# What would happen to pressure

We have a line with 1000# liquid pressure, we pump it into a container that is 100% non expandable and remove `100% of the air. We now cut off pressure using a 100% positive cut off. Would the pressure cease to exist the instant the pressure line was sealed off?

No. When you connect the line to the container, the material will flow from line to the container until they’re at an equal pressure.

When you seal off the pipe from the container, that container still holds the pressure it equalized at with the pipe.

Agreed, because liquids are still compressible (a bit).

I think we’d run into problems if we have a theoretically 100% incompressible liquid as well as a 100% rigid container, but since neither of those things can exist in reality, pressure is able to exist.

That makes sense to me even though I believed the answer to be zero.

This is how propane cylinders are filled.

Not exactly, propane is under pressure because it is heated past the boiling point.

What is the boiling point of propane at all the different pressures then? Will it boil at -273K at 1000 pounds of pressure in the tank?

I believe it will change to a gas at low temperatures but not that low.

When trucks pump the tank full of liquid propane, they do not first heat the propane nor the tank.

The answer to your original question is no: any substance that is free to move must necessarily have the same pressure everywhere within it. If it did not, the substance would flow from regions of high pressure to low pressure until equilibrium was reached. So when you connect your supply line to your container, fluid will flow until the pressure is the same throughout. If you cut off the flow before the pressure is equalized, the container will not be full. (It could be if you were dealing with a gas, because a gas always expands to fill the available volume – any amount of gas will “fill” any size container).

Nor is this any different if you are dealing with a substance that is completely incompressible. We have to think carefully about how the limit of incompressibility is taken, but there is no reason why the pressure, considered as a derivative of force with respect to compression, cannot remain finite while the force goes to infinity and the compression goes to zero.

Also keep in mind that pressure need not be generated by a restoring force, as it would be when squeezing an elastic substance. The pressure exerted by a gas, for example, has nothing to do with any elastic restoring force – it is a kinetic effect, caused by the bombardment of the walls of the container by the moving atoms or molecules of the gas. There is certainly a substantial kinetic component to the pressure exerted by a liquid, too.

Wow, I was right, but for the wrong reasons. I totally missed the noncompressable liquid aspect (thought only water was noncompressable). However, it does make sense that force can be carried by something other than compression.

Imagine a continuous jet of water shooting into that nonexpandable, completely full container; if you close the valve, that jet will bounce around in the tank until it’s released (though, along the way, it’ll scatter and create wakes until the entire contents of the container are pushing outwards).

I think propane start to boil at about -44 degrees at sea level. Below that you could poor it like any liquid.