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115-505: The quality of a partial vacuum refers to how closely it approaches a perfect vacuum. Other things equal, lower gas pressure means higher-quality vacuum. For example, a typical vacuum cleaner produces enough suction to reduce air pressure by around 20%. But higher-quality vacuums are possible. Ultra-high vacuum chambers, common in chemistry, physics, and engineering, operate below one trillionth (10) of atmospheric pressure (100 nPa), and can reach around 100 particles/cm. Outer space

230-553: A manometer . Depending on where the inlet holes are located on the probe, it can measure static pressures or stagnation pressures. 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: π = F l {\displaystyle \pi ={\frac {F}{l}}} and shares many similar properties with three-dimensional pressure. Properties of surface chemicals can be investigated by measuring pressure/area isotherms, as

345-453: A relative permittivity and relative permeability that are not identically unity. In the theory of classical electromagnetism, free space has the following properties: The vacuum of classical electromagnetism can be viewed as an idealized electromagnetic medium with the constitutive relations in SI units: relating the electric displacement field D to the electric field E and

460-579: A fundamental limit within which instantaneous position and momentum , or energy and time can be measured. This far reaching consequences also threatened whether the "emptiness" of space between particles exists. The strictest criterion to define a vacuum is a region of space and time where all the components of the stress–energy tensor are zero. This means that this region is devoid of energy and momentum, and by consequence, it must be empty of particles and other physical fields (such as electromagnetism) that contain energy and momentum. In general relativity ,

575-603: A good model for realizable vacuum, and agrees with a number of experimental observations as described next. QED vacuum has interesting and complex properties. In QED vacuum, the electric and magnetic fields have zero average values, but their variances are not zero. As a result, QED vacuum contains vacuum fluctuations ( virtual particles that hop into and out of existence), and a finite energy called vacuum energy . Vacuum fluctuations are an essential and ubiquitous part of quantum field theory. Some experimentally verified effects of vacuum fluctuations include spontaneous emission and

690-400: A gravitational well such as a planet, otherwise known as atmospheric pressure . In the case of planetary atmospheres , the pressure-gradient force of the gas pushing outwards from higher pressure, lower altitudes to lower pressure, higher altitudes is balanced by the gravitational force , preventing the gas from diffusing into outer space and maintaining hydrostatic equilibrium . In

805-423: A higher stagnation pressure when forced to a standstill. Static pressure and stagnation pressure are related by: p 0 = 1 2 ρ v 2 + p {\displaystyle p_{0}={\frac {1}{2}}\rho v^{2}+p} where 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

920-427: A liquid in liquid columns of constant density or at a depth within a substance is represented by the following formula: p = ρ g h , {\displaystyle p=\rho gh,} where: Horror vacui (physics) In philosophy and early physics , horror vacui ( Latin: horror of the vacuum ) or plenism ( / ˈ p l iː n ɪ z əm / )—commonly stated as " nature abhors

1035-431: A mass spectrometer must be used in conjunction with the ionization gauge for accurate measurement. Vacuum is useful in a variety of processes and devices. Its first widespread use was in the incandescent light bulb to protect the filament from chemical degradation. The chemical inertness produced by a vacuum is also useful for electron beam welding , cold welding , vacuum packing and vacuum frying . Ultra-high vacuum

1150-479: A measurable vacuum relative to the local environment. Similarly, much higher than normal relative vacuum readings are possible deep in the Earth's ocean. A submarine maintaining an internal pressure of 1 atmosphere submerged to a depth of 10 atmospheres (98 metres; a 9.8-metre column of seawater has the equivalent weight of 1 atm) is effectively a vacuum chamber keeping out the crushing exterior water pressures, though

1265-520: A measured, rather than defined, quantity. These manometric units are still encountered in many fields. Blood pressure is measured in millimetres (or centimetres) of mercury in most of the world, and lung pressures in centimetres of water are still common. Underwater divers use the metre sea water (msw or MSW) and foot sea water (fsw or FSW) units of pressure, and these are the units for pressure gauges used to measure pressure exposure in diving chambers and personal decompression computers . A msw

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1380-446: A minute drag on the Earth's orbit. While the Earth does, in fact, move through a relatively dense medium in comparison to that of interstellar space, the drag is so minuscule that it could not be detected. In 1912, astronomer Henry Pickering commented: "While the interstellar absorbing medium may be simply the ether, [it] is characteristic of a gas, and free gaseous molecules are certainly there". Thereafter, however, luminiferous aether

1495-411: A more complex dependence on the variables of state. Vapour pressure is the pressure of a vapour in thermodynamic equilibrium with its condensed phases in a closed system. All liquids and solids have a tendency to evaporate into a gaseous form, and all gases have a tendency to condense back to their liquid or solid form. The atmospheric pressure boiling point of a liquid (also known as

1610-405: A partial vacuum of about 10 Pa (0.1  Torr ). A number of electrical properties become observable at this vacuum level, which renewed interest in further research. While outer space provides the most rarefied example of a naturally occurring partial vacuum, the heavens were originally thought to be seamlessly filled by a rigid indestructible material called aether . Borrowing somewhat from

1725-424: A physical container, the pressure of the gas originates from the molecules colliding with the walls of the container. The walls of the container can be anywhere inside the gas, and the force per unit area (the pressure) is the same. If the "container" is shrunk down to a very small point (becoming less true as the atomic scale is approached), the pressure will still have a single value at that point. Therefore, pressure

1840-425: A proposed propulsion system for interplanetary travel . All of the observable universe is filled with large numbers of photons , the so-called cosmic background radiation , and quite likely a correspondingly large number of neutrinos . The current temperature of this radiation is about 3  K (−270.15  °C ; −454.27  °F ). The quality of a vacuum is indicated by the amount of matter remaining in

1955-420: A scalar, has no direction. The force given by the previous relationship to the quantity has a direction, but the pressure does not. 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 distributed to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point. It

2070-495: A suffix of "a", to avoid confusion, for example "kPaa", "psia". However, the US National Institute of Standards and Technology recommends that, to avoid confusion, any modifiers be instead applied to the quantity being measured rather than the unit of measure. For example, " p g = 100 psi" rather than " p = 100 psig" . Differential pressure is expressed in units with "d" appended; this type of measurement

2185-499: A vacuum ", for example by Spinoza —is a hypothesis attributed to Aristotle , later criticized by the atomism of Epicurus and Lucretius , that nature contains no vacuums because the denser surrounding material continuum would immediately fill the rarity of an incipient void. Aristotle also argued against the void in a more abstract sense: since a void is merely nothingness , following his teacher Plato , nothingness cannot rightly be said to exist. Furthermore, insofar as

2300-476: A vacuum if he wanted and the 1277 Paris condemnations of Bishop Étienne Tempier , which required there to be no restrictions on the powers of God, led to the conclusion that God could create a vacuum if he so wished. From the 14th century onward increasingly departed from the Aristotelian perspective, scholars widely acknowledged that a supernatural void exists beyond the confines of the cosmos itself by

2415-430: A vacuum is called outgassing . All materials, solid or liquid, have a small vapour pressure , and their outgassing becomes important when the vacuum pressure falls below this vapour pressure. Outgassing has the same effect as a leak and will limit the achievable vacuum. Outgassing products may condense on nearby colder surfaces, which can be troublesome if they obscure optical instruments or react with other materials. This

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2530-399: A vacuum was present, if only for an instant, between two flat plates when they were rapidly separated. There was much discussion of whether the air moved in quickly enough as the plates were separated, or, as Walter Burley postulated, whether a 'celestial agent' prevented the vacuum arising. Jean Buridan reported in the 14th century that teams of ten horses could not pull open bellows when

2645-549: A vanishing stress–energy tensor implies, through Einstein field equations , the vanishing of all the components of the Ricci tensor . Vacuum does not mean that the curvature of space-time is necessarily flat: the gravitational field can still produce curvature in a vacuum in the form of tidal forces and gravitational waves (technically, these phenomena are the components of the Weyl tensor ). The black hole (with zero electric charge)

2760-399: A void would be featureless, it could neither be encountered by the senses nor could its supposition lend additional explanatory power. Hero of Alexandria challenged the theory in the first century AD, but his attempts to create an artificial vacuum failed. The theory was debated in the context of 17th-century fluid mechanics , by Thomas Hobbes and Robert Boyle , among others, and through

2875-515: A wide array of vacuum technologies has since become available. The development of human spaceflight has raised interest in the impact of vacuum on human health, and on life forms in general. The word vacuum comes from Latin  'an empty space, void', noun use of neuter of vacuus , meaning "empty", related to vacare , meaning "to be empty". Vacuum is one of the few words in the English language that contains two consecutive instances of

2990-424: Is "p" or P . The IUPAC recommendation for pressure is a lower-case p . However, upper-case P is widely used. The usage of P vs p depends upon the field in which one is working, on the nearby presence of other symbols for quantities such as power and momentum , and on writing style. Mathematically: p = F A , {\displaystyle p={\frac {F}{A}},} where: Pressure

3105-453: Is 100 kPa (15 psi), a gas (such as helium) at 200 kPa (29 psi) (gauge) (300 kPa or 44 psi [absolute]) is 50% denser than the same gas at 100 kPa (15 psi) (gauge) (200 kPa or 29 psi [absolute]). Focusing on gauge values, one might erroneously conclude the first sample had twice the density of the second one. In a static gas , the gas as a whole does not appear to move. The individual molecules of

3220-463: Is a scalar quantity. It relates the vector area 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 F n = − p d A = − p n d A . {\displaystyle d\mathbf {F} _{n}=-p\,d\mathbf {A} =-p\,\mathbf {n} \,dA.} The minus sign comes from

3335-442: Is a fundamental parameter in thermodynamics , and it is conjugate to volume . The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N/m , or kg·m ·s ). This name for the unit was added in 1971; before that, pressure in SI was expressed in newtons per square metre. Other units of pressure, such as pounds per square inch (lbf/in ) and bar , are also in common use. The CGS unit of pressure

3450-418: Is a limit to the phenomenon. René Descartes proposed a plenic interpretation of atomism to eliminate the void, which he considered incompatible with his concept of space. The theory was rejected by later scientists, such as Galileo's pupil Evangelista Torricelli , who repeated his experiment with mercury . Blaise Pascal successfully repeated Galileo's and Torricelli's experiment and foresaw no reason why

3565-516: Is a non-SI unit): Vacuum is measured in units of pressure , typically as a subtraction relative to ambient atmospheric pressure on Earth. But the amount of relative measurable vacuum varies with local conditions. On the surface of Venus , where ground-level atmospheric pressure is much higher than on Earth, much higher relative vacuum readings would be possible. On the surface of the Moon with almost no atmosphere, it would be extremely difficult to create

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3680-416: Is a scalar quantity, not a vector quantity. It has magnitude but no direction sense associated with it. Pressure force 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 {\displaystyle \mathbf {F} } to

3795-512: Is an elegant example of a region completely "filled" with vacuum, but still showing a strong curvature. In classical electromagnetism , the vacuum of free space , or sometimes just free space or perfect vacuum , is a standard reference medium for electromagnetic effects. Some authors refer to this reference medium as classical vacuum , a terminology intended to separate this concept from QED vacuum or QCD vacuum , where vacuum fluctuations can produce transient virtual particle densities and

3910-449: Is an established constant. It is approximately equal to typical air pressure at Earth mean sea level and is defined as 101 325  Pa . 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., centimetres of water , millimetres of mercury or inches of mercury ). The most common choices are mercury (Hg) and water; water

4025-425: Is an even higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average in intergalactic space. Vacuum has been a frequent topic of philosophical debate since ancient Greek times, but was not studied empirically until the 17th century. Clemens Timpler (1605) philosophized about the experimental possibility of producing a vacuum in small tubes. Evangelista Torricelli produced

4140-537: Is connected to the region of interest. Any fluid can be used, but mercury is preferred for its high density and low vapour pressure. Simple hydrostatic gauges can measure pressures ranging from 1 torr (100 Pa) to above atmospheric. An important variation is the McLeod gauge which isolates a known volume of vacuum and compresses it to multiply the height variation of the liquid column. The McLeod gauge can measure vacuums as high as 10 torr (0.1 mPa), which

4255-485: Is defined as 0.1 bar (= 10,000 Pa), is not the same as a linear metre of depth. 33.066 fsw = 1 atm (1 atm = 101,325 Pa / 33.066 = 3,064.326 Pa). The pressure conversion from msw to fsw is different from the length conversion: 10 msw = 32.6336 fsw, while 10 m = 32.8083 ft. Gauge pressure is often given in units with "g" appended, e.g. "kPag", "barg" or "psig", and units for measurements of absolute pressure are sometimes given

4370-434: Is equal to the displacement of a millimeter of mercury ( mmHg ) in a manometer with 1 torr equaling 133.3223684 pascals above absolute zero pressure. Vacuum is often also measured on the barometric scale or as a percentage of atmospheric pressure in bars or atmospheres . Low vacuum is often measured in millimeters of mercury (mmHg) or pascals (Pa) below standard atmospheric pressure. "Below atmospheric" means that

4485-473: Is impossible. It is plain, then, that if there is a time in which it will move through any part of the void, this impossible result will follow: it will be found to traverse a certain distance, whether this be full or void, in an equal time; for there will be some body which is in the same ratio to the other body as the time is to the time. Plenism means "fullness", from Latin plēnum , English "plenty", cognate via Proto-Indo-European to "full". In Ancient Greek ,

4600-421: 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. In non- SI technical work, a gauge pressure of 32 psi (220 kPa) is sometimes written as "32 psig", and an absolute pressure as "32 psia", though the other methods explained above that avoid attaching characters to

4715-426: Is most often the compressive stress at some point within a fluid . (The term fluid refers to both liquids and gases – for more information specifically about liquid pressure, see section below .) Fluid pressure occurs in one of two situations: Pressure in open conditions usually can be approximated as the pressure in "static" or non-moving conditions (even in the ocean where there are waves and currents), because

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4830-409: Is no friction, it is inviscid (zero viscosity ). The equation for all points of a system filled with a constant-density fluid is p γ + v 2 2 g + z = c o n s t , {\displaystyle {\frac {p}{\gamma }}+{\frac {v^{2}}{2g}}+z=\mathrm {const} ,} where: Explosion or deflagration pressures are

4945-406: Is no ratio by which it exceeds 0; for that which exceeds must be divisible into the excess + that which is exceeded, so that will be what it exceeds 0 by + 0. For this reason, too, a line does not exceed a point unless it is composed of points! Similarly the void can bear no ratio to the full, and therefore neither can movement through the one to movement through the other, but if a thing moves through

5060-437: Is nontoxic and readily available, while mercury's high density allows a shorter column (and so a smaller manometer) to be used 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 , where g is the gravitational acceleration . Fluid density and local gravity can vary from one reading to another depending on local factors, so

5175-573: Is not used. High vacuum systems must be clean and free of organic matter to minimize outgassing. Ultra-high vacuum systems are usually baked, preferably under vacuum, to temporarily raise the vapour pressure of all outgassing materials and boil them off. Once the bulk of the outgassing materials are boiled off and evacuated, the system may be cooled to lower vapour pressures and minimize residual outgassing during actual operation. Some systems are cooled well below room temperature by liquid nitrogen to shut down residual outgassing and simultaneously cryopump

5290-419: Is of great concern to space missions, where an obscured telescope or solar cell can ruin an expensive mission. The most prevalent outgassing product in vacuum systems is water absorbed by chamber materials. It can be reduced by desiccating or baking the chamber, and removing absorbent materials. Outgassed water can condense in the oil of rotary vane pumps and reduce their net speed drastically if gas ballasting

5405-437: Is referred to as ' QED vacuum ' to distinguish it from the vacuum of quantum chromodynamics , denoted as QCD vacuum . QED vacuum is a state with no matter particles (hence the name), and no photons . As described above, this state is impossible to achieve experimentally. (Even if every matter particle could somehow be removed from a volume, it would be impossible to eliminate all the blackbody photons .) Nonetheless, it provides

5520-584: Is related to energy density and may be expressed in units such as joules per cubic metre (J/m , which is equal to Pa). Mathematically: p = F ⋅ distance A ⋅ distance = Work Volume = Energy (J) Volume  ( m 3 ) . {\displaystyle p={\frac {F\cdot {\text{distance}}}{A\cdot {\text{distance}}}}={\frac {\text{Work}}{\text{Volume}}}={\frac {\text{Energy (J)}}{{\text{Volume }}({\text{m}}^{3})}}.} Some meteorologists prefer

5635-463: Is the barye (Ba), equal to 1 dyn·cm , or 0.1 Pa. Pressure is sometimes expressed in grams-force or kilograms-force per square centimetre ("g/cm " or "kg/cm ") 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 deprecated in SI. The technical atmosphere (symbol: at) is 1 kgf/cm (98.0665 kPa, or 14.223 psi). Pressure

5750-434: Is the air pressure in an automobile tire , which might be said to be "220  kPa (32 psi)", but is actually 220 kPa (32 psi) above atmospheric pressure. Since atmospheric pressure at sea level is about 100 kPa (14.7 psi), the absolute pressure in the tire is therefore about 320 kPa (46 psi). In technical work, this is written "a gauge pressure of 220 kPa (32 psi)". Where space

5865-453: Is the lowest direct measurement of pressure that is possible with current technology. Other vacuum gauges can measure lower pressures, but only indirectly by measurement of other pressure-controlled properties. These indirect measurements must be calibrated via a direct measurement, most commonly a McLeod gauge. The kenotometer is a particular type of hydrostatic gauge, typically used in power plants using steam turbines. The kenotometer measures

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5980-410: Is thinner, in time E (if the length of B is equal to D ), in proportion to the density of the hindering body. For let B be water and D air; then by so much as air is thinner and more incorporeal than water, A will move through D faster than through B . Let the speed have the same ratio to the speed, then, that air has to water. Then if air is twice as thin, the body will traverse B in twice

6095-443: Is used in the study of atomically clean substrates, as only a very good vacuum preserves atomic-scale clean surfaces for a reasonably long time (on the order of minutes to days). High to ultra-high vacuum removes the obstruction of air, allowing particle beams to deposit or remove materials without contamination. This is the principle behind chemical vapor deposition , physical vapor deposition , and dry etching which are essential to

6210-412: Is useful for flywheel energy storage and ultracentrifuges . Vacuums are commonly used to produce suction , which has an even wider variety of applications. The Newcomen steam engine used vacuum instead of pressure to drive a piston. In the 19th century, vacuum was used for traction on Isambard Kingdom Brunel 's experimental atmospheric railway . Vacuum brakes were once widely used on trains in

6325-401: Is useful when considering sealing performance or whether a valve will open or close. Presently or formerly popular pressure units include the following: 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,

6440-401: The vector area A {\displaystyle \mathbf {A} } via the linear relation F = σ A {\displaystyle \mathbf {F} =\sigma \mathbf {A} } . This tensor may be expressed as the sum of the viscous stress tensor minus the hydrostatic pressure. The negative of the stress tensor is sometimes called the pressure tensor, but in

6555-475: The Heading Indicator (HI) ) are typically vacuum-powered, as protection against loss of all (electrically powered) instruments, since early aircraft often did not have electrical systems, and since there are two readily available sources of vacuum on a moving aircraft, the engine and an external venturi. Vacuum induction melting uses electromagnetic induction within a vacuum. Maintaining a vacuum in

6670-500: The Lamb shift . Coulomb's law and the electric potential in vacuum near an electric charge are modified. Theoretically, in QCD multiple vacuum states can coexist. The starting and ending of cosmological inflation is thought to have arisen from transitions between different vacuum states. For theories obtained by quantization of a classical theory, each stationary point of the energy in

6785-453: The Sun and the dynamic pressure of the solar winds , so the definition of pressure becomes difficult to interpret. The thermosphere in this range has large gradients of pressure, temperature and composition, and varies greatly due to space weather . Astrophysicists prefer to use number density to describe these environments, in units of particles per cubic centimetre. But although it meets

6900-431: The condenser is an important aspect of the efficient operation of steam turbines . A steam jet ejector or liquid ring vacuum pump is used for this purpose. The typical vacuum maintained in the condenser steam space at the exhaust of the turbine (also called condenser backpressure) is in the range 5 to 15 kPa (absolute), depending on the type of condenser and the ambient conditions. Evaporation and sublimation into

7015-540: The configuration space gives rise to a single vacuum. String theory is believed to have a huge number of vacua – the so-called string theory landscape . Outer space has very low density and pressure, and is the closest physical approximation of a perfect vacuum. But no vacuum is truly perfect, not even in interstellar space, where there are still a few hydrogen atoms per cubic meter. Stars, planets, and moons keep their atmospheres by gravitational attraction, and as such, atmospheres have no clearly delineated boundary:

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7130-458: The magnetic field or H -field H to the magnetic induction or B -field B . Here r is a spatial location and t is time. In quantum mechanics and quantum field theory , the vacuum is defined as the state (that is, the solution to the equations of the theory) with the lowest possible energy (the ground state of the Hilbert space ). In quantum electrodynamics this vacuum

7245-424: The normal boiling point ) is the temperature at which the vapor pressure equals the ambient atmospheric pressure. With any incremental increase in that temperature, the vapor pressure becomes sufficient to overcome atmospheric pressure and lift the liquid to form vapour bubbles inside the bulk of the substance. Bubble formation deeper in the liquid requires a higher pressure, and therefore higher temperature, because

7360-430: The pneuma of Stoic physics , aether came to be regarded as the rarefied air from which it took its name, (see Aether (mythology) ). Early theories of light posited a ubiquitous terrestrial and celestial medium through which light propagated. Additionally, the concept informed Isaac Newton 's explanations of both refraction and of radiant heat. 19th century experiments into this luminiferous aether attempted to detect

7475-484: The 1 atm inside the submarine would not normally be considered a vacuum. Therefore, to properly understand the following discussions of vacuum measurement, it is important that the reader assumes the relative measurements are being done on Earth at sea level, at exactly 1 atmosphere of ambient atmospheric pressure. The SI unit of pressure is the pascal (symbol Pa), but vacuum is often measured in torrs , named for an Italian physicist Torricelli (1608–1647). A torr

7590-409: The 17th century. This idea, influenced by Stoic physics , helped to segregate natural and theological concerns. Almost two thousand years after Plato, René Descartes also proposed a geometrically based alternative theory of atomism, without the problematic nothing–everything dichotomy of void and atom. Although Descartes agreed with the contemporary position, that a vacuum does not occur in nature,

7705-536: The MFP of room temperature air is roughly 100 mm, which is on the order of everyday objects such as vacuum tubes . The Crookes radiometer turns when the MFP is larger than the size of the vanes. Vacuum quality is subdivided into ranges according to the technology required to achieve it or measure it. These ranges were defined in ISO 3529-1:2019 as shown in the following table (100 Pa corresponds to 0.75 Torr; Torr

7820-452: The UK but, except on heritage railways , they have been replaced by air brakes . Manifold vacuum can be used to drive accessories on automobiles . The best known application is the vacuum servo , used to provide power assistance for the brakes . Obsolete applications include vacuum-driven windscreen wipers and Autovac fuel pumps. Some aircraft instruments ( Attitude Indicator (AI) and

7935-424: The absolute pressure is equal to the current atmospheric pressure. In other words, most low vacuum gauges that read, for example 50.79 Torr. Many inexpensive low vacuum gauges have a margin of error and may report a vacuum of 0 Torr but in practice this generally requires a two-stage rotary vane or other medium type of vacuum pump to go much beyond (lower than) 1 torr. Many devices are used to measure

8050-516: 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 ); similarly, the pound-force per square inch ( psi , symbol lbf/in ) is the traditional unit of pressure in the imperial and US customary systems. Pressure may also be expressed in terms of standard atmospheric pressure ;

8165-447: The convention that the force is considered towards the surface element, while the normal vector points outward. The equation has meaning in that, for any surface S in contact with the fluid, the total force exerted by the fluid on that surface is the surface integral over S of the right-hand side of the above equation. It is incorrect (although rather usual) to say "the pressure is directed in such or such direction". The pressure, as

8280-522: The definition of outer space, the atmospheric density within the first few hundred kilometers above the Kármán line is still sufficient to produce significant drag on satellites . Most artificial satellites operate in this region called low Earth orbit and must fire their engines every couple of weeks or a few times a year (depending on solar activity). The drag here is low enough that it could theoretically be overcome by radiation pressure on solar sails ,

8395-417: The denser surrounding material continuum would immediately fill any incipient rarity that might give rise to a void. In his Physics , book IV, Aristotle offered numerous arguments against the void: for example, that motion through a medium which offered no impediment could continue ad infinitum , there being no reason that something would come to rest anywhere in particular. In the medieval Muslim world ,

8510-454: The density of atmospheric gas simply decreases with distance from the object. The Earth's atmospheric pressure drops to about 32 millipascals (4.6 × 10 psi) at 100 kilometres (62 mi) of altitude, the Kármán line , which is a common definition of the boundary with outer space. Beyond this line, isotropic gas pressure rapidly becomes insignificant when compared to radiation pressure from

8625-464: The early 18th century by Sir Isaac Newton and Gottfried Leibniz . As advanced by Aristotle in Physics : In a void , no one could say why a thing once set in motion should stop anywhere; for why should it stop here rather than here? So that a thing will either be at rest or must be moved ad infinitum , unless something more powerful gets in its way. Further, things are now thought to move into

8740-701: The fabrication of semiconductors and optical coatings , and to surface science . The reduction of convection provides the thermal insulation of thermos bottles . Deep vacuum lowers the boiling point of liquids and promotes low temperature outgassing which is used in freeze drying , adhesive preparation, distillation , metallurgy , and process purging. The electrical properties of vacuum make electron microscopes and vacuum tubes possible, including cathode-ray tubes . Vacuum interrupters are used in electrical switchgear. Vacuum arc processes are industrially important for production of certain grades of steel or high purity materials. The elimination of air friction

8855-453: The first century AD. Following Plato , however, even the abstract concept of a featureless void faced considerable skepticism: it could not be apprehended by the senses, it could not, itself, provide additional explanatory power beyond the physical volume with which it was commensurate and, by definition, it was quite literally nothing at all, which cannot rightly be said to exist. Aristotle believed that no void could occur naturally, because

8970-444: The first laboratory vacuum in 1643, and other experimental techniques were developed as a result of his theories of atmospheric pressure. A Torricellian vacuum is created by filling with mercury a tall glass container closed at one end, and then inverting it in a bowl to contain the mercury (see below). Vacuum became a valuable industrial tool in the 20th century with the introduction of incandescent light bulbs and vacuum tubes , and

9085-480: The flat edge is used, force is distributed over a larger surface area resulting in less pressure, and it will not cut. Whereas using the sharp edge, which has less surface area, results in greater pressure, and so the knife cuts smoothly. This is one example of a practical application of pressure For gases, pressure is sometimes measured not as an absolute pressure , but relative to atmospheric pressure ; such measurements are called gauge pressure . An example of this

9200-430: The flexure of the diaphragm, which results in a change in capacitance. These gauges are effective from 10 torr to 10 torr, and beyond. Thermal conductivity gauges rely on the fact that the ability of a gas to conduct heat decreases with pressure. In this type of gauge, a wire filament is heated by running current through it. A thermocouple or Resistance Temperature Detector (RTD) can then be used to measure

9315-404: The fluid pressure increases above the atmospheric pressure as the depth increases. The vapor pressure that a single component in a mixture contributes to the total pressure in the system is called partial vapor pressure . When a person swims under the water, water pressure is felt acting on the person's eardrums. The deeper that person swims, the greater the pressure. The pressure felt is due to

9430-419: The following, the term "pressure" will refer only to the scalar pressure. According to the theory of general relativity , pressure increases the strength of a gravitational field (see stress–energy tensor ) and so adds to the mass-energy cause of gravity . This effect is unnoticeable at everyday pressures but is significant in neutron stars , although it has not been experimentally tested. Fluid pressure

9545-425: The gas density decreases, the MFP increases, and when the MFP is longer than the chamber, pump, spacecraft, or other objects present, the continuum assumptions of fluid mechanics do not apply. This vacuum state is called high vacuum , and the study of fluid flows in this regime is called particle gas dynamics. The MFP of air at atmospheric pressure is very short, 70  nm , but at 100  mPa (≈ 10  Torr )

9660-425: The gas, however, are in constant random motion . Because there are an extremely large number of molecules and because the motion of the individual molecules is random in every direction, no motion is detected. When the gas is at least partially confined (that is, not free to expand rapidly), the gas will exhibit a hydrostatic pressure. This confinement can be achieved with either a physical container of some sort, or in

9775-416: The gases being measured. Ionization gauges are used in ultrahigh vacuum. They come in two types: hot cathode and cold cathode. In the hot cathode version an electrically heated filament produces an electron beam. The electrons travel through the gauge and ionize gas molecules around them. The resulting ions are collected at a negative electrode. The current depends on the number of ions, which depends on

9890-548: 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, except aviation where the hecto- prefix is commonly used. The inch of mercury is still used in the United States. Oceanographers usually measure underwater pressure in decibars (dbar) because pressure in the ocean increases by approximately one decibar per metre depth. The standard atmosphere (atm)

10005-412: 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. One millimetre of mercury is approximately equal to one torr . The water-based units still depend on the density of water,

10120-490: The hemispheres, teams of horses could not separate two hemispheres from which the air had been partially evacuated. Robert Boyle improved Guericke's design and with the help of Robert Hooke further developed vacuum pump technology. Thereafter, research into the partial vacuum lapsed until 1850 when August Toepler invented the Toepler pump and in 1855 when Heinrich Geissler invented the mercury displacement pump, achieving

10235-454: The motions create only negligible changes in the pressure. Such conditions conform with principles of fluid statics . The pressure at any given point of a non-moving (static) fluid is called the hydrostatic pressure . Closed bodies of fluid are either "static", when the fluid is not moving, or "dynamic", when the fluid can move as in either a pipe or by compressing an air gap in a closed container. The pressure in closed conditions conforms with

10350-418: The moving body differs from the other owing to excess of weight or of lightness. Now the medium causes a difference because it impedes the moving thing, most of all if it is moving in the opposite direction, but in a secondary degree even if it is at rest; and especially a medium that is not easily divided, i.e. a medium that is somewhat dense. A , then, will move through B in time G , and through D , which

10465-410: The physicist and Islamic scholar Al-Farabi wrote a treatise rejecting the existence of the vacuum in the 10th century. He concluded that air's volume can expand to fill available space, and therefore the concept of a perfect vacuum was incoherent. According to Ahmad Dallal , Abū Rayhān al-Bīrūnī states that "there is no observable evidence that rules out the possibility of vacuum". The suction pump

10580-410: The port was sealed. The 17th century saw the first attempts to quantify measurements of partial vacuum. Evangelista Torricelli 's mercury barometer of 1643 and Blaise Pascal 's experiments both demonstrated a partial vacuum. In 1654, Otto von Guericke invented the first vacuum pump and conducted his famous Magdeburg hemispheres experiment, showing that, owing to atmospheric pressure outside

10695-428: The pressure in a vacuum, depending on what range of vacuum is needed. Hydrostatic gauges (such as the mercury column manometer ) consist of a vertical column of liquid in a tube whose ends are exposed to different pressures. The column will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube. The simplest design is a closed-end U-shaped tube, one side of which

10810-627: The pressure in the gauge. Hot cathode gauges are accurate from 10 torr to 10 torr. The principle behind cold cathode version is the same, except that electrons are produced in a discharge created by a high voltage electrical discharge. Cold cathode gauges are accurate from 10 torr to 10 torr. Ionization gauge calibration is very sensitive to construction geometry, chemical composition of gases being measured, corrosion and surface deposits. Their calibration can be invalidated by activation at atmospheric pressure or low vacuum. The composition of gases at high vacuums will usually be unpredictable, so

10925-415: The principles of fluid dynamics . The concepts of fluid pressure are predominantly attributed to the discoveries of Blaise Pascal and Daniel Bernoulli . Bernoulli's equation can be used in almost any situation to determine the pressure at any point in a fluid. The equation makes some assumptions about the fluid, such as the fluid being ideal and incompressible. An ideal fluid is a fluid in which there

11040-486: The ratio which the time E bears to the time H . For if the body Z be as much thinner than D as E exceeds H , A , if it moves through Z , will traverse it in a time inverse to the speed of the movement, i.e. in a time equal to H . If, then, there is no body in Z , A will traverse Z still more quickly. But we supposed that its traverse of Z when Z was void occupied the time H . So that it will traverse Z in an equal time whether Z be full or void. But this

11155-428: The result of the ignition of explosive gases , mists, dust/air suspensions, in unconfined and confined spaces. While pressures are, in general, positive, there are several situations in which negative pressures may be encountered: 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

11270-405: The success of his namesake coordinate system and more implicitly, the spatial–corporeal component of his metaphysics would come to define the philosophically modern notion of empty space as a quantified extension of volume. By the ancient definition however, directional information and magnitude were conceptually distinct. Medieval thought experiments into the idea of a vacuum considered whether

11385-439: The system, so that a high quality vacuum is one with very little matter left in it. Vacuum is primarily measured by its absolute pressure , but a complete characterization requires further parameters, such as temperature and chemical composition. One of the most important parameters is the mean free path (MFP) of residual gases, which indicates the average distance that molecules will travel between collisions with each other. As

11500-408: The system. Fluids cannot generally be pulled, so a vacuum cannot be created by suction . Suction can spread and dilute a vacuum by letting a higher pressure push fluids into it, but the vacuum has to be created first before suction can occur. The easiest way to create an artificial vacuum is to expand the volume of a container. For example, the diaphragm muscle expands the chest cavity, which causes

11615-454: The temperature of the filament. This temperature is dependent on the rate at which the filament loses heat to the surrounding gas, and therefore on the thermal conductivity. A common variant is the Pirani gauge which uses a single platinum filament as both the heated element and RTD. These gauges are accurate from 10 torr to 10 torr, but they are sensitive to the chemical composition of

11730-472: The term for the void is τὸ κενόν ( to kenón ). The idea was restated as "Natura abhorret vacuum" by François Rabelais in his series of books titled Gargantua and Pantagruel in the 1530s. The theory was supported and restated by Galileo Galilei in the early 17th century as "Resistenza del vacuo" . Galileo was surprised by the fact that water could not rise above a certain level in an aspiration tube in his suction pump , leading him to conclude that there

11845-495: The thickest medium such and such a distance in such and such a time, it moves through the void with a speed beyond any ratio. For let Z be void, equal in magnitude to B and to D . Then if A is to traverse and move through it in a certain time, H , a time less than E , however, the void will bear this ratio to the full. But in a time equal to H , A will traverse the part O of A . And it will surely also traverse in that time any substance Z which exceeds air in thickness in

11960-399: 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 . The negative gradient of pressure is called the force density . Another example is a knife. If

12075-406: The time that it does D , and the time G will be twice the time E . And always, by so much as the medium is more incorporeal and less resistant and more easily divided, the faster will be the movement. Now there is no ratio in which the void is exceeded by body, as there is no ratio of 0 to a number. For if 4 exceeds 3 by 1, and 2 by more than 1, and 1 by still more than it exceeds 2, still there

12190-537: The two-dimensional analog of Boyle's law , πA = k , at constant temperature. Surface tension is another example of surface pressure, but with a reversed sign, because "tension" is the opposite to "pressure". In an ideal gas , molecules have no volume and do not interact. According to the ideal gas law , pressure varies linearly with temperature and quantity, and inversely with volume: p = n R T V , {\displaystyle p={\frac {nRT}{V}},} where: Real gases exhibit

12305-435: The unit 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. Pressure is the amount of force applied perpendicular to the surface of an object per unit area. The symbol for it

12420-416: The unit of pressure are preferred. 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

12535-430: The vacuum in the steam space of the condenser, that is, the exhaust of the last stage of the turbine. Mechanical or elastic gauges depend on a Bourdon tube, diaphragm, or capsule, usually made of metal, which will change shape in response to the pressure of the region in question. A variation on this idea is the capacitance manometer , in which the diaphragm makes up a part of a capacitor. A change in pressure leads to

12650-424: The void because it yields; but in a void this quality is present equally everywhere, so that things should move in all directions. Further, the truth of what we assert is plain from the following considerations. We see the same weight or body moving faster than another for two reasons, either because there is a difference in what it moves through, as between water, air, and earth, or because, other things being equal,

12765-418: The volume of the lungs to increase. This expansion reduces the pressure and creates a partial vacuum, which is soon filled by air pushed in by atmospheric pressure. Pressure 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. Gauge pressure (also spelled gage pressure) is the pressure relative to

12880-442: The vowel u . Historically, there has been much dispute over whether such a thing as a vacuum can exist. Ancient Greek philosophers debated the existence of a vacuum, or void, in the context of atomism , which posited void and atom as the fundamental explanatory elements of physics. Lucretius argued for the existence of vacuum in the first century BC and Hero of Alexandria tried unsuccessfully to create an artificial vacuum in

12995-463: The weight of the water above the person. As someone swims deeper, there is more water above the person and therefore greater pressure. The pressure a liquid exerts depends on its depth. Liquid pressure also depends on the density of the liquid. If someone was submerged in a liquid more dense than water, the pressure would be correspondingly greater. Thus, we can say that the depth, density and liquid pressure are directly proportionate. The pressure due to

13110-498: Was described by Arab engineer Al-Jazari in the 13th century, and later appeared in Europe from the 15th century. European scholars such as Roger Bacon , Blasius of Parma and Walter Burley in the 13th and 14th century focused considerable attention on issues concerning the concept of a vacuum. The commonly held view that nature abhorred a vacuum was called horror vacui . There was even speculation that even God could not create

13225-461: Was discarded. Later, in 1930, Paul Dirac proposed a model of the vacuum as an infinite sea of particles possessing negative energy, called the Dirac sea . This theory helped refine the predictions of his earlier formulated Dirac equation , and successfully predicted the existence of the positron , confirmed two years later. Werner Heisenberg 's uncertainty principle , formulated in 1927, predicted

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