This article is about pressure in the physical sciences. For other uses, see Pressure (disambiguation).
Force distributed continuously over an area
Pressure 

Common symbols  p, P 

SI unit  Pascal [Pa] 

In SI base units  1 N/m^{2}, 1 kg/(m·s^{2}), or 1 J/m^{3} 

Derivations from other quantities  p = F / A 

Dimension  M 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 / Nonequilibrium

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
 Property diagrams
 Intensive and extensive properties

Process functions 


Functions of state 

 Temperature / Entropy (introduction)
 Pressure / Volume
 Chemical potential / Particle number
 Vapor quality
 Reduced properties


Material properties
Specific heat capacity 
$c=$ 
$T$  $\mathrm{\partial}S$  $N$  $\mathrm{\partial}T$ 

Compressibility 
$\beta =$ 
$1$  $\mathrm{\partial}V$  $V$  $\mathrm{\partial}p$ 

Thermal expansion 
$\alpha =$ 
$1$  $\mathrm{\partial}V$  $V$  $\mathrm{\partial}T$ 


Equations
 Carnot's theorem
 Clausius theorem
 Fundamental relation
 Ideal gas law
 Maxwell relations
 Onsager reciprocal relations
 Bridgman's equations
 Table of thermodynamic equations

Potentials
 Internal energy
$U(S,V)$  Enthalpy
$H(S,p)=U+pV$  Helmholtz free energy
$A(T,V)=UTS$  Gibbs free energy
$G(T,p)=HTS$

History 

 "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



 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



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/m^{2}); similarly, the poundforce 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 ^{1}⁄_{760} 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.