What pressure not increases if I seal the container?

  • #1

Main Question or Discussion Point

CASE1:
A container with some water is open to atmosphere, pressure is definitely atmospheric, no somehow I put cover over it and sealed it also. Pressure would increase because new equilibrium will establish between water and its vapour so pressure will increase in the closed container.

CASE2:
There is an empty container that means only air, now I cover it. Pressure will not increase; but my doubt is; as atmospheric pressure is defined as the weight of atmosphere over it divided by the surface area. As I close the container the weight is acting on the cover that load will be taken by the whole container so how the particles inside the container will feel the atmospheric pressure? Cover is not a fluid that means no pascals law; so how pressure reaches inside the container?
 

Answers and Replies

  • #2
jbriggs444
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CASE1:
A container with some water is open to atmosphere, pressure is definitely atmospheric, no somehow I put cover over it and sealed it also. Pressure would increase because new equilibrium will establish between water and its vapour so pressure will increase in the closed container.
Before any hypothetical vapor is emitted from the surface of the sealed-in water, that water is under atmospheric pressure. It was at atmospheric pressure before it was sealed in so it will be at atmospheric pressure immediately after.

If any vapor were to come out, it would be under at least one atmosphere of pressure. But under one atmosphere of pressure, that vapor would be condensing. So no vapor can come out. So pressure remains at one atmosphere.
CASE2:
There is an empty container that means only air, now I cover it. Pressure will not increase; but my doubt is; as atmospheric pressure is defined as the weight of atmosphere over it divided by the surface area. As I close the container the weight is acting on the cover that load will be taken by the whole container so how the particles inside the container will feel the atmospheric pressure? Cover is not a fluid that means no pascals law; so how pressure reaches inside the container?
Pressure is not defined as the weight of atmosphere divided by surface area. It is defined as force per unit area regardless of what is exerting that force.
 
  • #3
In first case; evaporation rate will become slower and more number of vapor will be above water surface. As diffusion of vapor is now stopped.

In second case; I don't understand why pressure is same. What is the new force that come into play for creating same pressure.
 
  • #4
jbriggs444
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In first case; evaporation rate will become slower and more number of vapor will be above water surface. As diffusion of vapor is now stopped.
Did you seal it with zero head space or with some air-filled head space?
 
  • #5
Did you seal it with zero head space or with some air-filled head space?
some air filled head space
 
  • #6
jbriggs444
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some air filled head space
Oh, that makes sense then. Yes. If the head space is dry air then vapor can evaporate into the head space and increase the pressure within the container.
 
  • #7
Help me in the second case :frown:
 
  • #8
jbriggs444
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Help me in the second case :frown:
Boyle's law. If you start with enough gas in the container to maintain atmospheric pressure at its current temperature then closing the lid will not change that.
 
  • #9
Boyle's law. If you start with enough gas in the container to maintain atmospheric pressure at its current temperature then closing the lid will not change that.
Didn't get you; sorry
 
  • #10
jbriggs444
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Judging by the rapidity of your response, you did not Google Boyle's law. Accordingly, I will stop helping you.
 
  • #11
Actually Sir, I know Boyle's law as follows; Pressure is inversely proportional to the volume, if temperature remains contant. But I was not able to relate this concept here
 
  • #12
jbriggs444
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The volume of the chamber does not change just because you close the lid. Accordingly, the pressure does not change just because you close the lid.
 
  • #13
I always used to relate atmospheric pressure due to weight of the air. What I understand atmosphere is hydrostatic that means each part of it is at 1atm; neglect pressure change due to height as density is low.

But something hollow solid container is placed in that sea of atmosphere and as solid will not follow pascal's law it, air inside the container will not experience the same pressure as outside. But here thermodynamics come into play. Due to isothermal condition. I think I am almost convinced but something is missing in my conceptualization.
 
  • #14
jbriggs444
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Suppose you seal the container as before. Then you take the container and fly it out into space. Do you expect the pressure within the container to change?
 
  • #15
Suppose you seal the container as before. Then you take the container and fly it out into space. Do you expect the pressure within the container to change?
You have a point. I got it; pressure will be same; that's how pressure is maintained in airplanes at high altitudes.

I think I am lacking in something very primitive concept. But now what I think let me tell you. When container was open; air above it also playing role in the pressure of the air inside the container; but when I put lid over it, I created a new wall for the collisions of the molecules inside the container that is compensating the less pressure due to no interaction with air outside the container.
 

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