# Pressure in a fluid

1. Feb 6, 2012

### aaaa202

Suppose I have a fluid in a system like the one on the picture. The fluid is lower in one of the pipes because the pressure in this pipe is bigger than the atmospheric pressure.
But is it then so that the pressure in the top of the water of pipe 1 equals the pressure of the water in the same level in pipe 2?
I really have a hard time understanding pressure in fluids. I mean how can you talk about for instance the pressure being bigger in the bottom of a fluid because it is a rest - indeed the fluid is not at rest since the particles in it are continuously moving around..

2. Feb 6, 2012

### geekedsloth

I'm not sure what picture you are talking about, but if I am not mistaken the water would not actually compress(I don't think you can have compressed water under normal circumstances, but I am probably wrong), but rather the air in the pipe is pressurized. When the air is pressurized it will try to expand, so to speak. Since the air and water can not occupy the same space, the air will push the water out of the way. So, in summary, if I am not mistaken the water itself does not have a pressure, but rather the pressurized air exerts a force on the water. As for your last question, I believe when we say the water is at rest we are not referring to anything as small as particles, but as a whole. By that logic no object is ever at rest. I think when we say water in a cup is at rest, it simply means the water is not moving around in the cup(but the particles the water consist of are). I hope that helps.
Edit: didn't read the op too well, for some reason I thought you were talking about water, so what I said probably doesn't apply here.

3. Feb 6, 2012

### aaaa202

okay thanks. I forgot to attach the picture so here it is. Does that make you change explanation? Indeed the pressure on the surface of the water in pipe 1 must be as big as the upwards pressure from the water below. But is this pressure then equal to ρgh, where h is the difference in height between the water in the two pipes?

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4. Feb 6, 2012

### nasu

The pressure on the surface of the liquid in tube 1 is equal to
$$\rho g h + P_{atm}$$
$$\rho g h$$ is the pressure difference.

5. Feb 6, 2012

### aaaa202

Hmm how can I realize that is true? Can you show me in terms of forces..? :)