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Article: Pressure


Definition and Units of Pressure

  • Pressure is the amount of force applied perpendicular to the surface of an object per unit area.
  • The symbol for pressure is p or P.
  • The SI unit for pressure is the pascal (Pa), which is equal to one newton per square meter (N/m²).
  • Other common units of pressure include pounds per square inch (psi), bar, millimeters of mercury (mmHg), and torr.
  • The CGS unit of pressure is the barye (Ba), equal to 1 dyn·cm² or 0.1 Pa.
  • Pressure can be expressed in joules per cubic meter (J/m³) to relate it to energy density.
  • Meteorologists often use hectopascals (hPa) for atmospheric air pressure.

Pressure Distribution and Scalar Nature

  • Pressure is distributed to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections.
  • It is a fundamental parameter in thermodynamics and is conjugate to volume.
  • Pressure has no direction as a scalar quantity, but the force it relates to has a direction.
  • Changing the orientation of the surface element changes the direction of the normal force, but the pressure remains the same.
  • In a static gas, the gas as a whole does not appear to move.
  • The individual molecules of the gas are in constant random motion.
  • The gas exhibits hydrostatic pressure when at least partially confined.
  • Pressure is a scalar quantity, not a vector quantity.
  • Pressure force acts in all directions at a point inside a gas.

Types of Pressure

  • Fluid pressure is the compressive stress within a fluid.
  • Fluid pressure occurs in open conditions (e.g., the ocean) and closed conditions (e.g., a water line).
  • Pressure in open conditions can be approximated as static pressure.
  • Closed bodies of fluid can be static or dynamic.
  • Bernoulli's equation can be used to determine pressure in a fluid.
  • Stagnation pressure: The pressure a fluid exerts when it is forced to stop moving. It is related to static pressure and flow velocity.
  • Vapour pressure: The pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. It depends on temperature and the tendency of liquids and solids to evaporate or condense.
  • Liquid pressure: The pressure exerted by a liquid depends on its depth and density. It can be calculated using the formula p = ρgh.

Negative Pressure

  • Negative pressures can be encountered in certain situations.
  • Negative pressures may be described when dealing with relative (gauge) pressures.
  • Negative absolute pressures are possible and can be tension.
  • Abdominal decompression is an example of applying negative gauge pressure.
  • Bulk solids and bulk liquids can be put under negative absolute pressure.
  • Negative pressure in materials: Negative pressure can be sustained in solids and liquids due to attractive interactions between molecules. However, materials under negative pressure are in a metastable state and can be fragile.
  • Negative liquid pressures: Liquid mercury has been observed to sustain negative pressures up to -425atm in clean glass containers. Negative pressures are also thought to be involved in the ascent of sap in tall plants.
  • Vacuum pressure: The Casimir effect can create a small attractive force known as vacuum pressure, which is not the same as the negative gauge pressure of a vacuum.
  • Negative stress in rigid bodies: Non-isotropic stresses in rigid bodies can have components of positive and negative stress along different surface normals. The pressure is then defined as the average of the three principal stresses.
  • Negative pressure in cosmology: Dark energy creates a small yet significant amount of negative pressure, which contributes to the acceleration of the universe's expansion.

Surface Pressure and Stagnation Pressure

  • Surface pressure: The two-dimensional analog of pressure, denoted by π, is the lateral force per unit length applied on a line perpendicular to the force.
  • Investigation of surface chemicals: Surface pressure can be measured to investigate properties of surface chemicals using pressure/area isotherms, similar to Boyle's law in two dimensions.
  • Surface tension: Another example of surface pressure, but with a reversed sign, as tension is the opposite of pressure.
  • Stagnation pressure: The pressure a fluid exerts when it is forced to stop moving.
  • Relationship with static pressure: Stagnation pressure and static pressure are related by the equation p0 = (1/2)ρov^2 + p, where p0 is the stagnation pressure, ρo is the density, v is the flow velocity, and p is the static pressure.
  • Measurement using Pitot tube: Stagnation pressure of a moving fluid can be measured using a Pitot tube or its variations connected to a manometer.
  • Importance in fluid dynamics: Stagnation pressure is crucial in analyzing fluid flow and determining factors such as velocity and kinetic energy.

Pressure Mentions

Pressure Data Sources

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