physics question about gravity and pressure

[tenbyfifty]

Say an object is 1km under the deep Atlantic - that means there's a km of water in all directions and its under a certain amount of pressure. If you have a spherical container of radius 1km filled with water and put the object at the centre of it and put the whole lot out in deep space , is it at the same pressure?

I know pressure acts in all directions equally btw, but how is it influenced by gravity?

I'm trying to get my head around what pressure actually means WRT the interior of stars -

is it just gravity or is it gravity plus something else?

If you keep making that sphere's radius bigger and bigger at what point will pressure start altering

the water inside it at a molecular level asssuming the wall of the sphere will hold.

Stop snoring at the back, Jennings!!!

[The Warthog]

I'm probably stepping into territory that an Arts and Humanities student shouldn't, but fools rush in, eh? Water is incompressible, so the pressure at 1km depth is a result of the weight of the water above that point pressing down on the object. If you put the object in a 55 gallon drum and sealed it, and took it into space, the pressure on the object would be zero, or near enough. If you did this with air, at the surface of the earth, the pressure on the object in space would still be 14lb/sq.in., as air is compressible. Nevertheless, the pressure of air at any altitude is the result of the weight of the air above it.

If you filled your drum with air at 1km below the sea, it would have tremendous pressure in space, equal to the pressure of the water when you filled the drum, because of the compressibility of air.

[themos]

Say an object is 1km under the deep Atlantic - that means there's a km of water in all directions and its under a certain amount of pressure.

If you have a spherical container of radius 1km filled with water and put the object at the centre of it and put the whole lot out in deep space , is it at the same pressure?

No. It's under no pressure whatsoever. Oh alright, a tiny bit, as much as the gravity that this mass alone can generate.

[tenbyfifty]

Water is incompressible, so the pressure at 1km depth is a result of the weight of the water above that point pressing down on the object.

In actual fact it would experience the same pressure from all directions not just the water above it.

This is a fundamental priciple about the way pressure works.

As for water being incompressible, ok , but what happens when you keep cranking up the pressure - something

surely must give at a molecular level. In neutron stars, protons and electrons get fused into neutrons

so i am sure water must have a breaking point.

what i am trrying to undertsand is how gravity and pressure interact in very massive objects, like stars.

pressure, ofcourse, not being a fundamental force like gravity but a macro level electromagnetic effect.

[tenbyfifty]

Say an object is 1km under the deep Atlantic - that means there's a km of water in all directions and its under a certain amount of pressure.

If you have a spherical container of radius 1km filled with water and put the object at the centre of it and put the whole lot out in deep space , is it at the same pressure?

No. It's under no pressure whatsoever. Oh alright, a tiny bit, as much as the gravity that this mass alone can generate.

are you confusing gravity with pressure. there is zero gravity at the centre of the earth but i reckon you'd still be under a lot of pressure down there.

after all the atmospheric pressure exerted on your body is due to the density of the air, not the gravity of the earth.

and ofcourse, just because you're weighless , it doesn't mean you can't be under pressure - ask astronauts - pressure

keeps them alive.

[themos]

are you confusing gravity with pressure. there is zero gravity at the centre of the earth but i reckon you'd still be under a lot of pressure down there.

after all the atmospheric pressure exerted on your body is due to the density of the air, not the gravity of the earth.

Perhaps I am confused. There is a lot of pressure in the centre of the earth BECAUSE of the considerable gravity of the planet which pulls in all that matter.

The atmospheric pressure is proportional to the density of the air, indeed. But it's there because of gravity pulling the atmosphere down. If you could "turn up" gravity, you would turn up the atmospheric pressure.

Having said this, in absence of gravity, a huge blob of water would held together by a) its own very weak gravity and surface tension. Those two would create some kind of pressure in the liquid.

[mikkey99]

Ok i`ll have a go

as warthog said the pressure underwater is caused by the weight of water above (increases by 1 atmosphere for evey 10M of depth)

BUT the weight is relative to the gravitational force exerted by the earth so in space no gravity = no weight = no pressure.

Air however is different as it can be compressed so at 1km depth it would be compressed to a pressure of 101 atm and if confined at this pressure would still be at 101 atm in space.

Mike

[themos]

something surely must give at a molecular level. In neutron stars, protons and electrons get fused into neutrons

so i am sure water must have a breaking point.

Ah yes, the electron degeneracy pressure. Look it up in wikipedia.

http://en.wikipedia.org/wiki/Electron_degeneracy_pressure

[tenbyfifty]

If you could "turn up" gravity, you would turn up the atmospheric pressure.

now this gets to the nub of my confusion, you can't, ofcourse, separate

gravity from the thing that creates gravity which is mass (let's forget dark matter for now),

but increasing mass would also create more pressure.

Pressure and gravity are clearly separate types of phenomena but they also interact

in massive objects and I'm just trying to get a picture of how it works.

at the moment the picture is a dog chasing its tail :?

btw, my bet WRT the giant sphere of water in space is that the object would be still be under considerable

pressure because of the amount of water, but less than on earth because, as you say, of very little gravity.

by how much I am not sure - knowing the answer to that would probably clear things up a bit.

[Cloudwatcher]

As for water being incompressible, ok , but what happens when you keep cranking up the pressure - something

surely must give at a molecular level. In neutron stars, protons and electrons get fused into neutrons

so i am sure water must have a breaking point.

As I recall a fluid under sufficient pressure will assume an altered state changing from a liquid to a gaseous form. In the case of water the heat generated by the pressure will result in water vapour/steam.

CW

[tenbyfifty]

so at 1km depth it would be compressed to a pressure of 101 atm and if confined at this pressure would still be at 101 atm in space.

so the object at the centre of a contained 1km sphere of water in space would have nowhere near the same pressure as on earth until you made the sphere big enough to approach a decent planetary mass and crank the gravity up a bit?

[themos]

my bet WRT the giant sphere of water in space is that the object would be still be under considerable

pressure because of the amount of water, but less than on earth because, as you say, of very little gravity.

by how much I am not sure - knowing the answer to that would probably clear things up a bit.

It won't be considerable, it will be minute. Firstly, calculate the gravity induced by the mass of water itself. One km radius sphere of water vs 6500 km radius sphere of earth. Earth's gravity is 6500(radial ratio)*5(density ratio)= 32500 times stronger than the water blob's.

According to this http://en.wikipedia.org/wiki/Surface_tension

a 1 km blob of water will have surface tension pressure of 1.4 nano atmospheres. That's a billionth.

[AstroTiger]

interesting question.

Placing anything in the ocean esp 1km will place it under pressure, esp with atmopspheric pressure also bearing down on it.

Pressure on it in outer space would be zero, there is nothing or negligble atmosphere in space so nothing to provide the pressure.

All pressure is is the air molecules (if in air) or water molecules (if in water) beating against the object in all directions.

Like sticking a satnav cradle on ur wind screen, it is atmospheric pressure holding that to your window, and its strong enough to take the satnav!

take the object to outerspace (ignoring the temp diff that would make the water freeze anyway) prob the boiling point of the liquid would be much lower, thus if anything increading the pressure of the water on the inner walls of the ball. Hence it might explode.

hope tht makes some sense

AT

[tenbyfifty]

It won't be considerable, it will be minute. Firstly, calculate the gravity induced by the mass of water itself. One km radius sphere of water vs 6500 km radius sphere of earth. Earth's gravity is 6500(radial ratio)*5(density ratio)= 32500 times stronger than the water blob's.

According to this http://en.wikipedia.org/wiki/Surface_tension

a 1 km blob of water will have surface tension pressure of 1.4 nano atmospheres. That's a billionth.

remember i didn't say a blob of water, i said a spherical container of water of 1 km radius.

does that make a difference to the internal pressure?

[themos]

remember i didn't say a blob of water, i said a spherical container of water of 1 km radius.

does that make a difference to the internal pressure?

It could. If you pressed more water molecules through a hole and then sealed it, you could build up to any pressure that the vessel can stand.

[Astroman]

Space is a vacuum. Water will not be stable there, unless isolated. The atmospheric pressure of Earth, combined with the temperature makes liquid water possible. If we ignore the lack of pressure in space, the water would form a sphere all on its own due to surface tension. You can see this effect in video taken by Skylab in the 70's, where there is atmospheric pressure, but no (or negligible) gravity. The pressure within a star is due to the enormous amount of compressible gas in its atmosphere. Enough gas and it forms into a sphere because of gravity. It is the gravity that contracts the star enough to compress the inner layers enough to heat them and cause fusion. The balance between the inward crunch of gravity and the outward push of radiation maintains the star's shape.

TBF is confusing pressure with gravity. All mass has a gravitational attraction with relation to other mass. Pressure results from atmosphere stacked above an object that is restrained by something below. Atmospheric pressure is a measure of how much atmosphere is contained in a column above the object. Internal pressure is a measure of the difference all around an object. A balloon has internal pressure as compared to the atmosphere. As a balloon rises, the internal pressure remains the same, while atmospheric pressure decreases, making the balloon bigger. Pressure under the ocean is seen from all directions, due to the viscosity and surface tension of the water. The greatest pressure however, is measured from above, due to gravity.

[The Warthog]

Looks like the Arts and Humanities student had it right, for the most part.

[themos]

Space is a vacuum. Water will not be stable there, unless isolated. The atmospheric pressure of Earth, combined with the temperature makes liquid water possible.

A bit of water will not be stable, it will evaporate, I can see that now. But a LOT of water that was able to generate enough gravity to capture a water vapour atmosphere and build up some pressure? I think that could.

[Macavity]

A bit of water will not be stable, it will evaporate, I can see that now. But a LOT of water that was able to generate enough gravity to capture a water vapour atmosphere and build up some pressure? I think that could.

Sounds reasonable - As in the "Ice Giants" of the solar system. Though, in fainess, these appear to have been "seeded" by a rocky core. Still, the comets, which seemed to be, at least once, "dirty snowballs" - Or is that vice versa, according to contemporary theory?

[themos]

I just found this discussion on what happens to water in space

http://www.bautforum.com/questions-answers/29137-water-space-what-happens.html

and this phase diagram for water

http://www.lsbu.ac.uk/water/phase.html

[AstroTiger]

I just found this discussion on what happens to water in space

http://www.bautforum.com/questions-answers/29137-water-space-what-happens.html

and this phase diagram for water

http://www.lsbu.ac.uk/water/phase.html

its been a few years since i last saw one fo those!

The thing to remember is that Water has hydrogen bonding. This enables it to form a sphere in outerspace!

Also the thing to remember is the balance between internal pressure and external.

Like all here for eg, we have dissolved gases in our blood which remains so becuse of atmospheric pressure. It is y we suffer the bends if we suddenly arise from a deep sea dive, or indeed would explode if we went into outerspace without a space suite on.

So the water in the sea would be pressured, but for it to retain its form, it must match that pressure internally.

Let us also remember that boiling only occurs when vapour pressure (internal if u like) exceeds atospheric. Reduce atmospheric, reduce the boiling point.

Which is y chemists sometimes perform distillations underpressure!

yes i am a geek of an ex chemist!

ok u can wake up now!

AT

[Cloudwatcher]

........... chemists sometimes perform distillations under pressure!

Would the 'pressure' be the risk of being caught with a few litres of 'Moonshine' in the lab?

CW

[Kaptain Klevtsov]

Eeek! Phase diagrams, here? What the flip?

That brought back all sorts of memories from thermodynamics that I thought I'd recovered from. Whatever next, psychrometric charts?

Kaptain Klevtsov

[beamish]

I assume that means you understood !! :shock:

Karlo