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Pressure exerted by a liquid is different to gas?

  1. Nov 9, 2009 #1
    Hello everyone,

    I have made number of topics recently I think they have all stem from my poor understanding in this matter.

    Ok in a gas, molecules can take up a volume and exert a pressure. Inside a fixed container when you increase the temperature pressure increases because more gas molecules are hitting the walls.

    Now in a liquid the molecules are close together and volume is determined by the shape of the container. So inside the container liquid molecules don't hit the walls and exert pressure some other way. What is this way? Is this called hydrostatic pressure? Also when you make the molecules move faster in a liquid would that increase the pressure like in a gas? According to what I have read it actually decrease hydrostatic pressure. However I have a picture in mind more kinetic more molecules hitting the walls higher the pressure. Maybe I'm just not understanding how gases and liquids behave in a molecular level when it comes to pressure.

    So please help me with this. I would greatly appreciate. Thanks :smile:
  2. jcsd
  3. Nov 9, 2009 #2


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    In a gas the molecules move about freely and hit all the walls equally - it is this motion that causes pressure which is the same everywhere in the container. The pressure only depends on the number and speed of the impacts - so only on the amount of gas and it's temperature.

    In a liquid the molecules near the walls push against the wall and are pushed by the other molecules above/behind them. The deeper you go the more molecules are above you and the more force they push with and so the pressure depends on the depth (and the density of the liquid).
  4. Nov 9, 2009 #3
    So why does hyrdostatic pressure decrease when kinetic energy increases when blood vessels constrict?

    wikipedia states otherwise

    "Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries, small arterioles and veins. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in staunching hemorrhage and acute blood loss. When blood vessels constrict, the flow of blood is restricted or decreased, thus, retaining body heat or increasing vascular resistance. Cutaneously, this makes the skin turn paler because less blood reaches the surface, preventing the radiation of heat. On a larger level, vasoconstriction is one mechanism by which the body regulates and maintains mean arterial pressure.

    Substances causing vasoconstriction are called vasoconstrictors or vasopressors. Generalized vasoconstriction usually results in an increase in systemic blood pressure, but it may also occur in specific tissues causing a localized reduction in blood flow.
  5. Nov 9, 2009 #4


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    Ok the answer for liquids and gasses was a bit simplified for simple containers.
    With blood vessels it's a little more complicated because the volume of blood in the system and the pumping rate also change.

    If the blood vessels constrict, the volume gets smaller, with the same amount of blood present the pressure increases because the walls are pressing more strongly against the liquid and so the liquid is pressing more strongly against itself = increased pressure
  6. Nov 9, 2009 #5
    You've also got to be careful comapring liquid sitting in a jar to blood in the body because blood is flowing.

    Notice the second half of the word hydrostatic.
  7. Nov 9, 2009 #6
    So draw a FBD of a water tank and calc. the force at different levels in the tank. You'll notice as depth increases, your weight component on the FBD diagram increases (more water = more weight).

    As you heat water up, yes the molecules move more but begin evaporating. Thus less liquid present, thus less weight. Hydrostatic force decreases.
  8. Nov 9, 2009 #7


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    You're describing two separate cases there and that's making you think their behaviors are more different than they really are.

    Case 1: Open Container
    In an open container, gases and liquids behave very similarly:

    -Both can be "poured" into a their container, with excess spilling out of the container.
    -Both are kept in their container by gravity.
    -The pressure of each results from gravity.
    -The pressure in each is a function of depth.
    -The density of each is a function of depth.

    The primary difference is that gases are esentially infinitely capable of filling up space whereas liquids are not. As a result, the density gradient of gases is much steeper. For a liquid, the denisty at the "top" of the container may only be a few percent different than the density at the bottom. For a gas, the density at the bottom of the container is essentially infinitely greater than that at the top.

    Case 2: Closed Container

    There are similarities:

    -When completely full, the pressure is somewhat dependent on gravity, somewhat dependent on temperature and the random motion of molecules.
    -Pressure and density again both vary with depth.

    The primary difference here is that, again, a gas is essentially infinitely capable of filling an empty space, as determined by the random motion of molecules. In a liquid, molecules are molecularly bonded together and not free to expand infinitely (unless they boil and it ceases to be a liquid or if the pressure is so high the difference between a liquid and gas goes away).

    Hydrostatic pressure exists for both.
    In a closed container, yes.
    That question may have more to do with biology than physics, so you may want to post that in one of our biology forums.
  9. Nov 9, 2009 #8
    Thanks for the all answers but I'm still confused. If there is hyrdostatic pressure inside a container containing liquid. If is starts to move what do you call that moving pressure(hyrdokinetic pressure). Basically what are the types of pressures inside a liquid.

    This is some info on Bernoulli effect

    "When air travels through an airway at a CONSTANT FLOW RATE, the total energy of the fluid (PE + KE) decreases because friction converts some of this energy into heat. Since the fluid is moving at a constant flow rate, the loss is in potential energy (decrease in hydrostatic pressure)."

    "As we move down the airway, the tubes begin to narrow. When a tube narrows, fluid velocity increases causing an increase in KE at the expense of PE (ie. a compensatory decrease in hydrostatic pressure). This is referred to as the BERNOULLI EFFECT."

    Ok what I like to know is that even though hydrostatic pressure is decreases is someother pressure (caused by moving molecules) increase? Thanks!!
  10. Nov 9, 2009 #9


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    Also known simply as "static pressure" and "velocity pressure". Add them together and you get "total pressure". Bernoulli's principle is that the sum of the two - the total pressure - is constant along a streamline.
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