On this Wikipedia the language links are at the top of the page across from the article title. This article is about pressure in the physical sciences. A figure pressure cooker beef stew pressure exerted by particle collisions inside a closed container.

The collisions that exert the pressure are highlighted in red. Various units are used to express pressure. Pressure is the amount of force applied perpendicular to the surface of an object per unit area. The symbol for it is “p” or P. The IUPAC recommendation for pressure is a lower-case p.

The minus sign comes from the convention that the force is considered towards the surface element, while the normal vector points outward. The equation has meaning in that, for any surface S in contact with the fluid, the total force exerted by the fluid on that surface is the surface integral over S of the right-hand side of the above equation. The pressure, as a scalar, has no direction. The force given by the previous relationship to the quantity has a direction, but the pressure does not.

If we change the orientation of the surface element, the direction of the normal force changes accordingly, but the pressure remains the same. Pressure is distributed to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point. It is a fundamental parameter in thermodynamics, and it is conjugate to volume. Since a system under pressure has the potential to perform work on its surroundings, pressure is a measure of potential energy stored per unit volume. The inch of mercury is still used in the United States. Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely. Gauge pressure is often given in units with “g” appended, e.

Although the force applied to the surface is the same, the thumbtack applies more pressure because the point concentrates that force into a smaller area. If we try to cut with the flat edge, force is distributed over a larger surface area resulting in less pressure, and it will not cut. Whereas using the sharp edge, which has less surface area, results in greater pressure, and so the knife cuts smoothly. Gauge pressure is the relevant measure of pressure wherever one is interested in the stress on storage vessels and the plumbing components of fluidics systems. However, whenever equation-of-state properties, such as densities or changes in densities, must be calculated, pressures must be expressed in terms of their absolute values. In a static gas, the gas as a whole does not appear to move.

The individual molecules of the gas, however, are in constant random motion. Because we are dealing with an extremely large number of molecules and because the motion of the individual molecules is random in every direction, we do not detect any motion. This tensor may be expressed as the sum of the viscous stress tensor minus the hydrostatic pressure. The negative of the stress tensor is sometimes called the pressure tensor, but in the following, the term “pressure” will refer only to the scalar pressure. Fluid pressure is most often the compressive stress at some point within a fluid. An open condition, called “open channel flow”, e. A closed condition, called “closed conduit”, e.

Such conditions conform with principles of fluid statics. Closed bodies of fluid are either “static”, when the fluid is not moving, or “dynamic”, when the fluid can move as in either a pipe or by compressing an air gap in a closed container. The pressure in closed conditions conforms with the principles of fluid dynamics. The concepts of fluid pressure are predominantly attributed to the discoveries of Blaise Pascal and Daniel Bernoulli. Bernoulli’s equation can be used in almost any situation to determine the pressure at any point in a fluid. They are effectively tension, and both bulk solids and bulk liquids can be put under negative absolute pressure by pulling on them.

Microscopically, the molecules in solids and liquids have attractive interactions that overpower the thermal kinetic energy, so some tension can be sustained. For non-isotropic stresses in rigid bodies, depending on how the orientation of a surface is chosen, the same distribution of forces may have a component of positive pressure along one surface normal, with a component of negative pressure acting along another surface normal. In cosmology, dark energy creates a very small yet cosmically significant amount of negative pressure, which accelerates the expansion of the universe. Stagnation pressure is the pressure a fluid exerts when it is forced to stop moving. Consequently, although a fluid moving at higher speed will have a lower static pressure, it may have a higher stagnation pressure when forced to a standstill. The pressure of a moving fluid can be measured using a Pitot tube, or one of its variations such as a Kiel probe or Cobra probe, connected to a manometer. Surface tension is another example of surface pressure, but with a reversed sign, because “tension” is the opposite to “pressure”.

In an ideal gas, molecules have no volume and do not interact. T is the absolute temperature, V is the volume, R is the ideal gas constant. Vapour pressure is the pressure of a vapour in thermodynamic equilibrium with its condensed phases in a closed system. The vapor pressure that a single component in a mixture contributes to the total pressure in the system is called partial vapor pressure. When a person swims under the water, water pressure is felt acting on the person’s eardrums. The deeper that person swims, the greater the pressure. The pressure felt is due to the weight of the water above the person.