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Suppose i have a rigid container(cylinder) open from top. I put it in large reservoir having h column of liquid. There is no air in the container during the process of placement of the container inside the liquid. The container placed upside down at the bottom of the reservoir and it is leakproof such that whatever amount of liquid entered is cut off from the liquid surround the container , means , the liquid inside container is not in contact with the liquid outside it. My question here is that ,enter image description herewill the pressure of liquid inside the container remain same as it was before or it will change?

My intuition says that the pressure inside the liquid is due to the weight of the water column. Now that the liquid inside container is completely cut off from the surrounding liquid, it should be independent of weight of surround water column. Hence the pressure inside container should change and it would depend upon the weight of water column inside it. Pls clarify where am i getting wrong? All conditions are ideal, neglect compressibility and other things.

My initial question closed due to some unexplainable reasons so i am trying to clarify more about what i am trying to understant here, My doubt is that, pressure in fluids at a point is due to the weight of the fluid column above it. So will the pressure change if we somehow able to compensate the above weight (by completely blocking up from all sides a particular part of the fluid)? example, you have open container kept on the surface, atmospheric pressure acts on it(even inside the container as it is open). Now if you put a lid on the container such that it becomes leakproof( whatever air is inside remains inside and has no interaction with the outer air. Will the pressure now change in the container? enter image description here

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  • $\begingroup$ i don't know why it has been closed, i am really looking for the answer. I tried my best to explain it. $\endgroup$
    – PinkAura
    Dec 9, 2023 at 20:09
  • $\begingroup$ You may consider submarines as examples, or am I mistaken? $\endgroup$
    – Toffomat
    Dec 12, 2023 at 14:15
  • $\begingroup$ @Toffomat not submarine, i was considering an ideal case where you have a perfectly rigid container and somehow you flip it at the bottom surface, such that some liquid traps in it and any other liquid won't be able to come inside the container or go outside it, so i had a doubt whether pressure would be same inside it (as that of outside) or not.. $\endgroup$
    – PinkAura
    Dec 13, 2023 at 7:07
  • $\begingroup$ I wanna ask you, we are surrounded by atmosphere which exert some atmospheric pressure, do all closed containers on earth have pressure inside them equal to atmospheric pressure? Isn't it thing of daily experience? How did you even got the question? $\endgroup$
    – Qwerty
    Dec 13, 2023 at 12:41
  • $\begingroup$ my question was different, and it was too ideal to be true, so you cannot relate this to real life i guess... in real life we do not really have perfectly leak proof objects . there is some air going in and out $\endgroup$
    – PinkAura
    Dec 13, 2023 at 12:49

1 Answer 1

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If the small container with no bottom is introduced when the liquid is already there, then the small container will not change the liquid's pressure. It will be what it was initially.

If you have a completely sealed small container (rigid) with its own liquid in it, and place it as shown, and then somehow delete the bottom surface while maintaining a perfect seal, then the inner liquid will have a pressure corresponding to its original pressure, unaffected by the external liquid. E.g. you can even have a rigid container that had vacuum on the inside, and sink it into a body of water. As long as the seal holds indefinitely and the rigid walls are strong enough, the pressure inside will be unaffected.

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  • $\begingroup$ "If the small container with no bottom is introduced when the liquid is already there, then the small container will not change the liquid's pressure. It will be what it was initially" can you pls explain why this is so and where am i getting wrong? I think pressure is due to weight of liquid column above it and if you place a rigid container upside down at the bottom (such that it is leakproof), shouldn't it kind of a "protect" the liquid inside from the weight of liquid column above it,Thus changing pressure inside? $\endgroup$
    – PinkAura
    Dec 9, 2023 at 18:25
  • $\begingroup$ anyway, i got the answer i guess. initially the liquid part was in contact with the surrounding liquid , thinking pressure as energy density, the energy is transferred to them until the container blocks the liquid. now the inside energy is same as outside. correct me if i am wrong $\endgroup$
    – PinkAura
    Dec 10, 2023 at 18:42
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    $\begingroup$ When pressure is applied to any liquid, it compresses slightly (even at the ppm level). Before the small rigid container is introduced, the liquid has a density corresponding to the pressure at that depth. If you then cover that region with an inverted cup shape as you describe, the liquid inside would have to increase in volume (perhaps only slightly) to decrease its pressure. The rigid container walls will prevent this; therefore the pressure remains unchanged. $\endgroup$
    – RC_23
    Dec 12, 2023 at 7:04
  • $\begingroup$ If the container walls were not perfectly rigid, their flexing would allow the outside water to exert its pressure on the inner water, and again the pressure would remain the same. $\endgroup$
    – RC_23
    Dec 12, 2023 at 7:06
  • $\begingroup$ ohh got it , thanks $\endgroup$
    – PinkAura
    Dec 13, 2023 at 7:02

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