Perpetual motion

[joekgamer]

In your first post you say pressure and in the second you say not.

 

While the water pressure would stay the same, it is still the force that is 'breaking' the surface tension.

 

The surface tension can never be broken because the pressure of the atmosphere is greater than the pressure in the column of water

 

Put the system in a vacuum. Or at least, put it in a very low-pressure atmosphere.

[modest]

While the water pressure would stay the same, it is still the force that is 'breaking' the surface tension.

 

The meniscus doesn't rise to a breaking level because the water-glass surface tension is less than the water-air surface tension.

 

Put the system in a vacuum. Or at least, put it in a very low-pressure atmosphere.

 

Water is a gas in a vacuum.

 

~modest

[Janhatesme]

I know this seems a bit silly, but I've got an idea using magnets that seems feasible, at least in my mind. I just haven't had time to try it. Is it truly impossible, or simply not yet done? Does perpetual motion break any laws of physics, or is it only not proven possible mathematically?

 

Note: I will move this to Inventions and gadgets when Tormod gets a reply to his guitar rig question. I didn't want to bump that down the list.

 

 

I just will say that I had this exact same idea about ten years ago (perpetual motion involving magnets) and everyone I talked to about it said it was impossible.

[joekgamer]

The meniscus doesn't rise to a breaking level because the water-glass surface tension is less than the water-air surface tension.

You don't need to use glass, and there has to be some point at which the water pressure from the increased height of the colum would be more than the surface tension could hold.

 

Water is a gas in a vacuum.

How about almost a vacuum? Or a hydrogen/helium atmosphere? The pressure doesn't need to be that low. Or, you could (if you had either an extremely effecient cooling system or an environment-based cooling system) cool down the system to make the water stay a liquid.

[modest]

You don't need to use glass, and there has to be some point at which the water pressure from the increased height of the colum would be more than the surface tension could hold.

 

I'm sorry, Polymath, I just don't have time to look into all the math for it. I think the wiki page on capillary action has relevant formulas. You might work out a diagram and an exact proposal.

 

How about almost a vacuum? Or a hydrogen/helium atmosphere?

 

No matter how low the press of the atmosphere is, the pressure in the liquid is less than atmospheric if it is raised by capillary action.

 

It is just like sucking on a straw. The pressure in the liquid in the straw is less than the pressure of the atmosphere right outside the straw.

 

~modest

[CraigD]

Water is a gas in a vacuum.

How about almost a vacuum? Or a hydrogen/helium atmosphere? The pressure doesn't need to be that low. Or, you could (if you had either an extremely effecient cooling system or an environment-based cooling system) cool down the system to make the water stay a liquid.

As long as it doesn't react chemically with it, the composition of the gas containing the water isn't important, only its pressure.

 

Below its triple point pressure of about 611.73 pa (0.006 atmospheres), solid water (ice) changed gaseous water (steam) without passing through its liquid state.

 

So there's a well-defined minimum atmospheric pressure required to have liquid water.

[joekgamer]

How about the pressure being at or near that point? Or how about a different liquid?

 

Also, how does the pressure in the tube become less than atmospheric pressure? I looked up the concept, but I couldn't find it. Also, the gavitational force helps out because the last stretch of tubing being straight down.

[Rade]

If you build a machine to serve you and it continues to serve [with no additional input of energy] until the day you die, I would argue you have built a perpetual motion machine.

[joekgamer]

If a machine used permanent magnets (which draw their force from the alignment of their atoms' domains) in an attempt to achieve perpetual motion, would it still be considered perpetual motion? After a while, the strain would break down the magnet enough for it to no longer be considered a magnet, so would the machine be considered to be using the alignment of the magnets' atoms' domains as fuel?

[CraigD]

If a machine used permanent magnets (which draw their force from the alignment of their atoms' domains) in an attempt to achieve perpetual motion, would it still be considered perpetual motion? After a while, the strain would break down the magnet enough for it to no longer be considered a magnet, so would the machine be considered to be using the alignment of the magnets' atoms' domains as fuel?

You're correct, Polymath: if a machine uses anything as a fuel, it's not a perpetual motion machine. However, as I wrote (with pretty little pictures of magnets :)) in this 2008 post and a few that follow it, the physics of extracting energy from permanent magnets by weakening their fields is exactly backwards. To extract useful energy from magnetic material, you must rearrange its molecules in a way that strengthens its magnetic field.

 

Though possible in principle, I've never seen a published "magnetic material fueled" machine design that gets this physics right. Nearly every PM machine enthusiast (or in many cases, fanatic ;)) design I've seen assumes the incorrect, backwards physics.

 

I've not tried calculating the energy of an example material to see if a "magnet fueled" engine would be practically sensible - this would be an good exercise for anyone interested.

[CarlNGraham]

How a bout a machine that does the 'work' of telling the time by just watch astronomical object travel across a scale?

[CraigD]

How a bout a machine that does the 'work' of telling the time by just watch astronomical object travel across a scale?

In the context of discussing perpetual motion, "work" is the mechanical, [imath]W = F \cdot d[/imath], kind. The common meaning of "work", as in "memorizing 'Macbeth' was a lot of work!", can be inappropriate when talking about physical mechanical subjects, because it's metaphorical, not physical.

 

There's something mechanical underlying these metaphors. The metaphorical "work" of memorizing Shakespeare requires real physical movement of neurons and neurochemicals in the brain, body and eye muscles exerting forces, etc, on the order of about 100 W (100 J/s), which requires energy in the form of the food we eat.

 

So a machine that "tells the time" by, say, tracking the Sun with an automatic sextant and recording the data somewhere, does real physical work - it needs batteries, a wind up clockwork, an external energy supply, or something similar. A passive device like a sundial doesn't - the physical work of telling time with it is done by the system of person who looks at it, again, is a food-powered biological machine.

[CarlNGraham]

In the context of discussing perpetual motion, "work" is the mechanical, [imath]W = F \cdot d[/imath], kind. The common meaning of "work", as in "memorizing 'Macbeth' was a lot of work!", can be inappropriate when talking about physical mechanical subjects, because it's metaphorical, not physical.

 

There's something mechanical underlying these metaphors. The metaphorical "work" of memorizing Shakespeare requires real physical movement of neurons and neurochemicals in the brain, body and eye muscles exerting forces, etc, on the order of about 100 W (100 J/s), which requires energy in the form of the food we eat.

 

So a machine that "tells the time" by, say, tracking the Sun with an automatic sextant and recording the data somewhere, does real physical work - it needs batteries, a wind up clockwork, an external energy supply, or something similar. A passive device like a sundial doesn't - the physical work of telling time with it is done by the system of person who looks at it, again, is a food-powered biological machine.

 

Sorry, next time I will put the :rolleyes: on it. Not everybody gets my sense of humour :(

[modest]

Erasmus wrote an interesting post comparing thermodynamic and information entropy showing that the work done storing information complies with the laws of thermodynamics: Thermodynamic and Information Entropy

Very cool.

 

~modest