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Why doesn't a barometer with vacuum get crushed by the atmosphere?

  1. Sep 17, 2013 #1
    Why doesn't a mercury barometer containing vacuum get crushed by the atomospheric pressure?

    Here is what I think might be the reason.

    Although there is vacuum inside a mercury barometer, that vacuum is pressurized by the atmosphere that pushes mercury from the reservoir. That pressure is cancelled out by the pressure exerted by the atmosphere from outside onto the barometer tube containing the vacuum.

    Did I get it right?

    Additional question:

    Suppose I take a test-tube that can contain only 700 mm of mercury.

    Then, I fill this test-tube up with mercury, and place it in the reservoir.

    Now, since the atmosphere from outside is putting pressure on the reservoir at 1 atmospheric pressure, should this pressure make the mercury kept in the reservoir try to rise the level of the mercury inside the test-tube, and having failed to do so, expand the volume of the test-tube or break it apart?
     
    Last edited: Sep 17, 2013
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  3. Sep 17, 2013 #2

    Nugatory

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    The shell of the barometer is strong enough to stand up to the force of atmospheric pressure.

    If it were made of some more flimsy material, it would be crushed. For example, a thin-walled plastic bottle will collapse if you suck the air out of it but a glass or metal bottle will not.
     
  4. Sep 17, 2013 #3
    Thank you very much. So, glass is pretty strong then.

    But I have a question. Isn't the vacuum under pressure by the mercury inside the tube ( which is pushed above by the atmosphere outside? ) Because if it was not, wouldn't then the mercury inside the tube keep falling down util totally emptying the tube?
     
  5. Sep 17, 2013 #4

    SteamKing

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    You can't put pressure on a vacuum. It's either empty space or it's not a vacuum.

    The mercury column doesn't fall down completely because of the atmospheric pressure holding it up, remember? That's why barometers were developed in the first place.
     
  6. Sep 17, 2013 #5
    Yes, now I think there is no pressure on the vacuum too. Because if there was any pressure, there wouldn't be any static level of vacuum, which would then rise instead.

    Thank you very much for your answer.
     
  7. Sep 17, 2013 #6
    I thought the glass isn't strong enough, because I head that there was a scientist ( the inventor of air pump, I think ) who pumped air out of a metal sphere that was made up of two hemispheres joined together. The hemispheres couldn't be separated even with a number of horses trying to pull them apart. I thought, if atmospheric pressure was so much that even a number of horses' strength was relatively weaker, the glass should easily get crushed.
     
  8. Sep 17, 2013 #7
    Turns out a thick layer of glass can be made strong enough to hold it
     
  9. Sep 17, 2013 #8
    Thank you very much.

    It's interesting that although even a brick kept on a glass tube can break it ( I haven't done any experiment though. But even if a brick cannot break the glass tube, I think standing on a test-tube surely will ), 1 atmospheric pressure can not. I heard somewhere that the weight of an elephant is less than 1 atmospheric pressure. So, if weight of a little brick or a person can break the glass, why can't 1 atmospheric pressure?
     
  10. Sep 17, 2013 #9
    If your weight was distributed uniformly over the whole surface of the glass pointing inwards as is the case for the atmospheric pressure, than you wouldn't break the glass either.
     
  11. Sep 17, 2013 #10
    Pressure is force divided by area. Elephants have enormous feet; if they stand on flat ground, the pressure is less than atmospheric pressure. If an elephant stood on a tiny test tube, its weight would be concentrated on the much smaller area of the test tube, and the pressure would be much higher than atmospheric pressure.

    A lot of laboratory glassware is strong enough to withstand 1 atmosphere of pressure, so that it can be used in procedures conducted under low pressure. It doesn't actually have to be that thick. You can still break this glassware by dropping it or standing on it, since the resulting pressure is much more than 1 atmosphere.
     
  12. Sep 17, 2013 #11

    sophiecentaur

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    I have read through this thread and can't find any reference to the fact that a tube with circular cross section is, inherently, very strong. If you wanted a square section tube, it would need to be made of very much thicker glass.
    Also, a tube is much stronger in compression than if you pumped it up to + 1 Atmosphere, internal pressure.
     
  13. Sep 17, 2013 #12

    cjl

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    Also keep in mind that a cylindrical or spherical chamber is extremely strong against a completely uniform compressive load. The air pressure is uniform - it pushes inwards on all sides equally. If you stand on it however, the force might be smaller, but it is also not uniform. A cylinder is much weaker when loaded on one side than it is when loaded on all sides equally.
     
  14. Sep 17, 2013 #13

    russ_watters

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    It does depend on the material though.
     
  15. Sep 18, 2013 #14
    Thank you very much for your answer. But sorry as I didn't get it. When you try to crush an empty can of pepsi with your hand, isn't it easier if you try to crush the can by pressing on it from all sides?
     
  16. Sep 18, 2013 #15
    Thank you very much for your answer.

    So the credit goes to the cylindrical-ness of the tube?

    I'm sorry as I did not get the part ''Also, a tube is much stronger in compression than if you pumped it up to + 1 Atmosphere, internal pressure.'' Would you please care to explain what you said in layman terms?
     
  17. Sep 18, 2013 #16

    sophiecentaur

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    When a cylindrical tube is subjected to a high external pressure, the forces are all in a direction which compresses the material (strongest mode). When you 'pump up' the internal pressure, the forces are all tending to stretch the material (weaker mode).

    Many materials are much stronger when you are pressing against them than when you try to pull them apart. Concrete is a great example of this. A block will withstand many tons of weight on top of it but will crack quite easily on the outside edge if you try to 'bend it'. Pre-stressed concrete beams use steel bars with plates, pulling the ends together, to ensure that, even when used as cantilevers (with overhang), the net forces over all of the the beam are still compressive. Lightweight arches are much better engineering than massive horizontal beams. Try breaking a chicken's egg by pressing it together (longways) between the palms of your hand - it's unbelievably strong.

    Re the pepsi can: it's hard to set up an even pressure around the sides of a can but if you take a loop of string and pass it round the sides of a can, then tighten it by twisting round a stick (a tourniquet) and you will find the can is really pretty strong compared with when you just poke it with a finger. There is a party trick in which you can stand on a light drinks can and it will support you (all your weight applied over a very small cross sectional area of metal and the forces are all compressive). Bend down (balancing requires practice) and tap the sides of the can, to form a dimple, and the can collapses because the forces are no longer just compressive but can bend the material.
     
  18. Sep 18, 2013 #17

    CWatters

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    Air pressure is essentially caused by the molecules of gas that make up air (Nitrogen, oxygen etc)bouncing off each other and the surface of the vessel.



    A true vacuum contains no molecules so can't create a pressure or be pressurised.
     
    Last edited by a moderator: Apr 17, 2017
  19. Sep 19, 2013 #18
    Thank you very much. Yes, I was wrong to assume that.
     
    Last edited by a moderator: Apr 17, 2017
  20. Sep 19, 2013 #19
    Thank you very much for your explanation.

    Though I did not understand what you said about compressive force, concrete beams, arched beams etc, since I am not familiar with these engineering terms; I would love to know the basics required to understand this kind of stuff. Can you suggest me any book in which these are explained in easy language?

    I am sorry that I did not understand the procedure ''take a loop of string and pass it round the sides of a can, then tighten it by twisting round a stick (a tourniquet)'', possibly due to my poor ability to form image from words, but I really appreciate your efforts to explain it. Thanks a lot for that.

    I will definitely try crushing an egg, and doing that pepsi can trick.

    Thank you very much, again.
     
  21. Sep 19, 2013 #20

    A.T.

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    Think of a thin plastic pipe. It's easy to break it by bending. But it's difficult to crush it by compressing it from all sides uniformly.
     
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