume no attrition, it is possible that the oscillation will continue indefinitely; but that's hardly surprising, and will generate no "additional" energy - it would be a perpetual motion only in the sense in which a planet rotating around a sun in perfect vacuum is perpetual motion, and it would violate no physical laws.

Re: idea about perpetual motion and want to know why wont work

Posted: Wed Oct 29, 2008 6:53 pm UTC
by Monty40xi
Friction kills most perpetual motion ideas, and it'll hit your idea too. There will be friction in your central axel, steadily bleeding off the kinetic energy and turning it into heat. Nothing in your system would turn that heat back into kinetic energy (can't be done with 100% efficiency so don't get pulled down that tangent), so over time your system has to lose its energy.

...plus, as the others have pointed out, the magnetic fields won't actually give you the torque you're thinking they will. Try building the outer ring with its 8 magnets and then using iron filings to look at the field lines. That should help things make sense.

Re: idea about perpetual motion and want to know why wont work

Posted: Wed Oct 29, 2008 9:28 pm UTC
by danpilon54
Monty40xi wrote:Friction kills most perpetual motion ideas


This would be true if "perpetual motion" meant simply moving forever. This can easily be achieved by being in orbit etc. Its more about doing work, but needing no energy input, ie the creation of energy. [/nitpickiness]

Also yes, there will be an equilibrium there due to the positive poles that must exist and the whole counter-rotational invariance thing.

Re: idea about perpetual motion and want to know why wont work

Posted: Thu Oct 30, 2008 9:53 am UTC
by Bobber
HOW can you guys NOT realize that OP is a troll?

Shitty nickname, shitty postcount shitty spelling, shitty paint drawing and a shitty perpetual motion machine idea.

Seriously.

Re: idea about perpetual motion and want to know why wont work

Posted: Thu Oct 30, 2008 11:42 am UTC
by seladore
Bobber wrote:HOW can you guys NOT realize that OP is a troll?

Shitty nickname, shitty postcount shitty spelling, shitty paint drawing and a shitty perpetual motion machine idea.

Seriously.


Really? Quite a dedicated troll to make a diagram.

Re: idea about perpetual motion and want to know why wont work

Posted: Thu Oct 30, 2008 2:09 pm UTC
by Bluggo
Bobber wrote:HOW can you guys NOT realize that OP is a troll?

I dunno... he has been perfectly polite, as far as I can see, and his question was not really divisive or inflamatory - he just asked us a physics question, that's all.

Now, if he had said something like
Har, I have this megnificent perpetual motian idea, if u dont'understand it ur stupid... how do I maek $$$ w it???

you would have a point, but he did not... or at least, not yet ;-)

EDIT:
I have another question about this general topic.
Would a Brownian motor violate the second law of thermodynamics?
If not, why? As far as I can see, it extracts work from a single source of heat...

Re: idea about perpetual motion and want to know why wont work

Posted: Thu Oct 30, 2008 3:08 pm UTC
by Uber_Apple
Does a current circulating in a ring of superconductor circulate forever assuming that superconducting conditions can be kept forever [ignoring the energy required to cool down the superconductor/ using a very, very,very high temp (room temp) superconductor (probably doesn't exist and never will but... mebbe?)] as long as we don't try and extract energy from the current.

what causes the current to slow down and eventually stop if not?

Re: idea about perpetual motion and want to know why wont work

Posted: Thu Oct 30, 2008 4:35 pm UTC
by Bluggo
Vivek wrote:Does a current circulating in a ring of superconductor circulate forever assuming that superconducting conditions can be kept forever

Yes.
Wikipedia wrote:Experiments have demonstrated that currents in superconducting coils can persist for years without any measurable degradation. Experimental evidence points to a current lifetime of at least 100,000 years, and theoretical estimates for the lifetime of a persistent current exceed the estimated lifetime of the universe.

Re: idea about perpetual motion and want to know why wont work

Posted: Thu Oct 30, 2008 5:20 pm UTC
by Monty40xi
Bluggo wrote:
Vivek wrote:Does a current circulating in a ring of superconductor circulate forever assuming that superconducting conditions can be kept forever

Yes.
Wikipedia wrote:Experiments have demonstrated that currents in superconducting coils can persist for years without any measurable degradation. Experimental evidence points to a current lifetime of at least 100,000 years, and theoretical estimates for the lifetime of a persistent current exceed the estimated lifetime of the universe.

Huh. I would've thought random temperature fluctuations at a particle level would create enough short-lived resistances to gradually raise the overall temperature out of superconducting range a lot earlier than the lifetime of the universe.

Re: idea about perpetual motion and want to know why wont work

Posted: Sat Nov 01, 2008 8:32 pm UTC
by smw543
Monty40xi wrote:Huh. I would've thought random temperature fluctuations at a particle level would create enough short-lived resistances to gradually raise the overall temperature out of superconducting range a lot earlier than the lifetime of the universe.
First, the question asked whether it were true if one disregards the energy needed to maintain a cold enough temperature for superconductivity.

Also, by definition, a superconducting coil would never reduce the current. I believe the only reason the current would ever die (even if it took longer than the rest of time) has to do with some rather tricky concepts of quantum mechanics. Concerning high-temperature superconductivity, considerable progress has been made; mercury thallium barium calcium copper oxide is supercondictive at 135K (-135C.) The future looks promising.

Re: idea about perpetual motion and want to know why wont work

Posted: Sun Nov 02, 2008 5:13 pm UTC
by Hodan
Back to perpetual motion, this reminded me of a device I saw somewhere that seemed rather nifty. I'm not sure if it counts as a perpetual motion device though, so I ask.

Basically, it was a giant ferris wheel, with a certain number of weights on it and a switch at the top and the bottom. Here, I'll draw a picture of what I thought it was.
Image

Clearly I do not possess the same skill shown in xkcd, but, I think it gets the point across. The switch at the top "shifts" the weight forward, causing it to have a larger distance from the center and making the wheel move. The switch on the bottom shifts it back. I'm thinking this should work to keep it in motion, but is it perpetual motion?

edit: Eh... looking at the picture.. I'm not sure if I drew it correctly.. it looks like I'm shifting the weight backwards. The real device shifted the weight forwards by tilting it, from like | -> /

Re: idea about perpetual motion and want to know why wont work

Posted: Sun Nov 02, 2008 5:37 pm UTC
by Ashi
Hodan wrote:Back to perpetual motion, this reminded me of a device I saw somewhere that seemed rather nifty. I'm not sure if it counts as a perpetual motion device though, so I ask.

Basically, it was a giant ferris wheel, with a certain number of weights on it and a switch at the top and the bottom. Here, I'll draw a picture of what I thought it was.
<image>
Clearly I do not possess the same skill shown in xkcd, but, I think it gets the point across. The switch at the top "shifts" the weight forward, causing it to have a larger distance from the center and making the wheel move. The switch on the bottom shifts it back. I'm thinking this should work to keep it in motion, but is it perpetual motion?

edit: Eh... looking at the picture.. I'm not sure if I drew it correctly.. it looks like I'm shifting the weight backwards. The real device shifted the weight forwards by tilting it, from like | -> /


If you look a few posts up, it was stated by danpilon54 that,

This would be true if "perpetual motion" meant simply moving forever. This can easily be achieved by being in orbit etc. Its more about doing work, but needing no energy input, ie the creation of energy. [/nitpickiness]


It takes energy to shift the weights, so, no, it's not perpetual motion.

Re: idea about perpetual motion and want to know why wont work

Posted: Sun Nov 02, 2008 7:05 pm UTC
by Hodan
Mmmm, but what if you scaled that apparatus down, put a magnet on one of the weights, and then put it next to a coil? Wouldn't you then generate a current, which could be fed into a capacitor, essentially getting energy from no where?

Re: idea about perpetual motion and want to know why wont work

Posted: Sun Nov 02, 2008 9:02 pm UTC
by BlackSails
Hodan wrote:Mmmm, but what if you scaled that apparatus down, put a magnet on one of the weights, and then put it next to a coil? Wouldn't you then generate a current, which could be fed into a capacitor, essentially getting energy from no where?


Lenz's law

Re: idea about perpetual motion and want to know why wont work

Posted: Mon Nov 03, 2008 4:09 pm UTC
by Fnord
Please do remember that magnets are not an infinite source of energy.

Re: idea about perpetual motion and want to know why wont work

Posted: Mon Nov 03, 2008 4:42 pm UTC
by Velifer
Lenz's law


So just because one guy came up with a description of nature that holds true under repeated tests, we're gonna go make it a law? :wink:

All it takes is one good repeatable experiment where it doesn't hold true. Keep it up, you crazy perpetual-motion anarchists! Poke and prod at nature! Open those closed books! Others may think you are completely wasting your time, but I like to think you're keeping physics honest, and will be the first to know if the fundamental properties of the universe suddenly change. When your perpetual motion machines actually work, I want to know!

(That, and I want you safely off the streets.)

Re: idea about perpetual motion and want to know why wont work

Posted: Mon Nov 03, 2008 8:14 pm UTC
by seladore
Except that's not how physics works... Lenz's law isn't thought to be true just because it always happens, but because it is a mathematical result of deeper principles.

Perpetual motion fanatics invariably lack an understanding of these deeper principles, and are convinced that their next clever trick can work round it. Just like the circle squarers - it is mathematically impossible, but people carry on anyway because they don't understand the underlying structure. Perpetual motion machines don't fail because no-one has found the trick yet, they fail because they violate fundamental physical principles.

So, you could keep banging your head against a metaphorical wall, or you could "poke and prod at nature" in an effective and informed way that actually works. Like being a real researcher, a real physicist. This is open to anyone, and someone who can find a search for perpetual motion interesting can find a love for actual physics.

Re: idea about perpetual motion and want to know why wont work

Posted: Mon Nov 03, 2008 9:42 pm UTC
by Velifer
seladore wrote:There is no reason for the clockwise moments to be different from the anti-clockwise moments.


Coriolis forces. Duh.

Re: idea about perpetual motion and want to know why wont work

Posted: Tue Nov 04, 2008 3:22 am UTC
by smw543
seladore wrote:Except that's not how physics works... Lenz's law isn't thought to be true just because it always happens, but because it is a mathematical result of deeper principles.

Perpetual motion fanatics invariably lack an understanding of these deeper principles, and are convinced that their next clever trick can work round it. Just like the circle squarers - it is mathematically impossible, but people carry on anyway because they don't understand the underlying structure. Perpetual motion machines don't fail because no-one has found the trick yet, they fail because they violate fundamental physical principles.

So, you could keep banging your head against a metaphorical wall, or you could "poke and prod at nature" in an effective and informed way that actually works. Like being a real researcher, a real physicist. This is open to anyone, and someone who can find a search for perpetual motion interesting can find a love for actual physics.

Thank you for taking Velifer's witty post and throwing it to the floor, then stabbing it repeatedly in the eye in the name of maintaining the stereotype that engineers have no sense of humor. I greatly appreciate it.

EDIT: Sorry if this comes across as harsh. I don't mean to overstep my boundaries as a newbie with ~20 posts, this kind of thing just frustrates me.

Re: idea about perpetual motion and want to know why wont work

Posted: Tue Nov 04, 2008 9:45 am UTC
by seladore
smw543 wrote:
seladore wrote:Except that's not how physics works... Lenz's law isn't thought to be true just because it always happens, but because it is a mathematical result of deeper principles.

Perpetual motion fanatics invariably lack an understanding of these deeper principles, and are convinced that their next clever trick can work round it. Just like the circle squarers - it is mathematically impossible, but people carry on anyway because they don't understand the underlying structure. Perpetual motion machines don't fail because no-one has found the trick yet, they fail because they violate fundamental physical principles.

So, you could keep banging your head against a metaphorical wall, or you could "poke and prod at nature" in an effective and informed way that actually works. Like being a real researcher, a real physicist. This is open to anyone, and someone who can find a search for perpetual motion interesting can find a love for actual physics.

Thank you for taking Velifer's witty post and throwing it to the floor, then stabbing it repeatedly in the eye in the name of maintaining the stereotype that engineers have no sense of humor. I greatly appreciate it.

EDIT: Sorry if this comes across as harsh. I don't mean to overstep my boundaries as a newbie with ~20 posts, this kind of thing just frustrates me.


I saw no witty post, just an attempt to justify pseudoscience. Was the winking smiley a clue? If so, I offer my most humble grovelling apologies. I promise I will be sitting in a party hat for the next two hours, chair dancing to WHAM! in attempt to lighten up.

P.s. I ain't no stinkin' engineer.

Re: idea about perpetual motion and want to know why wont work

Posted: Tue Nov 04, 2008 2:43 pm UTC
by Monty40xi
seladore wrote:
smw543 wrote:
seladore wrote:Except that's not how physics works... Lenz's law isn't thought to be true just because it always happens, but because it is a mathematical result of deeper principles.

Perpetual motion fanatics invariably lack an understanding of these deeper principles, and are convinced that their next clever trick can work round it. Just like the circle squarers - it is mathematically impossible, but people carry on anyway because they don't understand the underlying structure. Perpetual motion machines don't fail because no-one has found the trick yet, they fail because they violate fundamental physical principles.

So, you could keep banging your head against a metaphorical wall, or you could "poke and prod at nature" in an effective and informed way that actually works. Like being a real researcher, a real physicist. This is open to anyone, and someone who can find a search for perpetual motion interesting can find a love for actual physics.

Thank you for taking Velifer's witty post and throwing it to the floor, then stabbing it repeatedly in the eye in the name of maintaining the stereotype that engineers have no sense of humor. I greatly appreciate it.

EDIT: Sorry if this comes across as harsh. I don't mean to overstep my boundaries as a newbie with ~20 posts, this kind of thing just frustrates me.


I saw no witty post, just an attempt to justify pseudoscience. Was the winking smiley a clue? If so, I offer my most humble grovelling apologies. I promise I will be sitting in a party hat for the next two hours, chair dancing to WHAM! in attempt to lighten up.

P.s. I ain't no stinkin' engineer.

Telling perpetual motion people that he wants them "safely off the streets" is an attempt to justify pseudoscience?

Re: idea about perpetual motion and want to know why wont work

Posted: Tue Nov 04, 2008 3:01 pm UTC
by seladore
OK, OK I take it all back.

The WHAM! is on, and I'm fast on my way to becoming a better person.

Apologies for missing the irony.

Re: idea about perpetual motion and want to know why wont work

Posted: Tue Nov 04, 2008 3:15 pm UTC
by Random832
urmommawasfun wrote:okay so a couple nights ago i had this idea for perpetual motion. I do not believe this will work now i am searching for a good explination on why it wont work.


You can't make a magnetic monopole by gluing two magnets together facing in opposite directions, which is what you're trying to do - doing that is going to result in an extremely weak field that vanishes at any significant distance from the assembly.

If one assumes it's entirely frictionless etc, it could probably spin indefinitely if started, but so can any object - you can't extract any energy from this without slowing it down.

Re: idea about perpetual motion and want to know why wont work

Posted: Tue Nov 04, 2008 3:28 pm UTC
by Velifer
people wrote:blahblahblah

S'ok. I'm a big boy. It was only slightly clever, and Seladore did make a good followup. Just be sure to wake me up before you go-go.

Re: idea about perpetual motion and want to know why wont work

Posted: Tue Nov 04, 2008 8:33 pm UTC
by smw543
seladore wrote:OK, OK I take it all back.

The WHAM! is on, and I'm fast on my way to becoming a better person.

Apologies for missing the irony.

Which album? I'm partial to Make It Big myself. Both because of the overall quality of the track selection and because I believe it was the exact quote from that incident in a Beverly Hills park restroom.

(By the way, I'm not an engineer either; not even sure why I said that.)