# water pressure - random science question.

#### beerwax

##### Enlightened
at the bottom of the ocean at depths of 1000s of feet water still occupies about the same volume. it does compress but not much. but the pressure is great.

how does this manifest itself at the molecular level. the molecules arent particularly closer together so they must be moving faster ?
and
does it take more energy to move a given object at 2000 feet below sealevel than it does at 2 feet ? that is to ask if i had a bucket of water under these sorts of pressures what would its consistency be like?

i dont have these answers , probably did once . anyone shed some light please ?
cheers

#### bbb74

##### Enlightened
at the bottom of the ocean at depths of 1000s of feet water still occupies about the same volume. it does compress but not much. but the pressure is great.

how does this manifest itself at the molecular level. the molecules arent particularly closer together so they must be moving faster ?
and
does it take more energy to move a given object at 2000 feet below sealevel than it does at 2 feet ? that is to ask if i had a bucket of water under these sorts of pressures what would its consistency be like?

i dont have these answers , probably did once . anyone shed some light please ?
cheers

I don't really know so I'll guess

No the molecules wouldn't "move faster" - that would be more to do with temperature.

No I don't think it would take more energy to move at those depths because, as you already said, its not much more dense down there as water doesn't really compress. In a submarine trying to stay quiet I think it would be an advantage because the higher water pressure would lead to less cavitation on the propeller.

At a molecular level I don't think there would be any change.

#### Steve K

##### Flashlight Enthusiast
wiki tells me this:

"The compressibility of water is a function of pressure and temperature. At 0 °C, at the limit of zero pressure, the compressibility is 5.1×10−10 Pa−1.[27] At the zero-pressure limit, the compressibility reaches a minimum of 4.4×10−10 Pa−1 around 45 °C before increasing again with increasing temperature. As the pressure is increased, the compressibility decreases, being 3.9×10−10 Pa−1 at 0 °C and 100 MPa.

The bulk modulus of water is 2.2 GPa.[28] The low compressibility of non-gases, and of water in particular, leads to their often being assumed as incompressible. The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.[28]"

At the molecular level? .. I assume that this means that the molecules are closer together. Don't know if the hydrogen atoms snuggle up a little closer to that oxygen atom or not... a gentleman never tells.

regards,
Steve K.

#### LukeA

##### Flashlight Enthusiast
Bear with me, I had a whole post written last night and then, because I was in Windows to use a piece of software and I neglected to click "keep me logged in", I lost the whole thing and went to sleep.

At 2000ft, water increases slightly in density, from 999.91kg/m^3 to 1002.3kg/m^3 (at a constant temperature of ~45F). The pressure required to do that is 6.23MPa, which is about 65atm. Now, it takes energy to compress the water, and that energy is called PV work. Add PV work to internal energy, which is together the energy in the bonds of all the atoms and their kinetic energy, and you get enthalpy, which is the total energy content of the system per unit mass. In this case, the internal energy actually decreases slightly because the water molecules have less room to move around, so they have a lower average kinetic energy, but a bunch of PV work has been added so the enthalpy is significantly higher at depth.

Additionally, NIST12 predicts that the viscosity deep down is slightly lower than at the surface, which, getting back to the original question, means that pushing an object horizontally through water would be slightly easier at depth than near the surface, but I'm not sure the difference would be perceptible.

An interesting tidbit: the speed of sound is a little (10m/s) faster at depth, probably because the water is more dense.

NIST12 data
Code:
``````Temp(K) P(MPa) Density  Int. En.  Enthalpy Entropy  sound speed viscosity
280.00	0.10132	999.91   28.794	 28.895   0.10411  1434.3       1433.5
280.00	6.2280	1002.9   28.714	 34.924   0.10379  1444.2       1425.1``````

#### beerwax

##### Enlightened
thanks guys. if density and viscocity arent much different i guess this means that water at 2000 feet looks and feels like water at 2 feet. so for a submarine , if it can avoid crumpling , and leaking , its business as usual. i still cant think of a practicle demonstration of this where you can see it. i have seen footage on the 'tv' of a seal swimming at 1100 feet supposedly, and it looked to be moving normally.

wiki tells me many things , including that as the water cools and gets below 4 degrees C it then starts to expand slightly and become less dense.

and 2000 feet isnt deep, the ocean goes way deeper than that. we can put a man on the moon, but can we put him on the ocean floor.

#### elgarak

##### Flashlight Enthusiast
As far as pressure is concerned, spaceflight is much easier. Max pressure difference between inside and outside is 1 atmosphere for a spacecraft. Less if you use a oxygen only atmosphere, like they did on Apollo, or for the EVA suits now.