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Encyclopedia > Pressure The use of water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin in Canberra, Australia.

Pressure (symbol: p) is the force per unit area applied on a surface in a direction perpendicular to that surface. Peer pressure comprises a set of group dynamics whereby a group in which one feels comfortable may override personal habits, individual moral inhibitions or idiosyncratic desires to impose a group norm of attitudes and/or behaviors. ... Image File history File links Download high resolution version (1600x1062, 294 KB) Captain Cook Memorial Fountain and National Library - Lake Burley Griffin Canberra File links The following pages link to this file: Pressure Lake Burley Griffin ... Image File history File links Download high resolution version (1600x1062, 294 KB) Captain Cook Memorial Fountain and National Library - Lake Burley Griffin Canberra File links The following pages link to this file: Pressure Lake Burley Griffin ... Sunset over Lake Burley Griffin, viewed from the Commonwealth Bridge Lake Burley Griffin is a lake in the centre of Canberra, Australias federal capital city. ... For other uses, see Canberra (disambiguation). ... In physics, a net force acting on a body causes that body to accelerate; that is, to change its velocity. ... Area is a physical quantity expressing the size of a part of a surface. ... A surface normal, or just normal to a flat surface is a three-dimensional vector which is perpendicular to that surface. ...

Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.

Formulaic

Conjugate variables
of thermodynamics
Pressure Volume
(Stress) (Strain)
Temperature Entropy
Chem. potential Particle no.

Mathematically: Thermodynamic potentials Maxwell relations Bridgmans equations Exact differential (edit) In thermodynamics, the internal energy of a system is expressed in terms of pairs of conjugate variables such as pressure/volume or temperature/entropy. ... The volume of a solid object is the three-dimensional concept of how much space it occupies, often quantified numerically. ... Stress is a measure of force per unit area within a body. ... This article is about the deformation of materials. ... Fig. ... Ice melting - classic example of entropy increasing described in 1862 by Rudolf Clausius as an increase in the disgregation of the molecules of the body of ice. ... In thermodynamics and chemistry, chemical potential, symbolized by Î¼, is a term introduced in 1876 by the American mathematical physicist Willard Gibbs, which he defined as follows: Gibbs noted also that for the purposes of this definition, any chemical element or combination of elements in given proportions may be considered a... The particle number, N, is the number of so called elementary particles (or elementary constituents) in a thermodynamical system. ...

where:

p is the pressure,
F is the normal force,
A is the area.

Pressure is a scalar, and has SI units of pascals; 1 Pa = 1 N/m2. Fn represents the normal force. ... In physics, a scalar is a simple physical quantity that does not depend on direction, and therefore does not depend on the choice of a coordinate system. ... Look up si, Si, SI in Wiktionary, the free dictionary. ... The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Youngs modulus and tensile strength). ...

Pressure is transmitted 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. Thermodynamics (from the Greek Î¸ÎµÏÎ¼Î·, therme, meaning heat and Î´Ï…Î½Î±Î¼Î¹Ï‚, dunamis, meaning power) is a branch of physics that studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale by analyzing the collective motion of their particles using statistics. ... Thermodynamic potentials Maxwell relations Bridgmans equations Exact differential (edit) In thermodynamics, the internal energy of a system is expressed in terms of pairs of conjugate variables such as pressure/volume or temperature/entropy. ... The volume of a solid object is the three-dimensional concept of how much space it occupies, often quantified numerically. ...

Units

Non-SI measures (still in use in some parts of the world) include the pound-force per square inch (psi) and the bar. The pound-force is a non-SI unit of force or weight (properly abbreviated lbf or lbf). The pound-force is equal to a mass of one pound multiplied by the standard acceleration due to gravity on Earth (which is defined as exactly 9. ... An inch (plural: inches; symbol or abbreviation: in or, sometimes, â€³ - a double prime) is the name of a unit of length in a number of different systems, including English units, Imperial units, and United States customary units. ... A pressure gauge reading in PSI (red scale) and kPa (black scale) The pound-force per square inch (symbol: lbf/inÂ²) is a non-SI unit of pressure based on avoirdupois units. ... The bar (symbol bar), decibar (symbol dbar) and the millibar (symbol mbar, also mb) are units of pressure. ...

The cgs unit of pressure is the barye (ba). It is equal to 1 dyn·cm-2. This article or section is in need of attention from an expert on the subject. ... The barye (symbol: Ba) is the centimetre-gram-second (CGS) unit of pressure. ...

Pressure is still sometimes expressed in mmHg/cm2 or grams-force/cm2 (sometimes as kg/cm2 and g/mol2 without properly identifying the force units). But using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as a unit of force is expressly forbidden in SI; the unit of force in SI is the newton (N). The technical atmosphere (symbol: at) is 1 kgf/cm2. One way of defining pressure is in terms of the height of a column of fluid that may be supported by that pressure; or the height of a column of fluid that exerts that pressure at its base. ... A technical atmosphere (symbol: at) is a non-SI unit of pressure equal to 1 kilogram-force per square centimeter, i. ...

Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, which is equivalent to the older unit millibar (mbar). Similar pressures are given in kilopascals (kPa) in practically all other fields, where the hecto prefix is hardly ever used. In Canadian weather reports, the normal unit is kPa. The obsolete unit inch of mercury (inHg, see below) is still sometimes used in the United States. Meteorology is the scientific study of the atmosphere that focuses on weather processes and forecasting. ... A millibar (mbar, also mb) is 1/1000th of a bar, a unit for measurement of pressure. ... Inches of mercury or inHg is a non SI unit for pressure. ...

The standard atmosphere (atm) is an established constant. It is approximately equal to typical air pressure at earth mean sea level and is defined as follows. Atmospheric pressure is the pressure caused by the weight of air above any area in the Earths atmosphere. ...

standard atmosphere = 101325 Pa = 101.325 kPa = 1013.25 hPa.

Because pressure is commonly measured by its ability to displace a column of liquid in a manometer, pressures are often expressed as a depth of a particular fluid (e.g., inches of water). The most common choices are mercury (Hg) and water; water is nontoxic and readily available, while mercury's density allows for a shorter column (and so a smaller manometer) to measure a given pressure. The pressure exerted by a column of liquid of height h and density ρ is given by the hydrostatic pressure equation p = ρgh. The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Youngs modulus and tensile strength). ... A manometer is a pressure measuring instrument, often also called pressure gauge. ... General Name, Symbol, Number mercury, Hg, 80 Chemical series transition metals Group, Period, Block 12, 6, d Appearance silvery Standard atomic weight 200. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ...

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. When millimetres of mercury or inches of mercury are quoted today, these units are not based on a physical column of mercury; rather, they have been given precise definitions that can be expressed in terms of SI units. The water-based units still depend on the density of water, a measured, rather than defined, quantity. The torr (symbol: Torr) or millimeter of mercury (mmHg) is a non-SI unit of pressure. ... Inches of mercury or inHg is a non SI unit for pressure. ...

Although no longer favoured in physics, these manometric units are still encountered in many fields. Blood pressure is measured in millimetres of mercury in most of the world, and lung pressures in centimeters of water are still common. Scuba divers often use a manometric rule of thumb: the pressure exerted by ten metres depth of water is approximately equal to one atmosphere. A sphygmomanometer, a device used for measuring arterial pressure. ... Scuba diving is swimming underwater while using self-contained breathing equipment. ... A rule of thumb is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination. ...

Other units presently or formerly in use include the following:

• atmosphere
• manometric units:
• centimetre, inch, and millimetre of mercury (torr)
• millimetre, centimetre, metre, inch, and foot of water
• imperial units:
• kip, ton-force (short), ton-force (long), pound-force, ounce-force, and poundal per square inch
• pound-force, ton-force (short), and ton-force (long)
• non-SI metric units:
• bar, millibar
• force, or kilopond, per square centimetre (technical atmosphere)
• gram-force and tonne-force (metric ton-force) per square centimetre
• barye (dyne per square centimetre)
• kilogram-force and tonne-force per square metre
• sthene per square metre (pieze)
Pressure Units

pascal
(Pa)

bar
(bar)
technical atmosphere
(at)

atmosphere
(atm)

torr
(mmHg)
pound-force per
square inch

(psi)
1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 9.8692×10−6 7.5006×10−3 145.04×10−6
1 bar 100 000 ≡ 106 dyn/cm2 1.0197 0.98692 750.06 14.504
1 at 98 066.5 0.980665 ≡ 1 kgf/cm2 0.96784 735.56 14.223
1 atm 101 325 1.01325 1.0332 ≡ 1 atm 760 14.696
1 torr 133.322 1.3332×10−3 1.3595×10−3 1.3158×10−3 ≡ 1 mmHg 19.337×10−3
1 psi 6 894.76 68.948×10−3 70.307×10−3 68.046×10−3 51.715 ≡ 1 lbf/in2

Example reading:  1 Pa = 1 N/m2  = 10−5 bar  = 10.197×10−6 at  = 9.8692×10−6 atm, etc.
Note:  mmHg is an abbreviation for millimetres of mercury. Standard atmosphere (symbol: atm) is a unit of pressure. ... The torr (symbol: Torr) or millimeter of mercury (mmHg) is a non-SI unit of pressure. ... A centimeter (centimetre) of water or cmH2O is a somewhat outdated unit for pressure. ... In the United States, a kip is a unit of force that equals 1,000 pounds, i. ... Look up ton in Wiktionary, the free dictionary. ... The poundal is a non-SI unit of force. ... The bar (symbol bar), decibar (symbol dbar) and the millibar (symbol mbar, also mb) are units of pressure. ... A millibar (mbar, also mb) is 1/1000th of a bar, a unit for measurement of pressure. ... A technical atmosphere (symbol: at) is a non-SI unit of pressure equal to 1 kilogram-force per square centimeter, i. ... The barye (symbol: Ba) is the centimetre-gram-second (CGS) unit of pressure. ... In physics, the dyne is a unit of force specified in the centimetre-gram-second (cgs) system of units, symbol dyn. One dyne is equal to exactly 10-5 newtons. ... The sthene is the unit of force in the former Soviet mts system, 1933-1955. ... The pieze is the unit of pressure in the former Soviet mts system, 1933-1955. ... The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Youngs modulus and tensile strength). ... The bar (symbol bar), decibar (symbol dbar) and the millibar (symbol mbar, also mb) are units of pressure. ... A technical atmosphere (symbol: at) is a non-SI unit of pressure equal to 1 kilogram-force per square centimeter, i. ... Standard atmosphere (symbol: atm) is a unit of pressure. ... The torr (symbol: Torr) or millimeter of mercury (mmHg) is a non-SI unit of pressure. ... A pressure gauge reading in PSI (red scale) and kPa (black scale) The pound-force per square inch (symbol: lbf/inÂ²) is a non-SI unit of pressure based on avoirdupois units. ... The newton (symbol: N) is the SI derived unit of force, named after Sir Isaac Newton in recognition of his work on classical mechanics. ... In physics, the dyne is a unit of force specified in the centimetre-gram-second (cgs) system of units, symbol dyn. One dyne is equal to exactly 10-5 newtons. ... The deprecated unit kilogram-force (kgf) or kilopond (kp) is the force exerted by one kilogram of mass in standard Earth gravity (defined as exactly 9. ... Standard atmosphere (symbol: atm) is a unit of pressure. ... The torr (symbol: Torr) or millimeter of mercury (mmHg) is a non-SI unit of pressure. ... The pound-force is a non-SI unit of force or weight (properly abbreviated lbf or lbf). The pound-force is equal to a mass of one pound multiplied by the standard acceleration due to gravity on Earth (which is defined as exactly 9. ...

Examples

As an example of varying pressures, a finger can be pressed against a wall without making any lasting impression; however, the same finger pushing a thumbtack can easily damage the wall. 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. Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point. Unlike stress, pressure is defined as a scalar quantity. A brass thumbtack A thumbtack is a short nail or pin with a large, slightly rounded head made of metal which is used to fasten documents to a background for public display and which can easily be inserted or removed by hand. ... Stress is a measure of force per unit area within a body. ... In physics, a scalar is a simple physical quantity that does not depend on direction, and therefore does not depend on the choice of a coordinate system. ...

Gauge pressure is a critical 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. For instance, at an altitude of 112 m, the mean atmospheric pressure is 100 kPa. At this altitude, a pressure vessel containing any gas (such as helium) at 200 kPa (gauge), or 300 kPa (absolute), is 50 % more dense than at 100 kPa (gauge), 200 kPa (absolute); not double the density, as one might assume by focusing on gauge values.

Scalar nature

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. If we enclose the gas within a container, we detect a pressure in the gas from the molecules colliding with the walls of our container. We can put the walls of our container anywhere inside the gas, and the force per unit area (the pressure) is the same. We can shrink the size of our "container" down to an infinitely small point, and the pressure has a single value at that point. Therefore, pressure is a scalar quantity, not a vector quantity. It has a magnitude but no direction associated with it. Pressure acts in all directions at a point inside a gas. At the surface of a gas, the pressure force acts perpendicular to the surface. This article does not cite any references or sources. ... Three different views of Brownian motion, with 32 steps, 256 steps, and 2048 steps denoted by progressively lighter colors. ...

A closely related quantity is the stress tensor σ, which relates the vector force F to the vector area A via Stress is a measure of force per unit area within a body. ...

This tensor may be divided up into a scalar part (pressure) and a traceless tensor part shear. The shear tensor gives the force in directions parallel to the surface, usually due to viscous or frictional forces. The stress tensor is sometimes called the pressure tensor, but in the following, the term "pressure" will refer only to the scalar pressure. In mathematics, a tensor is (in an informal sense) a generalized linear quantity or geometrical entity that can be expressed as a multi-dimensional array relative to a choice of basis; however, as an object in and of itself, a tensor is independent of any chosen frame of reference. ... In physics and mechanics, shear refers to a deformation that causes parallel surfaces to slide past one another (as opposed to compression and tension, which cause parallel surfaces to move towards or away from one another). ...

Types

Explosion or Deflagration Pressures

Explosion or deflagration pressures are the result of the ignition of explosible gases, mists, dust/air suspensions, in unconfined and confined spaces.-1...

Negative pressures

While pressures are generally positive, there are several situations in which a negative pressure may be encountered:

• When dealing in relative (gauge) pressures. For instance, an absolute pressure of 80 kPa may be described as a gauge pressure of -21 kPa (i.e., 21 kPa below an atmospheric pressure of 101 kPa).
• When attractive forces (e.g., Van der Waals forces) between the particles of a fluid exceed repulsive forces. Such scenarios are generally unstable since the particles will move closer together until repulsive forces balance attractive forces. Negative pressure exists in the transpiration pull of plants.
• The Casimir effect can create a small attractive force due to interactions with vacuum energy; this force is sometimes termed 'vacuum pressure' (not to be confused with the negative gauge pressure of a vacuum).
• Depending on how the orientation of a surface is chosen, the same distribution of forces may be described either as a positive pressure along one surface normal, or as a negative pressure acting along the opposite surface normal.
• In the cosmological constant.

The title given to this article is incorrect due to technical limitations. ... Transpirational pull is the main phenomenon driving the flow of sap in the xylem tissues of large plants. ... In physics, the Casimir effect is a physical force exerted between separate objects, which is due to neither charge, gravity, nor the exchange of particles, but instead is due to resonance of all-pervasive energy fields in the intervening space between the objects. ... A surface normal, or just normal to a flat surface is a three-dimensional vector which is perpendicular to that surface. ... In physical cosmology, the cosmological constant (usually denoted by the Greek capital letter lambda: Î›) was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a stationary universe. ...

Hydrostatic pressure (head pressure)

Hydrostatic pressure is the pressure due to the weight of a fluid. Hydrostatic pressure is the pressure exerted by a fluid due to its weight. ... For other uses, see Weight (disambiguation). ...

where:

ρ (rho) is the density of the fluid (i.e., the practical density of fresh water is 1000 kg/m3);
g is the acceleration due to gravity (approximately 9.81 m/s2 on earth's surface);
h is the height of the fluid column (in metres). Feet can be used if the rest of the units used in the equation are defined in feet.

See also Pascal's law. 1. ... In physics, density is mass m per unit volume V. For the common case of a homogeneous substance, it is expressed as: where, in SI units: Ï (rho) is the density of the substance, measured in kgÂ·m-3 m is the mass of the substance, measured in kg V is... g (also gee, g-force or g-load) is a non-SI unit of acceleration defined as exactly 9. ... In the physical sciences, Pascals law or Pascals principle states that the fluid pressure at all points in a connected body of an incompressible fluid at rest, which are at the same absolute height, are the same, even if additional pressure is applied on the fluid at some...

Stagnation pressure

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. Static pressure and stagnation pressure are related by the Mach number of the fluid. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid. See Bernoulli's equation (note: Bernoulli's equation only applies for incompressible flow). Total Pressure redirects here. ... An F/A-18 Hornet breaking the sound barrier. ... In fluid dynamics, Bernoullis equation, derived by Daniel Bernoulli, describes the behavior of a fluid moving along a streamline. ...

The pressure of a moving fluid can be measured using a Pitot probe, or one of its variations such as a Kiel probe or Cobra probe, connected to a manometer. Depending on where the inlet holes are located on the probe, it can measure static pressure or stagnation pressure. A Pitot tube is a measuring instrument used to measure fluid flow. ... A Kiel probe is a device for measuring pressure in fluid dynamics. ... A Cobra probe is a device to measure the pressure of a moving fluid. ... A manometer is a pressure measuring instrument, often also called pressure gauge. ...

Surface pressure

There is a two-dimensional analog of pressure -- the lateral force per unit length applied on a line perpendicular to the force.

Surface pressure is denoted by π and shares many similar properties as three-dimensional pressure. Properties of surface chemicals can be investigated by measuring pressure/area isotherms, as the two-dimensional analog of Boyle's law, πA = k, at constant temperature. Boyles law (sometimes referred to as the Boyle Mariotte law) is one of the gas laws. ...

Diurnal (daily) rhythm of air pressure in northern Germany (black curve is air pressure) Atmospheric pressure is the pressure at any point in the Earths atmosphere. ... A sphygmomanometer, a device used for measuring arterial pressure. ... Boyles law (sometimes referred to as the Boyle Mariotte law) is one of the gas laws. ... The ideal gas law or equation is the equation of state of an ideal gas. ... Conversion of units refers to conversion factors between different units of measurement for the same quantity. ... Many, if not most, parameters and measurements in the physical sciences and engineering are expressed as a numerical quantity and a corresponding dimensional unit; for example: 1000 kg/mÂ³, 100 kPa/bar, 50 miles per hour, 1000 Btu/lb. ... Isotherms of an ideal gas The ideal gas law is the equation of state of a hypothetical ideal gas, first stated by BenoÃ®t Paul Ã‰mile Clapeyron in 1834. ... Kinetic theory attempts to explain macroscopic properties of gases, such as pressure, temperature, or volume, by considering their molecular composition and motion. ... In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ... Sound is a disturbance of mechanical energy that propagates through matter as a wave. ... â€œMicrophonesâ€ redirects here. ... Timeline of temperature and pressure measurement technology 1592 - Galileo Galilei builds a crude thermometer using the contraction of air to draw water up a tube 1612 - Santorre Santorio puts thermometer to medical use 1643 - Evangelista Torricelli invents the mercury barometer 1714 - Daniel Gabriel Fahrenheit invents the mercury in glass thermometer... Look up Vacuum in Wiktionary, the free dictionary. ... The Roots blower is one example of a vacuum pump A vacuum pump is a pump that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. ... Results from FactBites:

 Dealing With Peer Pressure (821 words) For example, a few kids in school might try to get you to cut class with them, your soccer friend might try to convince you to be mean to another player and never pass her the ball, or a kid in the neighborhood might want you to shoplift with him. Some kids give in to peer pressure because they want to be liked, to fit in, or because they worry that other kids may make fun of them if they don't go along with the group. For example, positive peer pressure can be used to pressure bullies into acting better toward other kids.
 Pressure (528 words) Pressure in a fluid can be seen to be a measure of energy per unit volume by means of the definition of work. Pressure in a fluid may be considered to be a measure of energy per unit volume or energy density. The most obvious application is to the hydrostatic pressure of a fluid, where pressure can be used as energy density alongside kinetic energy density and potential energy density in the Bernoulli equation.
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