Dictionary:

pressure

  (prĕsh'ər)

1. 1. The act of pressing.
   2. The condition of being pressed.
2. The application of continuous force by one body on another that it is touching; compression.
3. (Abbr. P) Physics. Force applied uniformly over a surface, measured as force per unit of area.
4. Meteorology. Atmospheric pressure.
5. A compelling or constraining influence, such as a moral force, on the mind or will: pressure to conform; peer-group pressure.
6. Urgent claim or demand: under the pressure of business; doesn't work well under pressure.
7. An oppressive condition of physical, mental, social, or economic distress.
8. A physical sensation produced by compression of a part of the body.
9. Archaic. A mark made by application of force or weight; an impression.

1. To force, as by overpowering influence or persuasion.
2. To pressurize.
3. To pressure-cook.

[Middle English, from Old French, from Latin pressūra, from pressus, past participle of premere, to press.]

Concept

Pressure is the ratio of force to the surface area over which it is exerted. Though solids exert pressure, the most interesting examples of pressure involve fluids—that is, gases and liquids—and in particular water and air. Pressure plays a number of important roles in daily life, among them its function in the operation of pumps and hydraulic presses. The maintenance of ordinary air pressure is essential to human health and well-being: the body is perfectly suited to the ordinary pressure of the atmosphere, and if that pressure is altered significantly, a person may experience harmful or even fatal side-effects.

How It Works

Force and Surface Area

When a force is applied perpendicular to a surface area, it exerts pressure on that surface equal to the ratio of F to A, where F is the force and A the surface area. Hence, the formula for pressure (p) is p = F/A. One interesting consequence of this ratio is the fact that pressure can increase or decrease without any change in force—in other words, if the surface becomes smaller, the pressure becomes larger, and vice versa.

If one cheerleader were holding another cheerleader on her shoulders, with the girl above standing on the shoulder blades of the girl below, the upper girl's feet would exert a certain pressure on the shoulders of the lower girl. This pressure would be equal to the upper girl's weight (F, which in this case is her mass multiplied by the downward acceleration due to gravity) divided by the surface area of her feet. Suppose, then, that the upper girl executes a challenging acrobatic move, bringing her left foot up to rest against her right knee, so that her right foot alone exerts the full force of her weight. Now the surface area on which the force is exerted has been reduced to half its magnitude, and thus the pressure on the lower girl's shoulder is twice as great.

For the same reason—that is, that reduction of surface area increases net pressure—a well-delivered karate chop is much more effective than an open-handed slap. If one were to slap a board squarely with one's palm, the only likely result would be a severe stinging pain on the hand. But if instead one delivered a blow to the board, with the hand held perpendicular—provided, of course, one were an expert in karate—the board could be split in two. In the first instance, the area of force exertion is large and the net pressure to the board relatively small, whereas in the case of the karate chop, the surface area is much smaller—and hence, the pressure is much larger.

Sometimes, a greater surface area is preferable. Thus, snowshoes are much more effective for walking in snow than ordinary shoes or boots. Ordinary footwear is not much larger than the surface of one's foot, perfectly appropriate for walking on pavement or grass. But with deep snow, this relatively small surface area increases the pressure on the snow, and causes one's feet to sink. The snowshoe, because it has a surface area significantly larger than that of a regular shoe, reduces the ratio of force to surface area and therefore, lowers the net pressure.

The same principle applies with snow skis and water skis. Like a snowshoe, a ski makes it possible for the skier to stay on the surface of the snow, but unlike a snowshoe, a ski is long and thin, thus enabling the skier to glide more effectively down a snow-covered hill. As for skiing on water, people who are experienced at this sport can ski barefoot, but it is tricky. Most beginners require water skis, which once again reduce the net pressure exerted by the skier's weight on the surface of the water.

Measuring Pressure

Pressure is measured by a number of units in the English and metric—or, as it is called in the scientific community, SI—systems. Because p = F/A, all units of pressure represent some ratio of force to surface area. The principle SI unit is called a pascal (Pa), or 1 N/m². A newton (N), the SI unit of force, is equal to the force required to accelerate 1 kilogram of mass at a rate of 1 meter per second squared. Thus, a Pascal is equal to the pressure of 1 newton over a surface area of 1 square meter.

In the English or British system, pressure is measured in terms of pounds per square inch, abbreviated as lbs./in². This is equal to 6.89 · 10³ Pa, or 6,890 Pa. Scientists—even those in the United States, where the British system of units prevails—prefer to use SI units. However, the British unit of pressure is a familiar part of an American driver's daily life, because tire pressure in the United States is usually reckoned in terms of pounds per square inch. (The recommended tire pressure for a mid-sized car is typically 30-35 lb/in².)

Another important measure of pressure is the atmosphere (atm), which the average pressure exerted by air at sea level. In English units, this is equal to 14.7 lbs./in², and in SI units to 1.013 · 10⁵ Pa—that is, 101,300 Pa. There are also two other specialized units of pressure measurement in the SI system: the bar, equal to 10⁵ Pa, and the torr, equal to 133 Pa. Meteorologists, scientists who study weather patterns, use the millibar (mb), which, as its name implies, is equal to 0.001 bars. At sea level, atmospheric pressure is approximately 1,013 mb.

The Barometer

The torr, once known as the "millimeter of mercury," is equal to the pressure required to raise a column of mercury (chemical symbol Hg) 1 mm. It is named for the Italian physicist Evangelista Torricelli (1608-1647), who invented the barometer, an instrument for measuring atmospheric pressure.

The barometer, constructed by Torricelli in 1643, consisted of a long glass tube filled with mercury. The tube was open at one end, and turned upside down into a dish containing more mercury: hence, the open end was submerged in mercury while the closed end at the top constituted a vacuum—that is, an area in which the pressure is much lower than 1 atm.

The pressure of the surrounding air pushed down on the surface of the mercury in the bowl, while the vacuum at the top of the tube provided an area of virtually no pressure, into which the mercury could rise. Thus, the height to which the mercury rose in the glass tube represented normal air pressure (that is, 1 atm.) Torricelli discovered that at standard atmospheric pressure, the column of mercury rose to 760 millimeters.

The value of 1 atm was thus established as equal to the pressure exerted on a column of mercury 760 mm high at a temperature of 0°C (32°F). Furthermore, Torricelli's invention eventually became a fixture both of scientific laboratories and of households. Since changes in atmospheric pressure have an effect on weather patterns, many home indoor-outdoor thermometers today also include a barometer.

Pressure and Fluids

In terms of physics, both gases and liquids are referred to as fluids—that is, substances that conform to the shape of their container. Air pressure and water pressure are thus specific subjects under the larger heading of "fluid pressure." A fluid responds to pressure quite differently than a solid does. The density of a solid makes it resistant to small applications of pressure, but if the pressure increases, it experiences tension and, ultimately, deformation. In the case of a fluid, however, stress causes it to flow rather than to deform.

There are three significant characteristics of the pressure exerted on fluids by a container. First of all, a fluid in a container experiencing no external motion exerts a force perpendicular to the walls of the container. Likewise, the container walls exert a force on the fluid, and in both cases, the force is always perpendicular to the walls.

In each of these three characteristics, it is assumed that the container is finite: in other words, the fluid has nowhere else to go. Hence, the second statement: the external pressure exerted on the fluid is transmitted uniformly. Note that the preceding statement was qualified by the term "external": the fluid itself exerts pressure whose force component is equal to its weight. Therefore, the fluid on the bottom has much greater pressure than the fluid on the top, due to the weight of the fluid above it.

Third, the pressure on any small surface of the fluid is the same, regardless of that surface's orientation. In other words, an area of fluid perpendicular to the container walls experiences the same pressure as one parallel or at an angle to the walls. This may seem to contradict the first principle, that the force is perpendicular to the walls of the container. In fact, force is a vector quantity, meaning that it has both magnitude and direction, whereas pressure is a scalar, meaning that it has magnitude but no specific direction.

Real-Life Applications

Pascal's Principle and the Hydraulic Press

The three characteristics of fluid pressure described above have a number of implications and applications, among them, what is known as Pascal's principle. Like the SI unit of pressure, Pascal's principle is named after Blaise Pascal (1623-1662), a French mathematician and physicist who formulated the second of the three statements: that the external pressure applied on a fluid is transmitted uniformly throughout the entire body of that fluid. Pascal's principle became the basis for one of the important machines ever developed, the hydraulic press.

A simple hydraulic press of the variety used to raise a car in an auto shop typically consists of two large cylinders side by side. Each cylinder contains a piston, and the cylinders are connected at the bottom by a channel containing fluid. Valves control flow between the two cylinders. When one applies force by pressing down the piston in one cylinder (the input cylinder), this yields a uniform pressure that causes output in the second cylinder, pushing up a piston that raises the car.

In accordance with Pascal's principle, the pressure throughout the hydraulic press is the same, and will always be equal to the ratio between force and pressure. As long as that ratio is the same, the values of F and A may vary. In the case of an auto-shop car jack, the input cylinder has a relatively small surface area, and thus, the amount of force that must be applied is relatively small as well. The output cylinder has a relatively large surface area, and therefore, exerts a relatively large force to lift the car. This, combined with the height differential between the two cylinders (discussed in the context of mechanical advantage elsewhere in this book), makes it possible to lift a heavy automobile with a relatively small amount of effort.

The Hydraulic Ram

The car jack is a simple model of the hydraulic press in operation, but in fact, Pascal's principle has many more applications. Among these is the hydraulic ram, used in machines ranging from bulldozers to the hydraulic lifts used by firefighters and utility workers to reach heights. In a hydraulic ram, however, the characteristics of the input and output cylinders are reversed from those of a car jack.

The input cylinder, called the master cylinder, has a large surface area, whereas the output cylinder (called the slave cylinder) has a small surface area. In addition—though again, this is a factor related to mechanical advantage rather than pressure, per se—the master cylinder is short, whereas the slave cylinder is tall. Owing to the larger surface area of the master cylinder compared to that of the slave cylinder, the hydraulic ram is not considered efficient in terms of mechanical advantage: in other words, the force input is much greater than the force output.

Nonetheless, the hydraulic ram is as well-suited to its purpose as a car jack. Whereas the jack is made for lifting a heavy automobile through a short vertical distance, the hydraulic ram carries a much lighter cargo (usually just one person) through a much greater vertical range—to the top of a tree or building, for instance.

Exploiting Pressure Differences

Pumps

A pump utilizes Pascal's principle, but instead of holding fluid in a single container, a pump allows the fluid to escape. Specifically, the pump utilizes a pressure difference, causing the fluid to move from an area of higher pressure to one of lower pressure. A very simple example of this is a siphon hose, used to draw petroleum from a car's gas tank. Sucking on one end of the hose creates an area of low pressure compared to the relatively high-pressure area of the gas tank. Eventually, the gasoline will come out of the low-pressure end of the hose. (And with luck, the person siphoning will be able to anticipate this, so that he does not get a mouthful of gasoline!)

The piston pump, more complex, but still fairly basic, consists of a vertical cylinder along which a piston rises and falls. Near the bottom of the cylinder are two valves, an inlet valve through which fluid flows into the cylinder, and an outlet valve through which fluid flows out of it. On the suction stroke, as the piston moves upward, the inlet valve opens and allows fluid to enter the cylinder. On the downstroke, the inlet valve closes while the outlet valve opens, and the pressure provided by the piston on the fluid forces it through the outlet valve.

One of the most obvious applications of the piston pump is in the engine of an automobile. In this case, of course, the fluid being pumped is gasoline, which pushes the pistons by providing a series of controlled explosions created by the spark plug's ignition of the gas. In another variety of piston pump—the kind used to inflate a basketball or a bicycle tire—air is the fluid being pumped. Then there is a pump for water, which pumps drinking water from the ground It may also be used to remove desirable water from an area where it is a hindrance, for instance, in the bottom of a boat.

Bernoulli's Principle

Though Pascal provided valuable understanding with regard to the use of pressure for performing work, the thinker who first formulated general principles regarding the relationship between fluids and pressure was the Swiss mathematician and physicist Daniel Bernoulli (1700-1782). Bernoulli is considered the father of fluid mechanics, the study of the behavior of gases and liquids at rest and in motion.

While conducting experiments with liquids, Bernoulli observed that when the diameter of a pipe is reduced, the water flows faster. This suggested to him that some force must be acting upon the water, a force that he reasoned must arise from differences in pressure. Specifically, the slower-moving fluid in the wider area of pipe had a greater pressure than the portion of the fluid moving through the narrower part of the pipe. As a result, he concluded that pressure and velocity are inversely related—in other words, as one increases, the other decreases.

Hence, he formulated Bernoulli's principle, which states that for all changes in movement, the sum of static and dynamic pressure in a fluid remain the same. A fluid at rest exerts static pressure, which is commonly meant by "pressure," as in "water pressure." As the fluid begins to move, however, a portion of the static pressure—proportional to the speed of the fluid—is converted to what is known as dynamic pressure, or the pressure of movement. In a cylindrical pipe, static pressure is exerted perpendicular to the surface of the container, whereas dynamic pressure is parallel to it.

According to Bernoulli's principle, the greater the velocity of flow in a fluid, the greater the dynamic pressure and the less the static pressure: in other words, slower-moving fluid exerts greater pressure than faster-moving fluid. The discovery of this principle ultimately made possible the development of the airplane.

As fluid moves from a wider pipe to a narrower one, the volume of that fluid that moves a given distance in a given time period does not change. But since the width of the narrower pipe is smaller, the fluid must move faster (that is, with greater dynamic pressure) in order to move the same amount of fluid the same distance in the same amount of time. One way to illustrate this is to observe the behavior of a river: in a wide, unconstricted region, it flows slowly, but if its flow is narrowed by canyon walls, then it speeds up dramatically.

Bernoulli's principle ultimately became the basis for the airfoil, the design of an airplane's wing when seen from the end. An airfoil is shaped like an asymmetrical teardrop laid on its side, with the "fat" end toward the airflow. As air hits the front of the airfoil, the airstream divides, part of it passing over the wing and part passing under. The upper surface of the airfoil is curved, however, whereas the lower surface is much straighter.

As a result, the air flowing over the top has a greater distance to cover than the air flowing under the wing. Since fluids have a tendency to compensate for all objects with which they come into contact, the air at the top will flow faster to meet with air at the bottom at the rear end of the wing. Faster airflow, as demonstrated by Bernoulli, indicates lower pressure, meaning that the pressure on the bottom of the wing keeps the airplane aloft.

Buoyancy and Pressure

One hundred and twenty years before the first successful airplane flight by the Wright brothers in 1903, another pair of brothers—the Mont-golfiers of France—developed another means of flight. This was the balloon, which relied on an entirely different principle to get off the ground: buoyancy, or the tendency of an object immersed in a fluid to float. As with Bernoulli's principle, however, the concept of buoyancy is related to pressure.

In the third century B.C., the Greek mathematician, physicist, and inventor Archimedes (c. 287-212 B.C.) discovered what came to be known as Archimedes's principle, which holds that the buoyant force of an object immersed in fluid is equal to the weight of the fluid displaced by the object. This is the reason why ships float: because the buoyant, or lifting, force of them is less than equal to the weight of the water they displace.

The hull of a ship is designed to displace or move a quantity of water whose weight is greater than that of the vessel itself. The weight of the displaced water—that is, its mass multiplied by the downward acceleration caused by gravity—is equal to the buoyant force that the ocean exerts on the ship. If the ship weighs less than the water it displaces, it will float; but if it weighs more, it will sink.

The factors involved in Archimedes's principle depend on density, gravity, and depth rather than pressure. However, the greater the depth within a fluid, the greater the pressure that pushes against an object immersed in the fluid. Moreover, the overall pressure at a given depth in a fluid is related in part to both density and gravity, components of buoyant force.

Pressure and Depth

The pressure that a fluid exerts on the bottom of its container is equal to dgh, where d is density, g the acceleration due to gravity, and h the depth of the container. For any portion of the fluid, h is equal to its depth within the container, meaning that the deeper one goes, the greater the pressure. Furthermore, the total pressure within the fluid is equal to dgh + p_external, where p_external is the pressure exerted on the surface of the fluid. In a piston-and-cylinder assembly, this pressure comes from the piston, but in water, the pressure comes from the atmosphere.

In this context, the ocean may be viewed as a type of "container." At its surface, the air exerts downward pressure equal to 1 atm. The density of the water itself is uniform, as is the downward acceleration due to gravity; the only variable, then, is h, or the distance below the surface. At the deepest reaches of the ocean, the pressure is incredibly great—far more than any human being could endure. This vast amount of pressure pushes upward, resisting the downward pressure of objects on its surface. At the same time, if a boat's weight is dispersed properly along its hull, the ship maximizes area and minimizes force, thus exerting a downward pressure on the surface of the water that is less than the upward pressure of the water itself. Hence, it floats.

Pressure and the Human Body

Air Pressure

The Montgolfiers used the principle of buoyancy not to float on the water, but to float in the sky with a craft lighter than air. The particulars of this achievement are discussed elsewhere, in the context of buoyancy; but the topic of lighter-than-air flight suggests another concept that has been alluded to several times throughout this essay: air pressure.

Just as water pressure is greatest at the bottom of the ocean, air pressure is greatest at the surface of the Earth—which, in fact, is at the bottom of an "ocean" of air. Both air and water pressure are examples of hydrostatic pressure—the pressure that exists at any place in a body of fluid due to the weight of the fluid above. In the case of air pressure, air is pulled downward by the force of Earth's gravitation, and air along the surface has greater pressure due to the weight (a function of gravity) of the air above it. At great heights above Earth's surface, however, the gravitational force is diminished, and, thus, the air pressure is much smaller.

In ordinary experience, a person's body is subjected to an impressive amount of pressure. Given the value of atmospheric pressure discussed earlier, if one holds out one's hand—assuming that the surface is about 20 in² (0.129 m²)—the force of the air resting on it is nearly 300 lb (136 kg)! How is it, then, that one's hand is not crushed by all this weight? The reason is that the human body itself is under pressure, and that the interior of the body exerts a pressure equal to that of the air.

The Response to Changes in Air Pressure

The human body is, in fact, suited to the normal air pressure of 1 atm, and if that external pressure is altered, the body undergoes changes that may be harmful or even fatal. A minor example of this is the "popping" in the ears that occurs when one drives through the mountains or rides in an airplane. With changes in altitude come changes in pressure, and thus, the pressure in the ears changes as well.

As noted earlier, at higher altitudes, the air pressure is diminished, which makes it harder to breathe. Because air is a gas, its molecules have a tendency to be non-attractive: in other words, when the pressure is low, they tend to move away from one another, and the result is that a person at a high altitude has difficulty getting enough air into his or her lungs. Runners competing in the 1968 Olympics at Mexico City, a town in the mountains, had to train in high-altitude environments so that they would be able to breathe during competition. For baseball teams competing in Denver, Colorado (known as "the Mile-High City"), this disadvantage in breathing is compensated by the fact that lowered pressure and resistance allows a baseball to move more easily through the air.

If a person is raised in such a high-altitude environment, of course, he or she becomes used to breathing under low air pressure conditions. In the Peruvian Andes, for instance, people spend their whole lives at a height more than twice as great as that of Denver, but a person from a low-altitude area should visit such a locale only after taking precautions. At extremely great heights, of course, no human can breathe: hence airplane cabins are pressurized. Most planes are equipped with oxygen masks, which fall from the ceiling if the interior of the cabin experiences a pressure drop. Without these masks, everyone in the cabin would die.

Blood Pressure

Another aspect of pressure and the human body is blood pressure. Just as 20/20 vision is ideal, doctors recommend a target blood pressure of "120 over 80"—but what does that mean? When a person's blood pressure is measured, an inflatable cuff is wrapped around the upper arm at the same level as the heart. At the same time, a stethoscope is placed along an artery in the lower arm to monitor the sound of the blood flow. The cuff is inflated to stop the blood flow, then the pressure is released until the blood just begins flowing again, producing a gurgling sound in the stethoscope.

The pressure required to stop the blood flow is known as the systolic pressure, which is equal to the maximum pressure produced by the heart. After the pressure on the cuff is reduced until the blood begins flowing normally—which is reflected by the cessation of the gurgling sound in the stethoscope—the pressure of the artery is measured again. This is the diastolic pressure, or the pressure that exists within the artery between strokes of the heart. For a healthy person, systolic pressure should be 120 torr, and diastolic pressure 80 torr.

Where to Learn More

"Atmospheric Pressure: The Force Exerted by the Weight of Air" (Web site). <http://kids.earth.nasa.gov/archive/air_pressure/> (April 7, 2001).

Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991.

"Blood Pressure" (Web site). <http://www.mckinley.uiuc.edu/health-info/dis-cond/bloodpr/bloodpr.html> (April 7, 2001).

Clark, John Owen Edward. The Atmosphere. New York: Gloucester Press, 1992.

Cobb, Allan B. Super Science Projects About Oceans. New York: Rosen, 2000.

"The Physics of Underwater Diving: Pressure Lesson" (Web site). <http://www.uncwil.edu/nurc/aquarius/lessons/pressure.html> (April 7, 2001).

Provenzo, Eugene F. and Asterie Baker Provenzo. 47 Easy-to-Do Classic Experiments. Illustrations by Peter A. Zorn, Jr. New York: Dover Publications, 1989.

"Understanding Air Pressure" USA Today (Web site). <http://www.usatoday.com/weather/wbarocx.html> (April 7, 2001).

Zubrowski, Bernie. Balloons: Building and Experimenting with Inflatable Toys. Illustrated by Roy Doty. New York: Morrow Junior Books, 1990.

The ratio of force to area. Atmospheric pressure at the surface of Earth is in the vicinity of 15 lbf/in.² (1.0 × 10⁵ Pa). Pressures in enclosed containers less than this value are spoken of as vacuum pressures; for example, the vacuum pressure inside a cathode-ray tube is 10⁻⁸ mmHg, meaning that the pressure is equal to the pressure that would be produced by a column of mercury, with no force acting above it, that is 10⁻⁸ mm high. This is absolute pressure measured above zero pressure as a reference level. Inside a steam boiler, the pressure may be 800 lbf/in.² (5.5 × 10⁶ Pa) or higher. Such pressure, measured above atmospheric pressure as a reference level, is gage pressure, designated psig. See also Pressure measurement.

noun

1. The act, condition, or effect of exerting force on someone or something: strain¹, stress, tension. See push/pull.
2. Power used to overcome resistance: coercion, compulsion, constraint, duress, force, strength, violence. See attack/defend.

verb

1. To cause (a person or thing) to act or move in spite of resistance: coerce, compel, constrain, force, make, obligate, oblige. See attack/defend.
2. To maintain normal air pressure in: pressurize. See push/pull.

Definition: demand, difficulty
Antonyms: ease, facility, peace

v

Definition: bother, urge
Antonyms: leave alone

A stress or strain that may occur by compression, pull, or thrust; an applied force.

For more information on pressure, visit Britannica.com.

The force per unit area exerted by a homogeneous liquid or gas on the walls of its container.

The force acting on a given area (pressure = force/area). Common units of pressure are newtons per square centimetre and Pascals. The ability to spread forces over a wide area and thereby reduce pressure has important applications to safety in sport. High jumpers, for example, spread their body forces by landing flat on their backs. In other sports, the use of equipment such as helmets also tends to spread forces and reduce pressure. Failure to minimize the pressure on any one part of the body can lead to serious injury.

The force exerted on a given area. (See atmospheric pressure.)

Stress or strain, by compression, expansion, pull, thrust or shear.

• arterial p. — the blood pressure in the arteries.
• atmospheric p. — the pressure exerted by the atmosphere, about 15 lb per square inch (2.17 kPa) at sea level.
• capillary p. — the blood pressure in the capillaries.
• central venous p. (CVP) — see central venous pressure.
• cerebrospinal p. — the pressure of the cerebrospinal fluid, normally 100 to 150 mmHg.
• diastolic p. — the lowest pressure recorded in the arterial blood pressure cycle. Represents the minimal pressure in the left ventricle which can maintain its ejection phase. See also blood pressure.
• p. gauge — a device attached to the outlet of gas tanks to measure internal pressure which indicates the quantity of gas remaining.
• p. gradient — the rate of increase (or decrease) in the magnitude of the pressure being measured.
• intracranial p. (ICP) — see intracranial pressure.
• intraocular p. (IOP) — the pressure exerted against the outer coats by the contents of the eyeball.
• p. load — see flowload.
• mean circulatory filling p. — a measure of the average (arterial and venous) pressure necessary to cause filling of the circulation with blood; it varies with blood volume and is directly proportional to the rate of venous return and thus to cardiac output.
• p. natriuresis — thought to participate in regulating the volume of extracellular fluid levels when the normal neurohumoral mediators are impaired; the increase in water and sodium ion excretions which occur when blood pressure is elevated because of an increase in the circulating blood volume.
• p. necrosis — necrosis of tissue caused by exclusion of circulation by external compression, e.g. in prolonged recumbency, or due to too-tight bandage, collar, harness.
• negative p. — pressure less than that of the atmosphere.
• oncotic p. — the osmotic pressure of a colloid in solution.
• osmotic p. — the potential pressure of a solution directly related to its solute osmolar concentration; it is the maximum pressure developed by osmosis in a solution separated from another by a semipermeable membrane, i.e. the pressure that will just prevent osmosis between two such solutions.
• p. point granuloma — see pressure points (below).
• p. point pyoderma — see pressure points (below).
• p. points — parts of the body subject to pressure when the animal is recumbent, wearing harness or saddlery, or during restraint. Usually bony prominences such as the point of the hock, hip, shoulder, elbow and lateral aspects of limbs. These are predisposed to callus formation, infection pyoderma and granulomas.
• positive p. — pressure greater than that of the atmosphere.
• pulse p. — difference between systolic and diastolic pressures in arteries.
• p. receptors — e.g. the blood pressure receptors in the aortic arch and the carotid sinus.
• p. sore — decubitus ulcer.
• systolic p. — the highest reading in the arterial blood pressure cycle. A reflection of the ejection pressure of left ventricular systole, and the elasticity of the arterial system.
• venous p. — the blood pressure in the veins. See also central venous pressure.
• wedge p. — intravascular pressure as measured by a swan–ganz catheter introduced into the pulmonary artery; it permits indirect measurement of the mean left atrial pressure.
• p. wrap — bandages which apply pressure to underlying tissues; used after trauma to limit the development of edema, and in the management of lymphedema.

Quotes:

"All pressure is self-inflicted. It's what you make of it or how you let it rub off on you." - Sebastian Coe

"Pressure is a word that is misused in our vocabulary. When you start thinking of pressure, it's because you've started to think of failure." - Tommy Lasorda

"The only pressure I'm under is the pressure I've put on myself." - Mark Messier

"I like added pressure. It makes me work harder." - Mary Lou Retton

"I'm careful not to give into theatrics when times are tough, I don't like it when somebody gives into outside pressure and puts on a show for others." - Tony La Russa

This article is about pressure in the physical sciences. For the psychological meaning, see Peer pressure. For the song, see Pressure (song).

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.

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

Pressure is the amount of force that presses on a certain area. If you make the force on an are bigger, you increase the pressure on the area. Making the area smaller and keeping the force the same also increases the pressure. Nigelleelee 08:24, 20 October 2007 (UTC)

Definition

Formulaic

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

Mathematically:

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.

(CHANGING PRESSURE)

High pressure can be a nuisance or an advantage. Wide tyres reduce pressure. They help to stop vehicles sinking into mud or snow. Walking in deep snow is made easier by wearing snow shoes. They spread a person's weight over a much bigger area than normal shoes. Pressure can be increased by making the area that the force presses on smaller. Nigelleelee 08:31, 20 October 2007 (UTC)

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.

(PRESSURE UNDER WATER)

The weight of an object pushes down on the surface it is resting on. This causes pressure on the surface. Imagine that object is replaced by a block of water. There would still be pressure. This is what happens under the water. The further down into water you go, the bigger the weight of water above you, and the greater the pressure is. The difference under water is that the pressure pushes down, up and sideways as well. In fact, it pushes in all directions. Nigelleelee 08:31, 20 October 2007 (UTC)

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). 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 some parts of the world. 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.

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 unit inch of mercury (inHg, see below) 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 metres depth of water is approximately equal to one atmosphere.

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 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. 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.

Presently or formerly popular pressure units 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, 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)

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.

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.

The gradient of pressure is called the force density. 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. 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. In non-SI technical work, a gauge pressure 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 that the first sample had twice the density of the second.

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.

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. 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 explosible gases, mists, dust/air suspensions, in unconfined and confined spaces.

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.

Hydrostatic pressure (head pressure)

Hydrostatic pressure is the pressure due to the weight of a fluid.

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). Other units can be used if the rest of the units used in the equation are defined in a consistent way.

See also Pascal's law.

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).

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. 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.

See also

• Atmospheric pressure
• Blood pressure
• Boyle's Law
• Combined gas law
• Conversion of units
• Units conversion by factor-label
• Ideal gas law
• Kinetic theory
• Partial pressure
• Sound pressure
• Microphone
• Timeline of temperature and pressure measurement technology
• Vacuum
• Vacuum pump

Notes

1. ^ The preferred spelling varies by country and even by industry. Further, both spellings are often used within a particular industry or country. Industries in British English-speaking countries typically use the "gauge" spelling. Many of the largest American manufacturers of pressure transducers and instrumentation use the spelling "gage pressure" in their most formal documentation (Honeywell-Sensotec’s FAQ page and Fluke Corporation’s product search page).
2. ^ NIST, Rules and Style Conventions for Expressing Values of Quantities, Sect. 7.4.

External links

• 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

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - pres, tryk, nød
v. tr. - presse, lægge pres på, fremskaffe overtryk

idioms:

• at high pressure    under højtryk
• high pressure    højtryk
• pressure cooker    trykkoger
• pressure gauge    trykmåler
• pressure group    pressionsgruppe
• pressure point    sted, hvor blødning kan standses ved kompression
• pressure suit    rumdragt med indbygget jordatmosfære

Nederlands (Dutch)
druk uitoefenen, koken in een hoge druk pan, druk, dwang

Français (French)
n. - (gén, Tech, Météo) pression, tension (artérielle), (fig) pression, flux
v. tr. - faire pression sur, pressuriser

idioms:

• at high pressure    tension, stress
• high pressure    haute pression
• pressure cooker    autocuiseur, cocotte-minute
• pressure gauge    manomètre, indicateur de pression
• pressure group    groupe de pression
• pressure point    point de compression
• pressure suit    combinaison pressurisée

Deutsch (German)
n. - Druck, Zwang
v. - unter Druck setzen

idioms:

• at high pressure    mit Hochdruck
• high pressure    Hochdruck
• pressure cooker    Schnellkochtopf
• pressure gauge    Druckluftmesser, Manometer
• pressure group    Pressure-group
• pressure point    Druckpunkt
• pressure suit    Druckanzug

Ελληνική (Greek)
n. - πίεση, σύνθλιψη, άσκηση πιέσεως
v. - πιέζω

idioms:

• at high pressure    εντατικά, πυρετωδώς
• high pressure    υψηλή πίεση
• pressure cooker    (μαγειρ.) χύτρα ταχύτητας
• pressure gauge    θλιβόμετρο, πιεσόμετρο, μανόμετρο
• pressure group    ομάδα πολιτικής πιέσεως
• pressure point    σημείο πάνω από αρτηρία όπου με πίεση μπορεί να ελεγχθεί αιμορραγία, σημείο άσκησης πίεσης
• pressure suit    στολή (αεροπόρου) υπό πίεση

Italiano (Italian)
far pressione su, costringere, costrizione, pressione

idioms:

• high pressure    alta pressione
• pressure cooker    pentola a pressione
• pressure gauge    manometro
• pressure group    gruppo di pressione, lobby
• pressure point    punto di compressione
• pressure suit    tuta pressurizzata

Português (Portuguese)
n. - pressão (f), constrangimento (m)
v. - pressionar

idioms:

• at high pressure    alta pressão
• high pressure    alta pressão
• pressure cooker    panela de pressão
• pressure gauge    manômetro
• pressure group    pressão social
• pressure point    ponto de pressão
• pressure suit    escafandro

Русский (Russian)
оказывать давление на, давление, воздействие, прессование

idioms:

• at high pressure    очень напряженно
• high pressure    высокое давление
• pressure cooker    скороварка
• pressure gauge    манометр
• pressure group    группа воздействия
• pressure point    точка на кот. нужно давить
• pressure suit    высотный компенсирующий костюм

Español (Spanish)
n. - presión, fuerza, tensión, apremio
v. tr. - presionar, ejercer presión sobre

idioms:

• at high pressure    a alta presión
• high pressure    alta presión
• pressure cooker    olla a presión
• pressure gauge    manómetro
• pressure group    grupo de presión
• pressure point    punto de compresión
• pressure suit    traje de presión compensada, vestido presurizado

Svenska (Swedish)
n. - tryck(ning), tryckande, pressning, påtryckning, betryck, trångmål, brådska
v. - utöva påtryckningar, pressa, tvinga

中文（简体） (Chinese (Simplified))
压, 按, 榨, 对...施加压力, 迫使

idioms:

• at high pressure    紧张地, 高压之下
• high pressure    高气压, 高度紧张, 高压
• pressure cooker    高压锅
• pressure gauge    压力计
• pressure group    压力集团
• pressure point    压点, 加压止血点, 受压点
• pressure suit    增压服

中文（繁體） (Chinese (Traditional))
n. - 壓, 按, �