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Confusion about liquid having more pressure at the bottom

  1. Jul 24, 2011 #1

    Femme_physics

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    I know that a liquid has more pressure at its bottom part than upper part. Does it mean molecularly than the molecules are more condensed and numerous at the bottom?

    Also, I watched this clip:




    This video clip says in 00:50 that "when a liquid is enclosed in a space, and a pressure is applied to the liquid, this pressure is transmitted equally to all parts of the liquid."

    And yes, we know for a fact there is more pressure at the BOTTOM of the liquid than at its upper part, so isn't more force transmitted to the bottom than upper part? Or is this quite unrelated?
     
    Last edited by a moderator: Sep 25, 2014
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  3. Jul 24, 2011 #2

    Femme_physics

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    Also, at 4:30



    The guy says that pressure will be equal at all those points. But that's not true, is it? Since we KNOW there's more at the bottom!
     
    Last edited by a moderator: Sep 25, 2014
  4. Jul 24, 2011 #3

    I like Serena

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    You understand everything already! :smile:

    The additional or external pressure is distributed equally.

    The pressure at any point in the liquid has 2 parts.
    One part is caused by the weight of the liquid above.
    The other part is caused by applying external pressure.

    Btw, in the video the holes are at the same height, so the pressure will be equal.
     
    Last edited: Jul 24, 2011
  5. Jul 24, 2011 #4

    Femme_physics

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    Makes perfect sense then :)

    Thank you!
     
  6. Jul 24, 2011 #5

    Femme_physics

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    Wait, but an ideal fluid cannot be compressed. So how is the weight being "felt" exactly by the bottom liquid if it's not compressed?
     
  7. Jul 24, 2011 #6

    I like Serena

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    The same way that external pressure is "felt".
    The weight of the fluid on top generates pressure to the fluid at the bottom, just like external pressure would.

    In reality the molecules are compressed as you already said.
    Not all liquids are the same and one can be compressed more easily than another.
    Still, the effect is the same.
    Consider an ideal liquid to be so hard to compress that it is unnoticeable.
    Beyond that, it's just another "ideal" situation like frictionless surfaces, ideal ropes, point masses, wires without resistance, vacuum, and whatnot.
     
  8. Jul 24, 2011 #7

    Femme_physics

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    Ah, so all fluids in reality are compressible. But we can ignore it like we can ignore friction. Makes perfect sense now :smile: Thanks!
     
  9. Jul 24, 2011 #8

    russ_watters

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    Compression isn't required. You don't get shorter when you pick up a heavy object, do you?
     
  10. Jul 24, 2011 #9

    olivermsun

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    Also, be careful -- a liquid is a fluid, but not all fluids are liquids. Some ideal fluids are all about compressibility!
     
  11. Jul 24, 2011 #10
    I do, I get shorter of breath.

    :rofl:
     
  12. Jul 26, 2011 #11

    rcgldr

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    Gravity is an external force that is responsible for the difference in pressure versus height of a fluid. If the fluid is enclosed in a container, then the pressure gradient will result in a net downforce on the container equal to the weight of the fluid.

    An increase in pressure means an increase in the average speed of the molecules (temperature) and/or the density (more collisions per unit time). For a real fluid, if temperature is the same thorughout the fluid, then the pressure is related to the density.

    link to some charts for water and some math

    density_temperature_pressure.htm

    The symbols for density and pressure look almost identical in that article. For the density and change in pressure equation, that equation with the density displayed as the smaller rho character ρ:

    ρ1 = ρ0 / (1 - (p1 - p0) / E)

    density_1 = density_0 / (1 - (pressure_1 - pressure_0) / E)

    Note that the math in the article doesn't work for an ideal fluid, because an ideal fluid is not compressable, so there would be no change in density regardless of pressure. An ideal fuild creates dilemmas like this when trying to explain some aspects of physics.
     
    Last edited: Jul 26, 2011
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