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Why does water with no gravity acting upon it form a sphere?

by vorcil
Tags: acting, form, gravity, sphere, water
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vorcil
#1
May8-09, 11:04 PM
P: 394
Why does water with no gravity acting upon it form a sphere?

i know it has something to do with the fluid tension but otherwise i have no idea why, in space, water forms a sphere?
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cesiumfrog
#2
May8-09, 11:29 PM
P: 2,050
Surface tension.

In other words, the molecules are attracted to each other, so they naturally form the shape which minimises the number of them that are not surrounded by others.
diazona
#3
May8-09, 11:45 PM
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Quote Quote by cesiumfrog View Post
Surface tension.

In other words, the molecules are attracted to each other, so they naturally form the shape which minimises the number of them that are not surrounded by others.
Nice explanation

And of course, a sphere has the smallest surface area of all shapes with a given volume.

Phrak
#4
May8-09, 11:56 PM
P: 4,512
Why does water with no gravity acting upon it form a sphere?

Like what cesiumfrog says. Volumetric tension (aka 'surface tension'). Every molecule pulls on every molecule that's its immediate neighbor, until it gets close enough, and then it stops pulling. However this could describe a solid too. The atoms in iron will all attract their neighboring atoms, but an iron rod won't flow into a ball. The property of internal tension isn't quite sufficient to tell us why a liquid will deform into a ball and a solid will not.

You're question is a good one. And the tension answer is the one I've always been satisfied with--until now.

Maybe someone with some better skills on the nature of materials can tell us why the tension in a liquid is different than the tension in a solid.

Why do the molecules in a liquid slide past each other while still maintaining cohesion, while in a solid they remain fixed?
vorcil
#5
May9-09, 01:53 AM
P: 394
Yeah i'm still not satisfied by surface tension?

for example if you made a huge unshaped blob of water in space, it will become a very good sphere,

where does the energy that moves the water molecules come from?
does the water lose energy(possibly thermal) while it reforms into a sphere?
A.T.
#6
May9-09, 02:16 AM
P: 4,049
Quote Quote by vorcil View Post
where does the energy that moves the water molecules come from?
does the water lose energy(possibly thermal) while it reforms into a sphere?
The system looses potential energy and transforms it into thermal energy. The sphere is the minimal potential energy configuration of the molecules. Potential energy is defined by the relative position of molecules to each other, and the forces between them (electromagnetic + gravitation).
vorcil
#7
May9-09, 05:38 AM
P: 394
does water freeze in space?
vorcil
#8
May9-09, 05:40 AM
P: 394
Does the first law of thermodynamics apply in space, or in a vaccum?
i.e will heat be lost without a medium or matter for that heat to pass to?
dx
#9
May9-09, 06:49 AM
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The first law of thermodynamics is conservation of energy. It applies everywhere.
vorcil
#10
May9-09, 08:35 AM
P: 394
Quote Quote by dx View Post
The first law of thermodynamics is conservation of energy. It applies everywhere.
what's that law that states that a desire for equlibrium must be achieved between two areas of different temperature then:?

and does it apply to matter in a vaccum and weightless environment?
Count Iblis
#11
May9-09, 08:50 AM
P: 2,157
Quote Quote by Phrak View Post
Like what cesiumfrog says. Volumetric tension (aka 'surface tension'). Every molecule pulls on every molecule that's its immediate neighbor, until it gets close enough, and then it stops pulling. However this could describe a solid too. The atoms in iron will all attract their neighboring atoms, but an iron rod won't flow into a ball. The property of internal tension isn't quite sufficient to tell us why a liquid will deform into a ball and a solid will not.

You're question is a good one. And the tension answer is the one I've always been satisfied with--until now.

Maybe someone with some better skills on the nature of materials can tell us why the tension in a liquid is different than the tension in a solid.

Why do the molecules in a liquid slide past each other while still maintaining cohesion, while in a solid they remain fixed?
Iron also has a surface tension. The Helmholtz free energy of some amount of water or iron is proportional to the volume (the bulk contribution) and the area separating the compound from the atmosphere.

The difference between iron and water is caused by the fact that the water molecles are not bound to a lattice. Then the shape of the surface can change. Therefore it will become a sphere as that minimizes the free energy.

In case of solid iron rod, the minimum free energy configuration cannot be reached as the rod is frozen into a local free energy minimum.
Andy Resnick
#12
May9-09, 12:27 PM
Sci Advisor
P: 5,523
Quote Quote by Phrak View Post
<snip>

Maybe someone with some better skills on the nature of materials can tell us why the tension in a liquid is different than the tension in a solid.

Why do the molecules in a liquid slide past each other while still maintaining cohesion, while in a solid they remain fixed?
Fluids flow under stress, solids do not.
Phrak
#13
May9-09, 01:44 PM
P: 4,512
Quote Quote by Count Iblis View Post
Iron also has a surface tension. The Helmholtz free energy of some amount of water or iron is proportional to the volume (the bulk contribution) and the area separating the compound from the atmosphere.

The difference between iron and water is caused by the fact that the water molecles are not bound to a lattice. Then the shape of the surface can change. Therefore it will become a sphere as that minimizes the free energy.

In case of solid iron rod, the minimum free energy configuration cannot be reached as the rod is frozen into a local free energy minimum.
This leaves leaves out amorphous solids--at least I think so.
rcgldr
#14
May9-09, 01:44 PM
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If you ever watch space station videos showing water or orange juice large droplets being released (and then swallowed), the large droplets quickly become spherical. It seems difficult to believe it's just a surface related effect.
A.T.
#15
May9-09, 02:55 PM
P: 4,049
Quote Quote by Jeff Reid View Post
If you ever watch space station videos showing water or orange juice large droplets being released (and then swallowed), the large droplets quickly become spherical. It seems difficult to believe it's just a surface related effect.
Cohesion forces are stronger than you think. Mercury forms balls & blobs even against gravity. But without gravity, cohesion forces have little to work against, so even water can do it.
D H
#16
May9-09, 03:32 PM
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Quote Quote by Phrak View Post
This leaves leaves out amorphous solids--at least I think so.
No. It just takes longer -- a lot, lot longer -- for glasses to flow (and form into a sphere).

In researching this answer, I was going to bring up panes of glass in medieval cathedrals that has over the years have become a bit thicker at the bottom of the pane than at the top. Alas, this is but a scientific myth. Medieval craftsman could not blow perfectly flat glass. The thickness had a gradient across a pane of glass. Craftsmen intentionally placed panes of glass with the thickest side along the bottom. Glass can flow at room temperature, but it takes a lot longer than 500 years for the flow to become observable. The relaxation time for glass at ambient temperature is on the order of 1032 years.

Zanotto, "Do Cathedral Glasses Flow?" Am. J. Phys., 66:392-396 (1998)
http://alevi.usc.edu/Html/publicatio...deralGlass.pdf
zoobyshoe
#17
May9-09, 03:52 PM
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Quote Quote by D H View Post
The relaxation time for glass at ambient temperature is on the order of 1032 years.
Seems to me you'd get some flow out of solid granite in that much time.
Gear300
#18
May9-09, 04:38 PM
P: 1,133
Quote Quote by vorcil View Post
Why does water with no gravity acting upon it form a sphere?

i know it has something to do with the fluid tension but otherwise i have no idea why, in space, water forms a sphere?
yup...it is more or less surface tension...also, because of the molecular interactions (along with the gravity of each individual particle), you could assume the forces of attraction to be between each individual molecule. If you model it as such, you could also assume that the sphere would be the ideally modeled shape (the pressure is equal on all sides, so with a spherical shape, it is in equilibrium).


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