# Pressure

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This article is about pressure in the physical sciences. For other uses, see Pressure (disambiguation).
Pressure
Common symbols
p, P
SI unitPascal [Pa]
In SI base unitsN/m2, 1 kg/(s2), or 1 J/m3
Derivations from
other quantities
p = F / A
DimensionM L−1 T−2
Pressure as exerted by particle collisions inside a closed container
Thermodynamics
The altical Carnot heat engine
Branches
• Classical
• Statistical
• Chemical
• Quantum thermodynamics
• Equilibrium / Non-equilibrium
Laws
• Zeroth
• First
• Second
• Third
Systems
State
• Equation of state
• Ideal gas
• Real gas
• State of matter
• Equilibrium
• Control volume
• Instruments
Processes
• Isobaric
• Isochoric
• Isothermal
• Adiabatic
• Isentropic
• Isenthalpic
• Quasistatic
• Polytropic
• Free expansion
• Reversibility
• Irreversibility
• Endoreversibility
Cycles
• Heat engines
• Heat pumps
• Thermal efficiency
System properties
Note: Conjugate variables in italics
 Process functions Property diagrams Intensive and extensive properties Work Heat Temperature / Entropy (introduction) Pressure / Volume Chemical potential / Particle number Vapor quality Reduced properties
Material properties
• Property databases
Specific heat capacity  ${\displaystyle c=}$
 ${\displaystyle T}$ ${\displaystyle \partial S}$ ${\displaystyle N}$ ${\displaystyle \partial T}$
Compressibility  ${\displaystyle \beta =-}$
 ${\displaystyle 1}$ ${\displaystyle \partial V}$ ${\displaystyle V}$ ${\displaystyle \partial p}$
Thermal expansion  ${\displaystyle \alpha =}$
 ${\displaystyle 1}$ ${\displaystyle \partial V}$ ${\displaystyle V}$ ${\displaystyle \partial T}$
Equations
• Carnot's theorem
• Clausius theorem
• Fundamental relation
• Ideal gas law
• Maxwell relations
• Onsager reciprocal relations
• Bridgman's equations
• Table of thermodynamic equations
Potentials
• Free energy
• Free entropy
• Internal energy
${\displaystyle U(S,V)}$
• Enthalpy
${\displaystyle H(S,p)=U+pV}$
• Helmholtz free energy
${\displaystyle A(T,V)=U-TS}$
• Gibbs free energy
${\displaystyle G(T,p)=H-TS}$
• History
• Culture
History
• General
• Entropy
• Gas laws
• "Perpetual motion" machines
Philosophy
• Entropy and time
• Entropy and life
• Brownian ratchet
• Maxwell's demon
• Heat death paradox
• Loschmidt's paradox
• Synergetics
Theories
• Caloric theory
• Theory of heat
• Vis viva ("living force")
• Mechanical equivalent of heat
• Motive power
Key publications
• "An Experimental Enquiry
Concerning ... Heat"
• "On the Equilibrium of
Heterogeneous Substances"
• "Reflections on the
Motive Power of Fire"
Timelines
• Thermodynamics
• Heat engines
• Art
• Education
• Maxwell's thermodynamic surface
• Entropy as energy dispersal
Scientists
• Bernoulli
• Boltzmann
• Carnot
• Clapeyron
• Clausius
• Carathéodory
• Duhem
• Gibbs
• von Helmholtz
• Joule
• Maxwell
• von Mayer
• Onsager
• Rankine
• Smeaton
• Stahl
• Thompson
• Thomson
• van der Waals
• Waterston
• Book
• Category
• v
• t
• e

Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed.:445[1] Gauge pressure (also spelled gage pressure)[a] is the pressure relative to the ambient pressure.

Various units are used to express pressure. Some of these derive from a unit of force divided by a unit of area; the SI unit of pressure, the pascal (Pa), for example, is one newton per square metre (N/m2); similarly, the pound-force per square inch (psi) is the traditional unit of pressure in the imperial and U.S. customary systems. Pressure may also be expressed in terms of standard atmospheric pressure; the atmosphere (atm) is equal to this pressure, and the torr is defined as ​1760 of this. Manometric units such as the centimetre of water, millimetre of mercury, and inch of mercury are used to express pressures in terms of the height of column of a particular fluid in a manometer.