# What causes water pressure? (Physics wise)

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?

Gravity

The weight of stacked, freely movable water molecules. Think - a column of marbles.

As said, gravity. If you had a big water tank in Earth orbit***** and it was filled with water and you had pressure sensors all thru it the pressure would be the same at the center as at the surface.

*****Technically it would be in free-fall not zero-G, but the principle would be (almost) identical if the tank were drifting in space (i.e. not accelerating nor decelerating) outside of any planet’s gravity well…

I think it’s an excellent question - the key part of which is “what is happening on a particle level?”

Of course, the pressure is a result of gravity acting on the mass of water and air above. And we have Pascal’s Law that tells us macroscopically about pressure transmission. But I don’t think any of that answer’s the OP’s quesion.

What’s a good intuitive picture of how, at a molecular level, pressure is transmitted through a liquid?

Thanks! Yes this is what I’m after.

Make a paper band and stand it on a flat surface.

Add marbles, one at a time, until the band is full of marbles.

Add another marble and push it down in amongst the others. The paper band bursts.

Gravity is indirectly what causes the pressure difference in a column of gass as well, with the measured pressure, as you write, being higher lower down due to more particles being involved.

But think what happens as the particles being closer and closer together and think about what incompressible means. The water molecules are packed almost as close as they can be packed, so to move one you have to move a whole column of molecules against gravity.

Interesting stuff but doesn’t answer the question on a particle level.

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 don’t think this is a good analogy for pressure in a liquid. Liquid behaves more like stationary marbles than ideal gas molecules. (Or vibrating marbles, if you like - but the vibration isn’t the mechanism for imparting pressure on the container.)

For example, heating a cup of water doesn’t cause the pressure at the bottom to rise.

Well they aren’t truly incompressible. Assume your particle is a tiny volume for the sake of argument for that tiny volume to be in equilibrium the forces on each side have to be equivalent. So for a stack of water even if individual particles are bouncing this way and that the weight of an element in the stack isn’t going to change because the averages of the particles moving this way and that way cancel out to give the same result as if the particles weren’t moving.

So on a particle level if you are making the assumption that a particle is somewhat point like pressure isn’t really useful as pressure is better though of as a force spread over an area. And at the particle level the collisions and the forces aren’t really measurable in a huge tub.

So look at pressure more as the aggregate of all those tiny forces of collision. In some ways asking for pressure at a particle level is like asking for temperature at the particle level. At least at my level of understanding.

In a sealed container heating liquids should cause the pressure to rise somewhat.

You can just treat the marbles as if they are the average spherical radius of vibration of some imaginary smaller particles.

If you want to model it with moving particles, you’re going to need to push wasps into a tissue paper bag, or some such. Good luck.

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.

Pressure is the amount of force acting per unit area, pounds per square inch or Newtons per square meter.

As above, this force is indeed the force of gravity, and pressure is the result of gravity acting on the entire fluid in question. Simply put, the pressure at any level is caused by the weight of the water (and air) in the column above it.

We do have particles colliding with our unit area located on our pressure gauge, but that’s more of a proxy for pressure. Pressure isn’t the collisions per se, rather it’s the force imparted on our gauge by these collisions. The actual physics is fairly squirrelly once we get into Navier/Stokes equations …

This, and the previous point that I am thinking of liquids as an ideal gas are helping me get there.

Ta again.

Very close, just remember that volume is constant, pressure varies with mass times temperature.

What is temperature at the particle level?