Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Expansion and contraction of a gas

  1. Jun 14, 2012 #1
    1. The problem statement, all variables and given/known data
    The picture shows the settings of the experiment.The aim of this experiment is to investigate about the expansion and contraction of gas.
    When the boiling tube is put into boiling water,the coloured water rises as air in the tube expands when heated
    When the boiling tube is put into melting water,the coloured water falls as air in the tube contracts when cooled.

    I would like to ask,why the coloured water in the narrow glass tube would not fall into the boiling tube due to gravity before putting the boiling tube into boiling water and melting water?

    2. Relevant equations

    3. The attempt at a solution

    Attached Files:

    • 141.jpg
      File size:
      62.6 KB
  2. jcsd
  3. Jun 14, 2012 #2


    User Avatar
    Homework Helper
    Gold Member

    I'd like to give you an answer, but that answer is central to the whole point of the experiment.

    But perhaps I help in a different way. Treat the boiling tube, the stopper and the narrow glass tube as a single, rigid structure. Here are some questions to think about:
    1. Underneath the drop of water, what else is inside the tube (i.e. inside the rigid, tube structure)? (Don't answer 'nothing', because that's not correct. There is something else in there.)
    2. What's the only path for this stuff to get out of the tube?
    3. If you were to use a force, such as gravity (or any other force) to push down on the drop of water, what would happen to the other stuff already in the tube?
  4. Jun 14, 2012 #3
    2.through the narrow tube to the surrounding environment
    3.i dun know
    Do you mean that,the pressure inside and outside the tube are equal,so the air particles from both sides can't "push" the drop of water towards?
    If my assumption is correct,I still have a question.
    Replace the drop of water by a cork that fits the shape of the narrow tube.
    It won't fall although there is gravity exerts on it.
    It is because there is friction between the surface of boiling tube and cork.
    friction = gravity,so the cork won't drop into the tube.
    This is related to force exerts on an object instead of the air pressure,when should we use "pressure" and "force" to explain such phenomenon?
    (sorry my physics is really poor,I hope sb can understand what I mean)
  5. Jun 15, 2012 #4


    User Avatar
    Homework Helper
    Gold Member

    That's right. :smile:
    Yes. :smile: But remember, the drop of water is in there too. So what must happen to the drop of water if the air already in the tube is to escape?
    Almost, yes. That is at least the approximate idea.
    No, I'd say ignore friction for this problem. The friction on the water is not important for this problem. Also ignore friction for the cork analogy. There is something else more significant.
    Yes! it is about pressure! You've got it. :approve:

    If you squeeze a gas within a container into a smaller volume, what happens to the gas' pressure?

    Force is equal to the pressure multiplied by the area upon which the pressure acts. The net force may involve different pressures and different corresponding areas.

    There is atmospheric pressure above the water droplet. And the pressure of the gas below the water droplet (in the tube structure) is almost identical to atmospheric pressure. But it's not exactly identical (assuming the force of gravity on the water droplet is acting in the direction down the tube).

    Can you determine how the difference in pressure above and below the water droplet cancels out the force of gravity on the water droplet? (This is a subjective, rhetorical question.)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook