I know that if you are inside of a hermetic room, the pressure is the same. But I can’t understand *why*. For example, if you enclose water from the bottom of the ocean in a box, shouldn’t it be suddenly at the same pressure than outside, due to the fact there is not the weight of the water column above ?
Thanks!
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I’m sure I don’t understand your question.
Water pressure is only acting on the outside of the container and not on the air inside of it. That’s how people can take submarines down to deep places like the Challenger Deep or the wreck of the Titanic, despite there being enough pressure down there to basically crush them like an empty pop can.
The water column weighs down on your enclosure, which in turn pushes on the water inside it. Only when you remove the enclosure from the bottom of the ocean, can the water expand and relieve the pressure. It then pushes on your container, which either expands as well, or pushes back on the water if it’s rigid enough.
Firstly there are two types of fluids, incompressible and compressible.
For all extents and purposes, water is incompressible. Whereas air is compressible.
For incompressible fluids, you are right in saying that pressure is from the hight of water above you, as it is pushing down and into you. The water isn’t higher pressure, it is just *under* higher pressure. So when you bring it to the surface, it will be the same.
For compressible fluids like air, when the air is under higher pressure, it becomes more compressed and thus higher pressure. So if you brought it to a less pressurised area, it would rush out.
Edit: I wondered for a while about your exact idea. I thought that if you had a roof over you, you shouldn’t feel the same pressure, but you do. It took me a few years of doing physics in school to understand it is the air next to you pushing you. Whereas underwater it is the water above you pushing you.
1. Regular buildings aren’t airtight, so air at atmospheric pressure always exists within and flows throughout the building. Remember, pressure is the aggregate force per unit area of the gases around you. It acts on every part of your body, not just the top.
2. In sealed chambers, we try to keep the pressure similar to atmospheric pressure because that’s what our bodies are designed to handle. We *can* make chambers with higher or lower pressure, but if we go too high or too low, humans can’t access and work in those chambers.
If you enclose the water at the bottom of the ocean, then you have enclosed water that is already pressurized, so it stays pressurized. If you brought it to the surface, it would still be pressurized, the container would try to expand outwards. I suppose at theoretically perfect liquid with no dissolved gasses might not still be pressurized, because it wouldn’t compress under load.
If you enclose water at the bottom of the ocean in a box, then the column of fluid above it is applying pressure to the box, which applies pressure to the water within in.
When you close the box, you lock in the pressure. If the box is rigid it’s stuck there. If it’s flexible then the box is just transmitting pressure through the wall. Either way the pressure doesn’t change.
If you take a rigid box, close it at the surface, and sink it then the pressure inside *doesn’t* come up. We call that a submarine.
If you close a rigid box at depth and then extract some fluid from inside the pressure *will* fall, until you’ve got a vacuum in the box or the box fails.
Imagine the water is a spring. You push down on it, squeezing the spring smaller, then you take the now-small spring and shove it in a box that’s barely any bigger than the squished spring.
Even though you aren’t pushing down on the spring, it’s still stuck in its compressed state because the box is holding it that way. Opening the box would release the pressure (uncompress the spring) but as long as the box is small enough that the spring doesn’t have room to decompress it will stay at its current pressure.
Water works the same way.
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