Pressure
A finger presses against a wall without leaving a mark. The same finger pushes a thumbtack and damages the surface instantly. The force applied remains identical in both scenarios. The difference lies entirely in how that force distributes across the contact area. Pressure equals the amount of force applied perpendicular to an object's surface divided by the unit area over which it spreads. Scientists denote this quantity with the symbol p or P. The International Union of Pure and Applied Chemistry recommends using a lower-case p, though upper-case P appears frequently in many fields. This choice depends on nearby symbols for power or momentum and specific writing styles. Pressure acts as a fundamental parameter in thermodynamics. It serves as the derivative of internal energy relative to volume when entropy and particle number remain fixed.
In a static gas, individual molecules move in constant random directions while the bulk gas appears motionless. These collisions generate hydrostatic pressure within any confined space. A physical container holds these molecules, but gravity can also create atmospheric pressure around a planet. The pressure at any point inside such a fluid possesses a single value regardless of scale. This makes pressure a scalar quantity rather than a vector one. It has magnitude but no inherent direction sense. Force acts in all directions at a point inside a gas. At the surface of a gas, the force acts perpendicular to the boundary. The minus sign in the governing equation reflects the convention where force points toward the surface element while the normal vector points outward. Changing the orientation of the surface alters the normal force direction but leaves the pressure unchanged. In general relativity, pressure increases gravitational field strength significantly within neutron stars.
The SI unit for pressure is the pascal, equal to one newton per square metre. This name entered official use in 1971 before that scientists expressed it simply as newtons per square metre. Other common units include pounds per square inch and bar. The CGS unit is the barye, equivalent to 0.1 pascals. Meteorologists often prefer hectopascals for atmospheric air pressure because they match older millibar measurements. Oceanographers measure underwater pressure in decibars since pressure rises by approximately one decibar per meter of depth. The standard atmosphere equals roughly typical air pressure at Earth mean sea level. One millimetre of mercury approximates one torr today. These manometric units remain popular despite lacking physical columns of liquid. Blood pressure measures in millimetres or centimetres of mercury globally. Lung pressures frequently appear in centimetres of water. Underwater divers rely on metres sea water and feet sea water units for exposure monitoring.
Blaise Pascal and Daniel Bernoulli established the core concepts of fluid pressure. Their work distinguishes between open conditions like oceans and closed systems like pipes. Fluid statics govern non-moving fluids even when waves exist. A damaged hydrant sprays water at high speed due to internal pressure. Bernoulli's equation determines pressure at any point within a system filled with constant-density fluid. It assumes an ideal fluid with zero viscosity and no friction. Real gases exhibit complex dependencies beyond simple linear relationships. When a person swims deeper, water pressure acts against their eardrums. This pressure depends directly on depth and density rather than total volume. A wide shallow lake exerts half the average pressure of a small deep pond. Water seeks its own level because pressure differences force sideways movement until equalization occurs. Pressure acts equally in all directions regardless of head tilt underwater.
Hydraulic brakes utilize fluid pressure to stop vehicles effectively. Artesian wells rely on natural underground pressure to bring water to the surface. Blood pressure measurements remain critical for medical diagnostics worldwide. Plant cell turgidity depends on internal liquid pressure maintaining structural integrity. Pythagorean cups demonstrate how fluid dynamics can create self-emptying mechanisms. Pressure washing machines use concentrated streams to clean surfaces aggressively. Abdominal decompression applies negative gauge pressure intermittently during obstetric procedures. Explosion pressures result from igniting gases or dust suspensions in confined spaces. The Casimir effect creates attractive forces through vacuum energy interactions. Surface tension represents a two-dimensional analog where lateral force per unit length opposes pressure. Engineers calculate stagnation pressure when fluids are forced to stop moving completely.
Absolute pressure of 80 kilopascals describes as negative gauge pressure below atmospheric levels. Bulk solids and liquids sustain negative absolute pressure by pulling on them. Liquid mercury has been observed sustaining up to minus 142 megapascals inside clean glass containers. Negative liquid pressures facilitate sap ascent in plants taller than ten meters. Thermodynamically, bulk materials under negative pressure exist in metastable states. Cavitation remains a constant threat to these fragile conditions. Dark energy generates cosmologically significant negative pressure accelerating universal expansion. This effect remains unnoticeable at everyday pressures but dominates cosmic scales. Non-isotropic stresses allow components of positive stress alongside negative stress depending on surface orientation. The average of three principal stresses defines pressure in rigid bodies. Electromagnetic field stresses generally show non-isotropic behavior with normal stress being negative while perpendicular elements remain positive.
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Common questions
What is the definition of pressure in physics?
Pressure equals the amount of force applied perpendicular to an object's surface divided by the unit area over which it spreads. Scientists denote this quantity with the symbol p or P, though the International Union of Pure and Applied Chemistry recommends using a lower-case p.
When did the SI unit for pressure become known as the pascal?
The name pascal entered official use in 1971 before that scientists expressed it simply as newtons per square metre. The SI unit for pressure is equal to one newton per square metre.
Who established the core concepts of fluid pressure?
Blaise Pascal and Daniel Bernoulli established the core concepts of fluid pressure. Their work distinguishes between open conditions like oceans and closed systems like pipes.
How does depth affect water pressure on a diver?
Water pressure depends directly on depth and density rather than total volume. When a person swims deeper, water pressure acts against their eardrums because pressure rises by approximately one decibar per meter of depth.
Why can liquid mercury sustain negative absolute pressure?
Liquid mercury has been observed sustaining up to minus 142 megapascals inside clean glass containers. Bulk solids and liquids sustain negative absolute pressure by pulling on them within metastable states.