I was thinking about deep sea, specifically. If take an air-filled balloon deeper and deeper it will get smaller and smaller. If I take a water filled balloon, it doesn’t change much (I guess temperature may have a small effect on density?).
If I take a thin aluminium air filled sphere it will crush at a certain depth. If it’s water filled it stays intact.
Here’s my question: if I enclose myself in a metal sphere that’s filled with water (so that it doesn’t crush), will I be subjected to the pressure of the water above me? If yes, how is the water pressure transmitted through the rigid sphere? If it was an air filled sphere, it would crush me, but does the water in the sphere somehow protect me?
I think I just haven’t got my head around a few key principles…
In: 1
If you are inside of a sufficiently thick metal sphere, your body will be isolated from the pressures of the water outside the sphere. The pressure forces outside the sphere will equalize with compressive forces within the metal of the spheres wall. As long as the sphere remains the same size, it does not change the volume inside of the sphere where your body is, therefore the pressure within the sphere stays the same.
All is well for you until a creepy phantom fish morphes through the wall and eats you.
Ok, the sphere will compress under the pressure of the water caused by the weight of water and depth. How much it will compress will depend on the thickness and diameter and specific metal of the sphere. Since the water will now occupy a smaller volume inside the sphere, it’s pressure will increase, but not necessarily all the way up to outside pressure. The thinner the walls, the closer to outside pressure the water will be, the thicker the walls the lower the pressure of the water will be inside the sphere.
For something without much internal strength like a balloon or a very thin metal foil sphere, the inside pressure will be the same as the outside pressure.
Water is an incompressible fluid. That is it will always maintain the same volume under pressure. This is due to the molecular structure of H2O. Pressure in the ocean comes from the weight of the water above and the compression resistance of the water around you. That weight and resistance would be transfered through the metal sphere you imagined, to the water inside. You would hear some cracking and creaking as the imperfections in the metal are pushed around, but as the weight presses on the sphere from the outside, the resistance to compression presses out from the water inside.
In your question, I assume that the sphere you are using is, indeed crushable, since you say it is filled with water and you’re inside it.
That being the case the answer is easy. As the pressure outside the sphere presses in, it is transferred to the water inside it and crushes you!
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