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Understanding pressure with barometer as example

  1. Mar 13, 2013 #1
    In a barometer where a tube filled with mercury is inverted in dish filled with mercury, the mercury in the tube is suspended/rises to 760 mm mark. I read this is due to pressure of weight of mercury being equal to pressure of atmosphere.

    My question is that, if i were to take the tube out of dish, will the mercury still stay in as atmosphere pressure has not changed.
    Whats the physics behind pipettes in which the liquid stays suspended and does not flow out even when its removed from the dish?

    For this, my view of pressure is that in barometer, mercury molecules exposed to surface are under force of atmosphere and mercury molecules exert same force back and consequently have same pressure through out mercury in dish. When the tube containing mercury is put in dish, the mercury in tube applies force to mercury in dish and force of mercury in tube is greater. Thus, mercury flows out until its mass decreases enough so that force of mercury in tube equals that of mercury in dish. And this seems to happen when mercury is 760 mm high in tube at atmospheric pressure.

    If my microscopic view is flawed, please correct it. And answer my question according to correct microscopic view of pressure.
  2. jcsd
  3. Mar 13, 2013 #2


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    Try the experiment yourself.
    BUT use something less volatile than mercury ... say water in a tube, in a dish of water

    Ahhh but will it really ?
    pull the tube up out of the dish of water and see what happens
    Consider the airpressure on the surface of the dish of liquid, and what effect it has on the column of liquid in the tube :)

  4. Mar 13, 2013 #3
    You need to consider what the pressure is at the top surface of the mercury column in the barometer and on the top surface of the water in the pipette.
  5. Mar 14, 2013 #4
    k so pressure on surface of dish would be same as atmospheric pressure 1 atm. pressure in tube at top would be 0 as its vacuum. So mercury stays suspended. But if i take the tube filled with mercury out of dish, pressure in tube at top will still be 0 and pressure at opening of tube (since its in contact with air) will be 1 atm. so shouldn't mercury still stay in and not flow out when i take the tube out of dish?
  6. Mar 14, 2013 #5
    Theoretically yes. But it may be unstable if you tilt the tube slightly off vertical. The pipette works better in this regard, even though there isn't a perfect vacuum above the liquid.
  7. Mar 14, 2013 #6
    There are two other forces you need to consider: surface tension (which is what keeps the liquid in the pipette) and gravity (which is what will make the mecury fall out of the tube).
  8. Mar 15, 2013 #7
    Thanks a Lot every one!
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