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Air Pressure Again

A metal box, opened on the top, in put on the ground, then it’s closed welding a metal surface on it, in order to obtain a parallelepiped full of air. Suppose that the box walls are so strong that they would be perfectly resistant to the outside air pressure even if the box was empty. Then the air inside the box isn’t subjected to any influence by the external air pressure. In other words, standing inside the box there is no way to understand if outside there is air or empty space. Now, according to Stevin’s law, the pressure on the bottom surface inside the box should be equal to dgh, where h is the box height, much less than the air pressure outside! But actually the internal pressure is the same as the external pressure. I can’t find the mistake. Moreover i can’t understand how the pressure p in the gas law pV=nRT is involved in all that. thank you.

Asked Alessandro Morelli
Answer

Air and Liquid Pressure

Does air pressure in a capped bottle is different as that of open bottle??

If not and the atmospheric pressure in a closed container is same as that of the surroundings let 1bar at sea level, if i consider a
tube both end open and dip one end in water (like pipette in chemistry lab) and close the other by thumb, water remain
hanged in the tube..  if we say it is because the atmosphere that pushes up on the water in the tube is same as that of remaining air in tube pushing down on the water..won’t the water fall out due to its own weight as the upward and downward pressure is balanced…

Please explain the whole process and compare weight of water with up and down pressure by atmosphere..

Again would liquid ‘ll flow out of a container through a hole in vacuum??

 

 Asked Shashank Patra

The Air Pressure in an open bottle is equal to the atmospheric pressure. The pressure inside a closed bottle can be different.

In the experiment described, when the tube is partially filled and the upper end is closed, the water tries to fall down creating a lower pressure above it inside the tube. This creates a pressure difference, the outside pressure greater than the pressure inside and the water can fall only upto the level where the weight of water column is balanced by the force due to difference in pressure created.
See one live demonstrations here


 

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