4.4.13

Pressure

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Pressure

Pressure is the force per unit of area.

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Force on container

  • In fluids (liquids or gases), the particles collide with the walls of their container.
  • The particle exerts a force on the container and the container exerts a force on the particle.
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Force at right angles

  • The force exerted on the surface in contact with the fluid particle will be at the normal to the surface (at right angles).
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Equation

  • Pressure is defined as the force divided by the area perpendicular to the force over which the force is applied, or:
    • P=FAP = \frac{F}{A}
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Units

  • The SI unit for pressure is the pascal, where:
    • 1 Pa = 1 N/m2.

Variation of Pressure with Depth

If your ears have ever popped on a plane flight or ached during a deep dive in a swimming pool, you have experienced the effect of depth on the pressure of a fluid.

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Pressure due to the weight of a fluid

  • The pressure exerted on the bottom of a container by the weight of the fluid is:
    • P=mgAP = \frac{mg}{A}
  • We can find the mass of the fluid from its volume and density:
    • m=ρVm = \rho V
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Pressure due to the weight of a fluid

  • The volume of the fluid is equal to V=AhV=Ah, where AA is the cross-sectional area and hh is the depth.
  • Substituting the equations for mass and volume into the pressure equation gives:
    • P=hρgP= h\rho g
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Atmospheric pressure

  • Atmospheric pressure is another example of pressure due to the weight of a fluid that is due to the weight of air above a given height.
  • The atmospheric pressure at the Earth's surface varies a little due to the large-scale flow of the atmosphere induced by the Earth's rotation.
  • However, the average pressure at sea level is given by the standard atmospheric pressure PatmP_{atm}.
    • PatmP_{atm} = 1 atmosphere = 1.01 × 105 N/m2 = 101 kPa

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1Physical Quantities & Units

2Measurement Techniques

3Kinematics

4Dynamics

5Gravitational Fields

6Deformation of Solids

7Thermal Physics

8Oscillations

9Communication

10Electric Fields

11Current Electricity

12Magnetic Fields

13Modern Physics

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