# What causes water pressure? (Physics wise) **URL:** \ **Category:** Factual Questions\ **Created:** [May 19, 2016, 5:32pm UTC](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174 "2016-05-19T17:32:34Z")\ **Posts on this page:** 13\ **Page:** 2
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [May 19, 2016, 10:56pm UTC](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174/21 "2016-05-19T22:56:53Z")
> [@octopus](#): > > What is temperature at the particle level? Total kinetic energy of all it’s motions; vibrations, electron orbits, twisting and it’s motion through space … for starters … ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [May 20, 2016, 11:28am UTC](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174/22 "2016-05-20T11:28:16Z")
> [@Petrobey\_Mavromihalis](#): > > Both these models treat the particles as stationary, but they are all moving. That’s what causes the pressure - particles colliding and causing a momentum change over an area. The change in momentum can only increase if the velocity of the particles changes, or their mass, or their number. Although liquids do compress I don’t see how this would be enough to cause a significant enough number change, so it’s got to be velocity (or mass?!). But how does this velocity change come about? > Ta for all answers by the way! I take a solid block of ice and place it on a surface. It exerts a force on the surface and divided by the area of the surface we get the pressure. Now place another equal block on top. Do you think the velocities now all double at the bottom surface? I.e. the temperature increases drastically? Then why doesn’t this increase in energy spread throughout the atoms in the blocks and leave us with the same pressure we had? Yes, I know ice is a solid and not a gas or a liquid, but do you think the atomic forces involved just vanish and get entirely replaced by collisions and momentum transfer when the ice melts? The number of atoms at the interface will increase and they will be moving faster when we go from ice to water, do you think the pressure increases significantly? ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [May 20, 2016, 12:10pm UTC](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174/23 "2016-05-20T12:10:35Z")
Liquids are not gases. Gasses obey the gas laws (PV = nRT etc) and those laws are derived from the statistical mechanics of particles bouncing off one another and their constraining boundaries. Liquids don’t. Liquids, like solids are not held apart by colliding constituent particles. They are held apart because they are composed of fermions. Your average bucket of bouncing atoms can be compressed only so far before the electron clouds start to intersect. The local negative electric charge starts to repel the atoms, and more importantly, the Paulli exclusion principle starts to come into effect. You can’t cram the electrons with the same quantum states together, and the effective force holding the atoms apart becomes so huge that compression effectively stops. Liquids are thus just like a bucket of marbles. (Indeed, a bucket of marbles is also slightly compressible, but it also does not compress like a gas. It compresses like a bucket of deforming glass spheres.) Eventually you can exert enough force to overcome the electron cloud. First you push all the electrons into their lowest energy states, at which point it is _really_ unhappy about compressing any further, but given enough pressure you can overcome that too. You can find the first state in white dwarfs, and the the second in neutron stars. Neither meet the OP’s criteria 😃 ---
**Author:** ![Ignotus](https://avatars.discourse-cdn.com/v4/letter/i/ecc23a/32.png) [@Ignotus](https://boards.straightdope.com/u/Ignotus)\ **Post date:** [May 20, 2016, 12:24pm UTC](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174/24 "2016-05-20T12:24:21Z")
> [@AdamF](#): > > Particle velocity stays the same, but the spacing between molecules decreases. With solids and liquids, the space between molecules is already much smaller than the diameter of a molecule. So it is more accurate to say that molecules will vibrate faster. So, what happens if you rapidly de-pressurize heavily pressurized water? Will it all freeze solid? Or will part of it turn into vapor, and the rest solidify (much like liquid CO[sub]2[/sub] in a bottle when you open the valve)? (Suppose no dissolved gasses, and ambient STP.) ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [May 20, 2016, 12:26pm UTC](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174/25 "2016-05-20T12:26:27Z")
> [@Petrobey\_Mavromihalis](#): > > Yes I understand it increases with the depth of the water but what is happening on a particle level? Pressure is caused by particle collisions - I can understand that in denser atmosphere the number of these increases, but liquids are fairly incompressible, so what is causing the increase? Particle velocity? Imagine atoms were like bar magnets,with a north and south end, and that the atoms were lined up alternatively \< N S \> \< S N \> \ \< S N \> Well if you push at one end, the other end is pushed, right ? They aren’t even touching… Well in substances magnetic field is irrelevant, gravity is irrelevant, and the fields are electric field, and the strong nuclear field and the weak nuclear field… pretty much between molecules its the electric field only. And there is no alignment of the electric fields,they dont line up that way, its mostly the way the nucleii don’t want to come close to another, as they are both positively charged… and partly the way the electrons don’t want to be squashed up. Its all considered pressure because that explains why the macroscopic force, eg 100 kg of your weight, is not all pushed down onto a single atom, and how even though all the atoms, molecules in fluid and gas (… and really solids aren’t totally solid, so you may as well call them liquid !) are moving, they are still transferring their own little bit of force each… Pressure also explains why gases want to expand to fill the space it can …it wants to move to reducing pressure… ( some liquids evaporate and some solids sublimate, similarly… ) ---
**Author:** ![AdamF](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/adamf/32/6246_2.png) [@AdamF](https://boards.straightdope.com/u/AdamF)\ **Post date:** [May 20, 2016, 3:08pm UTC](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174/26 "2016-05-20T15:08:12Z")
> [@Ignotus](#): > > So, what happens if you rapidly de-pressurize heavily pressurized water? Will it all freeze solid? Or will part of it turn into vapor, and the rest solidify (much like liquid CO[sub]2[/sub] in a bottle when you open the valve)? > (Suppose no dissolved gasses, and ambient STP.) There is not much energy in a pressurized liquid. That’s why pressure bottles are hydro-tested. ---
**Author:** ![Ignotus](https://avatars.discourse-cdn.com/v4/letter/i/ecc23a/32.png) [@Ignotus](https://boards.straightdope.com/u/Ignotus)\ **Post date:** [May 20, 2016, 6:26pm UTC](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174/27 "2016-05-20T18:26:46Z")
> [@AdamF](#): > > There is not much energy in a pressurized liquid. That’s why pressure bottles are hydro-tested. How come less energy is stored in a pressurized liquid than a pressurized gas? I’ve certainly done the same amount of work pressurizing them both! ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [May 20, 2016, 7:21pm UTC](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174/28 "2016-05-20T19:21:35Z")
> [@Ignotus](#): > > How come less energy is stored in a pressurized liquid than a pressurized gas? I’ve certainly done the same amount of work pressurizing them both! You definitely haven’t. The work done is dependent on the compression, since liquids are less compressible, you do less work. Say you have a cylinder with a vertical piston of a certain mass. You start with the piston in a neutral position, equal pressure inside the cylinder as outside. You let the piston go, it’s pulled down by gravity until the pressure\*area equals the force of gravity. The work done by gravity will be proportional to the distance of travel, which will be a lot longer for a gas than for a liquid. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [May 20, 2016, 7:33pm UTC](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174/29 "2016-05-20T19:33:37Z")
I think **AdamF** may be incorrect … at higher pressure, the system holds more energy … poke a hole in the container and this “pressure energy” is converted into kinetic energy for the fluid flowing out … the thicker container walls are so holes don’t get poke in it. That’s if we start with a gas, then we’ll end up with a gas. The question was what if we start with a liquid, will it too come out as a gas? This will depend on the the energy of the system in it’s initial state. Above some threshold level, the liquid will convert to a gas, below this level, the liquid stays a liquid. For an extreme example (hehe), a container of water at 100 atms and 1,000ºC depressurizing very quickly may well be causative of an insurance claim … for a final state of standard temperature and pressure … ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [May 20, 2016, 8:02pm UTC](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174/30 "2016-05-20T20:02:07Z")
> [@watchwolf49](#): > > I think **AdamF** may be incorrect … at higher pressure, the system holds more energy … poke a hole in the container and this “pressure energy” is converted into kinetic energy for the fluid flowing out … the thicker container walls are so holes don’t get poke in it. No, assuming no significant amount of liquid turns into gas or vice versa, **AdamF** is correct. The work put in to pressurize the system is equal to the energy stored. And work is force x distance. Since your gas and liquid are at the same pressure, you’ve got the same force more or less, but the more compressible gas will have moved farther, so more distance. If your pressurize something very compressible, it will store more energy than something relatively incompressible. That’s why we use rubber bands, carefully coiled springs, or (for toy air rockets) air, to store energy, rather than compressing/expanding solid blocks of metal (which are much less compressible/extensible). ---
**Author:** ![Ignotus](https://avatars.discourse-cdn.com/v4/letter/i/ecc23a/32.png) [@Ignotus](https://boards.straightdope.com/u/Ignotus)\ **Post date:** [May 20, 2016, 8:11pm UTC](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174/31 "2016-05-20T20:11:55Z")
> [@naita](#): > > You definitely haven’t. The work done is dependent on the compression, since liquids are less compressible, you do less work. > > Say you have a cylinder with a vertical piston of a certain mass. You start with the piston in a neutral position, equal pressure inside the cylinder as outside. You let the piston go, it’s pulled down by gravity until the pressure\*area equals the force of gravity. > > The work done by gravity will be proportional to the distance of travel, which will be a lot longer for a gas than for a liquid. Wow! You’re right! I took a paper and pencil and worked it out, and it’s totally true! I think the fact that it defies my intuition has something to do with my innate tendency to think in relative rather than absolute differences… Well, ignorance fought!🙂 ---
**Author:** ![AdamF](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/adamf/32/6246_2.png) [@AdamF](https://boards.straightdope.com/u/AdamF)\ **Post date:** [May 20, 2016, 10:36pm UTC](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174/32 "2016-05-20T22:36:17Z")
> [@watchwolf49](#): > > That’s if we start with a gas, then we’ll end up with a gas. The question was what if we start with a liquid, will it too come out as a gas? Example 1: You have a container full of cold water at 100 atm. Poke a small hole in the side. There will be a small quick burst of water as the container depressurizes. Water will then pour out slowly until the level drops to the hole. Example 2: You have a pipe full of cold water at 100 atm. Poke a small hole in the side (the pipe is large = pressure inside does not change much). There will be a very fast stream of liquid water coming out the hole. Example 3: You have a pipe full of 300°C water at 100 atm. Poke a small hole in the side. As liquid water starts to pass through the hole, its pressure drops to 1 atm and it flashes into steam, which flows out of the hole at the speed of sound. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [May 21, 2016, 1:11pm UTC](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174/33 "2016-05-21T13:11:37Z")
> [@AdamF](#): > > Example 1: You have a container full of cold water at 100 atm. Poke a small hole in the side. There will be a small quick burst of water as the container depressurizes. Water will then pour out slowly until the level drops to the hole. > > Example 2: You have a pipe full of cold water at 100 atm. Poke a small hole in the side (the pipe is large = pressure inside does not change much). There will be a very fast stream of liquid water coming out the hole. > > Example 3: You have a pipe full of 300°C water at 100 atm. Poke a small hole in the side. As liquid water starts to pass through the hole, its pressure drops to 1 atm and it flashes into steam, which flows out of the hole at the speed of sound. That’s the ticket … assuming constant temperature. [Previous page](https://boards.straightdope.com/t/what-causes-water-pressure-physics-wise/755174.md?page=1)