# Pressure (Thermodynamics)

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Pressure (the symbol: P) is the force per unit area applied in a direction perpendicular to the surface of an object. 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. Pressure is the amount of force acting on a unit area. The symbol of pressure is P. (a)

### Formula

Mathematically:

$P = \frac{F}{A}\ \mbox{or}\ P = \frac{dF_n}{dA}$

where:

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

Pressure is a scalar quantity. It relates the vector surface element (a vector normal to the surface) with the normal force acting on it. The pressure is the scalar proportionality constant that relates the two normal vectors:

$d\mathbf{F}_n=-P\,d\mathbf{A} = -P\,\mathbf{n}\,dA$

The minus sign comes from the fact that the force is considered towards the surface element, while the normal vector points outwards.

It is incorrect (although rather usual) to say "the pressure is directed in such or such direction". The pressure, as a scalar, has no direction. It is the force given by the previous relation the quantity that has a direction. If we change the orientation of the surface element the direction of the normal force changes accordingly, but the pressure remains the same.

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.

### Units

Mercury column.

The SI unit for pressure is the pascal (Pa), equal to one newton per square meter (N/m2 or kg·m-1·s-2). This special name for the unit was added in 1971;(1) before that, pressure in SI was expressed simply as N/m2.

Non-SI measures such as pounds per square inch and bar are used in some parts of the world, primarily in the United States of America. The cgs unit of pressure is the barye (ba), equal to 1 dyn·cm-2. 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. 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). Similar pressures are given in kilopascals (kPa) in most other fields, where the hecto- prefix is rarely used. The inch of mercury is still used in the United States. 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. Scuba divers often use a manometric rule of thumb: the pressure exerted by ten meters depth of water is approximately equal to one atmosphere. Americans learn that 34 feet of fresh water or 33 feet of sea water equals one atm.

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.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. 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. One mmHg (millimeter of mercury) is equal to one torr. 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.

Gauge pressure is often given in units with 'g' appended, eg 'kPag' or 'psig', and units for measurements of absolute pressure are sometimes given a suffix of 'a', to avoid confusion, for example 'kPaa', 'psia'.

Presently or formerly popular pressure units include the following:

• atmosphere (atm)
• manometric units:
• centimeter, inch, and millimeter of mercury (torr)
• millimeter, centimeter, meter, inch, and foot of water
• customary 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)
 pascal(Pa) bar(bar) technical atmosphere(at) atmosphere(atm) torr(Torr) pound-force persquare 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 14.5037744 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 Torr; ≈ 1 mmHg 19.337×10−3 1 psi 6.894×103 68.948×10−3 70.307×10−3 68.046×10−3 51.715 ≡ 1 lbf/in2

### 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 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

$\mathbf{F}=\mathbf{\sigma A}\,$

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.

## References

(1) 14th Conference of the International Bureau of Weights and Measures

## Notes

(a) Note the upper case P is also used for power.

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