Pressure (symbol: 'p') is the force over an area applied to an object in a direction perpendicular to the surface. In Physics, a force is whatever can cause an object with Mass to Accelerate. Area is a Quantity expressing the two- Dimensional size of a defined part of a Surface, typically a region bounded by a closed Curve. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.

Definition

Pressure is an effect which occurs when a Force is applied on a surface. The symbol of Pressure is p which can also be written as P .

Formula

Mathematically:

where:

p is the pressure,

F is the normal force,

A is the area. In Thermodynamics, the Internal energy of a system is expressed in terms of pairs of conjugate variables such as temperature/entropy or pressure/volume The volume of any solid plasma vacuum or theoretical object is how much three- Dimensional space it occupies often quantified numerically Stress is a measure of the average amount of Force exerted per unit Area. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy In Thermodynamics and Chemistry, chemical potential, symbolized by μ, is a term introduced by the American engineer chemist and mathematical The particle number, N, is the number of so called ' Elementary particles (or elementary constituents in a thermodynamical system. In Physics, the normal force F_n\ (or in some books N) is the component perpendicular to the surface of contact of the Contact force

Pressure is a scalar quantity, and has SI units of pascals; 1 Pa = 1 N/m², and has EES units of psi; 1 psi = 1 lb/in². US customary units, also known in the United States as English units or Imperial units (in reference to the British Empire) (but see English The pound per square inch or more accurately pound-force per square inch (symbol psi or lbf/in² or lbf/in²) is a unit of

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. In Physics, thermodynamics (from the Greek θερμη therme meaning " Heat " and δυναμις dynamis meaning " In Thermodynamics, the Internal energy of a system is expressed in terms of pairs of conjugate variables such as temperature/entropy or pressure/volume The volume of any solid plasma vacuum or theoretical object is how much three- Dimensional space it occupies often quantified numerically

Units

Mercury column

The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m^-2 or kg·m^-1·s^-2). The newton (symbol N) is the SI derived unit of Force, named after Isaac Newton in recognition of his work on Classical M^2 redirects here For other uses see M². CM2 redirects here This special name for the unit was added in 1971; before that, pressure in SI was expressed simply as N/m².

Non-SI measures such as pound per square inch (psi) and bar are used in parts of the world. This article deals with the unit of force For the unit of mass see Pound (mass. Inches redirects here To see the Les Savy Fav album see Inches. The pound per square inch or more accurately pound-force per square inch (symbol psi or lbf/in² or lbf/in²) is a unit of 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), equal to 1 dyn·cm^-2. The centimetre-gram-second system ( CGS) is a system of physical units. The barye (symbol Ba) was in France a centimetre-gram-second (CGS unit of Pressure. Pressure is sometimes expressed in grams-force/cm², or as kg/cm² and the like without properly identifying the force units. But using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as units of force is expressly forbidden in SI. The technical atmosphere (symbol: at) is 1 kgf/cm². A technical atmosphere (symbol at is a non- SI unit of Pressure equal to one Kilogram-force per square centimeter In US Customary units, it is 14. 696 psi.

Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, which is equivalent to the older unit millibar (mbar). Meteorology (from Greek grc μετέωρος metéōros, "high in the sky" and grc -λογία -logia) is the Interdisciplinary The bar (symbol bar) decibar (symbol dbar) and the millibar (symbol mbar, also mb are units of Pressure. Similar pressures are given in kilopascals (kPa) in most other fields, where the hecto prefix is rarely used. The unit inch of mercury (inHg, see below) is still used in the United States. Inches of mercury, inHg or "Hg is a measuring unit for Pressure. Oceanographers usually measure underwater pressure in decibars (dbar) because an increase in pressure of 1 dbar is approximately equal to an increase in depth of 1 meter. The bar (symbol bar) decibar (symbol dbar) and the millibar (symbol mbar, also mb are units of Pressure. Scuba divers often use a manometric rule of thumb: the pressure exerted by ten metres depth of water is approximately equal to one atmosphere. Scuba diving is swimming underwater, or taking part in another activity while using a Scuba set. A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation

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:

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. Many techniques have been developed for the measurement of Pressure and Vacuum. g. , inches of water). The most common choices are mercury (Hg) and water; water is nontoxic and readily available, while mercury's high density allows for a shorter column (and so a smaller manometer) to measure a given pressure. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum Water is a common Chemical substance that is essential for the survival of all known forms of Life. The pressure exerted by a column of liquid of height h and density ρ is given by the hydrostatic pressure equation p = ρgh. 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 millimeters 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 torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere. Inches of mercury, inHg or "Hg is a measuring unit for Pressure. The water-based units still depend on the density of water, a measured, rather than defined, quantity. These manometric units are still encountered in many fields. Blood pressure is measured in millimeters of mercury in most of the world, and lung pressures in centimeters of water are still common. Blood pressure is also the title of a short story by Damon Runyan in Guys and Dolls and Other Stories

Presently or formerly popular pressure units include the following:

• atmosphere
• manometric units:
  • centimeter, inch, and millimeter of mercury (torr)
  • millimeter, centimeter, meter, 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, decibar, millibar
  • kilogram-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)

Example reading:  1 Pa = 1 N/m²  = 10⁻⁵ bar  = 10. This article deals with the unit of force For the unit of mass see Pound (mass. 197×10⁻⁶ at  = 9. 8692×10⁻⁶ atm, etc.
Note:  mmHg is an abbreviation for millimetres of mercury.

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. Stress is a measure of the average amount of Force exerted per unit Area. In Physics, a scalar is a simple Physical quantity that is not changed by Coordinate system rotations or translations (in Newtonian mechanics or

Another example is of a common knife. If we try and cut a fruit with the flat side it obviously won't cut. But if we take the thin side, it will cut smoothly. The reason is, the flat side has a greater surface area and so it does not cut the fruit. When we take the thin side, the surface area is reduced and so it cuts the fruit easily and quickly. This shows one of the good effects of Pressure.

The gradient of pressure is called the force density. In Vector calculus, the gradient of a Scalar field is a Vector field which points in the direction of the greatest rate of increase of the scalar In Fluid mechanics, the force density has the physical dimensions of force per unit volume For gases, pressure is sometimes measured not as an absolute pressure, but relative to atmospheric pressure; such measurements are called gauge pressure (also sometimes spelled gage pressure). ^[1] An example of this is the air pressure in an automobile tire, which might be said to be "220 kPa", but is actually 220 kPa above atmospheric pressure. This article is about tires used on road Vehicles including pneumatic tires and solid tires. Since atmospheric pressure at sea level is about 100 kPa, the absolute pressure in the tire is therefore about 320 kPa. In technical work, this is written "a gauge pressure of 220 kPa". Where space is limited, such as on pressure gauges, name plates, graph labels, and table headings, the use of a modifier in parentheses, such as "kPa (gauge)" or "kPa (absolute)", is permitted. Many techniques have been developed for the measurement of Pressure and Vacuum. A name plate identifies and displays a person or product's name In non-SI technical work, a gauge pressure of 32 psi is sometimes written as "32 psig", though the other methods explained above that avoid attaching characters to the unit of pressure are preferred. ^[2]

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. For instance, if the atmospheric pressure is 100 kPa, a gas (such as helium) at 200 kPa (gauge) (300 kPa [absolute]) is 50 % more dense than the same gas at 100 kPa (gauge) (200 kPa [absolute]). Focusing on gauge values, one might erroneously conclude the first sample had twice the density of the second one.

Scalar nature

In a static gas, the gas as a whole does not appear to move. This page is about the physical properties of gas as a state of matter The individual molecules of the gas, however, are in constant random motion. This article is about the physical phenomenon for the stochastic process see Wiener process. 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 magnitude but no direction sense 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 (at right angle) to the surface.

A closely related quantity is the stress tensor σ, which relates the vector force F to the vector area A via

This tensor may be divided up into a scalar part (pressure) and a traceless tensor part shear. Stress is a measure of the average amount of Force exerted per unit Area. In Geometry, for a finite planar surface of scalar Area S the vector area \mathbf{S} is defined as a vector History The word tensor was introduced in 1846 by William Rowan Hamilton to describe the norm operation in a certain type of algebraic system (eventually 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.

Types

Explosion or deflagration pressures

Explosion or deflagration pressures are the result of the ignition of explosive gases, mists, dust/air suspensions, in unconfined and confined spaces. This page is about the physical properties of gas as a state of matter

Negative pressures

While pressures are generally positive, there are several situations in which negative pressures 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. The Van der Waals equation is an Equation of state that can be derived from a special form of the potential between a pair of molecules (hard-sphere repulsion 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. Transpirational pull is the main phenomenon driving the flow of Water in the Xylem tissues of large Plants Mechanisms Transpirational pull
• 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). In Physics, the Casimir effect and the Casimir-Polder force are physical forces arising from a quantized field. Vacuum energy is an underlying background Energy that exists in Space even when devoid of Matter (known as Free space)
• 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. In Physical cosmology, the cosmological constant (usually denoted by the Greek capital letter Lambda: Λ was proposed by Albert Einstein as a modification

Stagnation pressure

Stagnation pressure is the pressure a fluid exerts when it is forced to stop moving. In Fluid dynamics, stagnation pressure is the Pressure at a Stagnation point in a fluid flow where the kinetic energy is converted into pressure energy 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. Mach number (\mathrm{Ma} or M (generally ˈmɑːk sometimes /ˈmɑːx/ or /ˈmæk/ is the speed of an object moving through air or any 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). In Fluid dynamics, Bernoulli's principle states that for an Inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in

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. A Pitot (ˈpiːtoʊ tube is a Pressure measurement instrument used to measure Fluid flow Velocity. 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. Many techniques have been developed for the measurement of Pressure and Vacuum. Depending on where the inlet holes are located on the probe, it can measure static pressure or stagnation pressure.

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 with 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. Boyle's law (sometimes referred to as the Boyle-Mariotte law) is one of several Gas laws and a special case of the Ideal gas law.

See also

Notes

External links

• Pressure calculator
• A Java pressure simulation applet
• Thermodynamics - A chapter from an online textbook
• Introduction to Fluid Statics and Dynamics on Project PHYSNET
• An exercise in air pressure
• Pressure being a scalar quantity
• Online pressure converter for 52 different pressure units
• Pressure conversions - for both SI and non-SI units