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Barometers and Pascal's law

  1. Jan 19, 2016 #1
    I have a question about Pascal law. Recenly I was studying and I found this image in my physics book - a copy of the barometer which Torricelli used in his experiment (this is a redraw):


    Where the blue thing is the fluid (mercury), while the black thing is the container.

    According to my textbook, the pressure at point A is simply (p*g*h), the preasure caused by the fluid above it. However, why don't we also count the atmospheric pressure? Isn't it supossed to act in the entire fluid according to Pascal's law?

    Thanks in advance!
  2. jcsd
  3. Jan 19, 2016 #2
    There are no atmospheric pressure in the tube - it is closed tube and that white area at the top is vacuum. Therefore if you press liquid at the point B (atmospheric pressure), liquid will climb the tube at point A due to Pascal's law, until it counter weights the pressure;( here p*g*h is a weight of liquid in the tube).
  4. Jan 19, 2016 #3

    Buzz Bloom

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    Gold Member

    Hi Peter:

    The barometer is giving you information to tell you what the value of the unknown atmospheric pressure P is. See

  5. Jan 19, 2016 #4
    A common misconception is that Pascal's law states that in a fluid at rest, pressure is transmitted throughout the fluid uniformly. This is not correct. Pascal's law actually states that at a particular position within a fluid at rest, the pressure acts the same in all directions (i.e., it is isotropic). This says nothing about how it varies from position to position.
  6. Jan 19, 2016 #5
    Chestemiller, your comment surprises me! It appears my textbook has made a mistake with it's definition of Pascal's law! So, when is pressure transmitted though a fluid uniformly? And does this really have nothing to do with Pascal's law (what I mean is, do the special cases where pressure transmission is uniform follow from Pascal's law)?

    (Sorry if some of these questions are unclear. English is a second language for me!)

    Thanks for answers!
  7. Jan 19, 2016 #6
    In a gravitational field (i.e., with gravity), pressure varies with depth. So it is not uniform throughout the fluid. Pressure is transmitted uniformly throughout a fluid only if the fluid is in static equilibrium and there is no gravity. As I learned it, Pascal's law says only that, at a given spatial location in a fluid, pressure acts identically in all directions. That means that, if I place a tiny surface at an arbitrary location in a fluid that is in static equilibrium, the force per unit area is independent of the orientation of the surface, and acts perpendicular to the surface.
  8. Jan 19, 2016 #7
    there is mercury vapour above the liquid in the tube, the pressure at A is hρg + saturated pressure of mercury at the prevailing temperature.
    The space above the mercury is called a 'torrecelli vacuum'
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