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 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 Data Sources