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UNIT 7: PRESSURE

  • Pressure: It is the force per unit area.

P = F / A

  • Unit: Pa

  • Examples

    • Swords exert high pressure because they concentrate the force on a small area.

    • Skis exert low pressure on the snow because the weight is spread over a large area.

  • Pressure of liquids

    • Pressure of a liquid is due to weight of the liquid.

      P = h ρ g

      When something is immersed in a liquid, the liquid pushes it up. This upthrust is because the pressure near the top of the object is less and the pressure near the bottom of the object is more.

  • Hydraulic systems: In such systems, pressure is transmitted between pistons using liquids because liquids are incompressible.

     Py = Px
    Fy / Ay = Fx / Ax
    Fy = Fx ( Ay / Ax )

    This means that force Fx will result in a bigger force Fy if Ay > Ax.

  • Pressure of gases

    • Pressure of gases is due to molecular collisions.

    • Pressure of a gas can be changed by

      • changing temperature

      • changing mass

      • changing volume

    • Atmospheric pressure

      • At sea level it is:

        1 atmosphere = 1.0 x 105 Pa = 76cm of Hg = 760mm of Hg

      • On a mountain top it is less.

      • In a deep mine it is more.

  • Simple Mercury Barometer: A simple mercury (Hg) barometer can be used to measure atmospheric pressure.

    • Working: Pressure of atmosphere is equal to the pressure of (additional height of) liquid.

  • Manometer: A manometer can be connected to a gas container to measure the pressure of the gas inside the container.

    • Working:

      • PG = PL + P0

        OR

      • PG + PL = P0

  • Boyle’s Law: P1 V1 = P2 V2

    • This formula assumes that temperature or mass does not change.

  • Total pressure beneath the surface of a liquid is the sum of atmospheric pressure and the liquid’s pressure.

  • When a molecule hits a surface it exerts a force.

  • When many molecules hit a wall at the same time, their collective force per unit area is pressure ( P = F / A ).

  • Pressure depends on frequency of collisions.

    • Frequency of collisions is affected by change in mass, volume or temperature:

      • When mass increases, the average separation between molecules decreases. So more frequent collisions.

      • When volume decreases, the average separation between molecules decreases. So more frequent collisions.

      • When temperature increases, the speed of molecules increases. So more frequent and more forceful collisions.


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