PRESSURE

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.

KEY TERMS

ATMOSPHERE:

A measure of pressure, abbreviated "atm" and equal to the average pressure exerted by air at sea level. In English units, this is equal to 14.7 pounds per square inch, and in SI units to 101,300 pascals.

BAROMETER:

An instrument form easuring atmospheric pressure.

BUOYANCY:

The tendency of an objectimmersed in a fluid to float.

FLUID:

Any substance, whether gas or liquid, that conforms to the shape of itscontainer.

FLUID MECHANICS:

The study of the behavior of gases and liquids at rest and in motion.

HYDROSTATIC PRESSURE:

the pressure that exists at any place in a body of fluid due to the weight of the fluid above.

PASCAL:

The principle SI or metricunit of pressure, abbreviated "Pa" and equal to 1 N/m².

PASCAL'S PRINCIPLE:

A statement, formulated by French mathematician and physicist Blaise Pascal (1623-1662), which holds that the external pressure applied on a fluid is transmitted uniformly throughout the entire body of that fluid.

PRESSURE:

The ratio of force to surface area, when force is applied in a direction perpendicular to that surface. The formula for pressure (p ) is p = F /A, where F is force and A the surface area.

Pressure

History of measuring devices

The kinetic molecular theory of gases and pressure

Atmospheric pressure and common measuring units for pressure

Pressure in liquids

Pressure is a physical force exerted over a surface. Specifically, pressure (p) is defined as force (F) divided by unit area (A): p = F/A. When expressed relative to a pure vacuum, this quotient is referred to as absolute pressure. When referred to relative to atmospheric pressure it is called gauge pressure. Pressure is an important concept in many chemical and physical processes. For example, many types of reactor vessels and boilers must be monitored to ensure they do not exceed specific pressure limits, lest they rupture. In atmospheric science, an understanding of air pressure, also known as barometric pressure, is critical for making weather predictions. Acrobats and cheerleaders sometimes stand on each other’s shoulders to form a human tower. Even with perfect balance, there is a limit to how high such a tower can be built. Ultimately, the ability of the bottom person to bear the pressure, caused by the weight of all the people stacked above, is the limiting factor. Pressure, then, is the amount of force applied on a given area.

In this last example, increasing the number of people in the tower increases the amount of force applied to the shoulder area, which in turn causes the bottom person to be under greater pressure. However, pressure can also be increased without changing the amount of applied force. If the person standing directly above were to stand on one foot, thereby shifting all the weight onto a smaller area, the bottom person would feel increased pressure on that burdened shoulder.

Turning a nail upside down and driving its large, flat head through the wood by hammering its point, is a more difficult task than conventional nailing. Even if you were able to hammer the point with the same force, the flat head of the nail would spread this force over a relatively large surface area. As a result, there might not be enough pressure on the surface of the wood to cause penetration.

A force exerted over a small area causes more pressure than the same force applied over a large area. This principle explains why karate experts use the side of the hand when breaking a board, instead of the full palm which has more surface and would apply less pressure to the wood.

Similarly, a force exerted over a large area causes less pressure than the same force applied over a small area. This explains why it is possible to walk on top of deep snow with large, flat snowshoes when ordinary boots would cause a person to sink.

History of measuring devices

Historically, pressure has been measured with gauges that monitor the displacement of a mechanical element. One of the earliest measuring tools was a liquid-filled tube known as a manometer. Manometers were commonly used by scientists in the 1700 sand 1800s and are still used today to calibrate other pressure gauges. A manometer consists of a cylindrical glass U-shaped tube that is partially filled with liquid. One end of the tube is exposed to the process that is generating pressure; the other end may be sealed or left open depending on the design of the instrument. The pressure exerts a force on the surface of the liquid and causes it to move a specific distance that is proportional to the magnitude of the force. The pressure differential can be measured by comparing the height of the liquid to calibrated marks on the side of the tube. Another type of manometer is the inverted bell-type instrument that consists of two inverted U-shaped tubes, or bells, mounted on a balance beam. Each of these bells is located over a pressure inlet tube in such a way that the bell that is subjected to the highest pressure will rise. This type of apparatus is used to measure very small pressure changes such as those found in drying appliances like conveyor dryers and kilns.

Another type of pressure-sensing element is the Bourdon tube, which is a flexible tube made from steel, beryllium, copper, and other special alloys. The tube, which is flattened slightly and sealed on one end, is formed into a coil. When the open end is exposed to pressure, the tube uncoils to a degree that corresponds to the magnitude of the pressure. Positive pressure causes the tube to expand and a vacuum (negative pressure as compared to air pressure) causes it to contract. As the coil moves, it displaces a needle or other indicator that points along a pre-calibrated scale to give the pressure measurement. Bourdon tubes have the advantage of being low cost, simply designed, and useful for measuring both high and low pressure. They also have several disadvantages: they are not extremely accurate at low pressures, the mechanical linkage to the indicator needle may require amplification to give a meaningful reading, and process materials may accumulate inside the tube since one end is sealed.

Other important mechanical pressure sensors include the bellows and diaphragm type elements. The bellows employs a coiled spring instead of a coiled tube as the responsive element. When exposed to pressure, the spring stretches and moves a needle on a measuring gauge. The diaphragm is a flexible disk made of sheet metal with ridges carved into it. When the disk is exposed to pressure, it deforms to one side or the other, depending on which side is exposed to the higher pressure. The disks are very sensitive to small changes in pressure and therefore are useful in low pressure measurements as low as 13 Pa (0.1 torr).

Today, mechanical pressure devices have been supplemented with electrical sensors and transmitters. Certain devices, like piezoelectric crystals, may directly register pressure changes. These crystals operate on the principle that certain quartz compounds can be cut and aligned in such a way that they will trigger a small electric current when pressure is applied to them. Other electronic sensing devices depend on a mechanical sensor like a Bourdon tube or a diaphragm to register a change in pressure, and then they use electronic components to amplify and transmit the signal. The mechanical element transmits the pressure change to the magnetic coil of a transformer, which in turn translates the pressure to an electronic signal that can be relayed to a variety of output devices. Examples of this type of gauge include strain gauges, which measure the change in electrical resistance of a metal wire exposed to pressure via a mechanical sensor such as a bourdon tube or a bellows, and capacitive pressure transducers that register changes in capacitance when an elastic element is displaced by pressure.

The kinetic molecular theory of gases and pressure

According to the kinetic theory, gas, like all matter, is composed of many small, invisible particles that are in constant motion. In a child’s toy balloon, the amount of particle motion depends on the temperature of the gas trapped inside. The collision of the air particles with the walls of the balloon, accounts for the pressure.

Imagine a glass jar containing a few steel ball bearings. If one were to shake the jar, the steel balls would crash into the walls, and the sum of their forces would exert a pressure that might be enough to break the glass. Pressure depends on the total number of collisions and the intensity of the force with which each steel ball hits the glass. Both factors can be increased by shaking the jar more violently or in the case of the toy balloon, by increasing the temperature of the air trapped inside.

Atmospheric pressure and common measuring units for pressure

As humans living on the surface of Earth, people dwell at the bottom of an ocean of air. Each person supports on his or her shoulders the pressure caused by the weight of an air column that extends out to interstellar space.

Hold out the palm of a hand. Its area is approximately 20 sq in (120 sq cm) and the weight of the air resting upon it is nearly 300 lb (136 kg). Yet with all this weight, the hand is not crushed. This is because human bodies are used to living under such pressures. The liquids and solids inside the body grow to exert an equal pressure from the inside.

Air particles are constantly hitting every part of human bodies and the pressure they cause is known as atmospheric pressure. At high altitudes, such as one would find in places like Mexico City, Mexico, or Aspen, Colorado, there is less air above and, therefore, less atmospheric pressure. Breathing becomes more difficult, but throwing a baseball for distance is easier because there is less air resistance experienced by the moving baseball.

The barometer, invented by Italian physicist Evangelista Torricelli (1608–1647) in 1643, was the first instrument built to measure the pressure of the gases in Earth’s atmosphere. (The non-SI unit of pressure, torr, was named after Torricelli.) It consisted of a long glass tube closed at one end, filled with liquid mercury, and inverted into a dish of more mercury.

With this instrument, it has been observed that at sea level, atmospheric pressure can support the weight of about 760 mm of Hg (mercury). The exact figure depends on such things as weather conditions.

One standard atmosphere (1 atm) of pressure is the pressure exerted by a column of mercury that is 760 mm high at a temperature of 32°F (0°C). In the universe, pressure varies from very high pressures

KEY TERMS

Atmospheric pressure— Earth’s gravitational force pulls the surrounding air towards Earth. The force created by this action causes atmospheric pressure.

Kinetic molecular theory— The theory that explains the behavior of matter in terms of the motion of the particles that make it up.

Newton— The SI unit of force. One newton is roughly the force exerted by Earth on a 0.1 kg mass. This is about equal to the force exerted upward by the human hand when supporting a medium sized apple.

SI system— An abbreviation for Le Système International d’Unités, a system of weights and measures adopted in 1960 by the General Conference on Weights and Measures.

found the center of stars and other massive bodies to about one atmosphere on Earth’s surface and to approximately zero in the vacuum of outer space.

The pascal is the SI (International System of Units) unit of pressure. One pascal is equal to the force of one newton applied to a surface whose area is equal to one squared meter, 1.0 Pa = 1.0 N / m². One atmosphere of pressure is equal to approximately 101.3 KPa.

Pressure in liquids

According to the kinetic theory, liquids are also composed of many small particles, but in contrast to gases where the particles are very far apart, liquid particles are often touching.

Liquid water is much more dense than air, and one liter of it contains many more particles and much more mass than an equivalent volume of air. When a swimmer dives into a lake, the person can feel the pressure of the water above even if just a few meters below the surface because the body is supporting a lot of weight. Doubling the depth below the surface causes the pressure on the human body to also double.

Fill an empty juice can with water and put two holes down one side. Place one hole near the top of the can and one near the bottom. The water coming out of the bottom hole will shoot out much further than the water escaping from the hole near the top. This is because the water at the bottom of the can is supporting the weight of the water column above it and so it is under greater pressure.

Lou D’ Amore

Pressure

Pressure is the amount of force applied to a given area. Acrobats and cheerleaders sometimes stand on each other's shoulders to form a human tower. Even with perfect balance, there is a limit to how high such a tower can be built. Ultimately, the ability of the bottom person to bear the pressure, caused by the weight of all the people stacked above, is the limiting factor . Pressure, then, is the amount of force applied on a given area.

In this example, increasing the number of people in the tower increases the amount of force applied to the shoulder area, which in turn causes the bottom person to be under greater pressure. But pressure can also be increased without changing the amount of applied force. If the person standing directly above were to stand on one foot, thereby shifting all the weight onto a smaller area, the bottom person would feel increased pressure on that burdened shoulder.

Turning a nail upside down and driving its large, flat head through the wood by hammering its point, is a more difficult task than conventional nailing. Even if you were able to hammer the point with the same force, the flat head of the nail would spread this force over a relatively large surface area. As a result, there might not be enough pressure on the surface of the wood to cause penetration.

A force exerted over a small area causes more pressure than the same force applied over a large area. This principle explains why karate experts use the side of the hand when breaking a board, instead of the full palm which has more surface and would apply less pressure to the wood.

Similarly, a force exerted over a large area causes less pressure than the same force applied over a small area. This explains why it's possible to walk on top of deep snow with large, flat snowshoes when ordinary rubber boots would cause you to sink.

The kinetic molecular theory of gases and pressure

According to the kinetic theory, gas, like all matter, is composed of many small, invisible particles that are in constant motion . In a child's toy balloon , the amount of particle motion depends on the temperature of the gas trapped inside. The collision of the air particles with the walls of the balloon, accounts for the pressure.

Imagine a glass jar containing a few steel ball bearings. If you were to shake the jar, the steel balls would crash into the walls, and the sum of their forces would exert a pressure which might be enough to break the glass. Pressure depends on the total number of collisions and the intensity of the force with which each steel ball hits the glass. Both factors can be increased by shaking the jar more violently or in the case of the toy balloon, by increasing the temperature of the air trapped inside.

Atmospheric pressure and common measuring units for pressure

As humans living on the surface of the earth , we dwell at the bottom of an ocean of air. Each one of us supports on his or her shoulders the pressure caused by the weight of an air column that extends out to interstellar space.

Hold out the palm of your hand. Its area is approximately 20 in2 (129 cm2) and the weight of the air resting upon it is nearly 300 lb (136 kg). Yet with all this weight, your hand does not crush. This is because our bodies are used to living under such pressures. The liquids and solids inside your body grow to exert an equal pressure from the inside.

Air particles are constantly hitting every part of our bodies and the pressure they cause is known as atmospheric pressure . At high altitudes, such as you would find in places like Mexico City or Aspen, there is less air above you and therefore less atmospheric pressure. Breathing becomes more difficult, but throwing a baseball for distance is easier because there is less air resistance experienced by the moving baseball.

The barometer , invented by Evangelista Torricelli in 1643, was the first instrument built to measure the pressure of the gases in our atmosphere. It consisted of a long glass tube closed at one end, filled with liquid mercury, and inverted into a dish of more mercury.

With this instrument, it has been observed that at sea level , atmospheric pressure can support the weight of about 760 mm of Hg (mercury). The exact figure depends on such things as weather conditions.

One standard atmosphere (1 atm) of pressure is the pressure exerted by a column of mercury that is 760 mm high at a temperature of 32°F (0°C). In the Universe, pressure varies from about 1 atmosphere on the Earth's surface to approximately zero in the vacuum of outer space. Much higher pressures are found at the center of stars and other massive bodies.

The pascal is the SI unit of pressure. One pascal is equal to the force of one newton applied to a surface whose area is equal to one squared meter, 1.0 Pa = 1.0 N / m2. One atmosphere of pressure is equal to approximately 101.3 KPa.

Pressure in liquids

According to the kinetic theory, liquids are also composed of many small particles, but in contrast to gases where the particles are very far apart, liquid particles are often touching.

Liquid water is much more dense than air, and one liter of it contains many more particles and much more mass than an equivalent volume of air. When you dive into a lake , you can feel the pressure of the water above you even if you are just a few meters below the surface because your body is supporting a lot of weight. Doubling your depth below the surface causes the pressure on your body to also double.

Fill an empty juice can with water and put two holes down one side. Place one hole near the top of the can and one near the bottom. The water coming out of the bottom hole will shoot out much further than the water escaping from the hole near the top. This is because the water at the bottom of the can is supporting the weight of the water column above it and so it is under greater pressure.

Lou D'Amore

KEY TERMS

Atmospheric pressure

—Earth's gravitational force pulls the surrounding air towards Earth. The force created by this action causes atmospheric pressure.

Kinetic molecular theory

—The theory that explains the behavior of matter in terms of the motion of the particles that make it up.

Newton

—The SI unit of force. One newton is roughly the force exerted by the Earth on a 0.1 kg mass. This is about equal to the force exerted upward by your hand when supporting a medium sized apple.

SI system

—An abbreviation for Le Système International d'Unités, a system of weights and measures adopted in 1960 by the General Conference on Weights and Measures.

Pressure

Pressure is the amount of force applied to a given area. Pressure is expressed in units such as pounds per square inch in the English system or newtons per square meter in the metric system.

To understand the difference between force and pressure, consider a block of wood one foot on each side and weighing 40 pounds. The force exerted by that block of wood on a table top is equal to its weight: 40 pounds. But the pressure exerted by the wood is the force exerted on each square inch. Since the block of wood rests on an area of 144 square inches (12 inches by 12 inches), the pressure it exerts is 40 pounds ÷ 144 square inches = 0.28 pounds per square inch.

But now imagine that the same block of wood is cut apart and put back together in the shape of a pyramid. And imagine that a way can be found to balance that pyramid of wood on the table top on its point. Then, the pressure exerted by the wood block is quite different. Its weight remains the same, 40 pounds, but all of that weight rests on a single point. Imagine that the area of the point is 0.01 square inch. Then, the pressure exerted by the block is 40 pounds ÷ 0.01 square inch = 4,000 pounds per square inch.

Perhaps you can see why a sharp knife cuts better than a dull one, or why a nail has a sharp point rather than a flat one. The force exerted by the knife or the nail is focused on a small area, creating a large pressure and, therefore, a more effective cutting or driving force.

All forms of matter—solid, liquid, and gas—exert pressure. In the case of solids and liquids, that pressure is caused by the weight of an object and the area on which that weight acts. In the case of a gas, that pressure is caused by the motion of the gas particles. As gas particles travel through space, they collide with walls, table tops, ceilings, floors, and other objects. The collision of the gas particles against these objects causes gas pressure.

Atmospheric pressure

The form of pressure best known to most people is probably air pressure. Air exerts pressure, as do all gases, because of the movement of air particles and their collision with other objects. At sea level, this pressure has a value of approximately 760 millimeters of mercury (760 mm Hg). This unit may seem peculiar, but it represents the height of a column of mercury that can be held up by air pressure at sea level.

Atmospheric pressure is also measured in other units such as atmospheres, millibars, inches of mercury, and kilopascals. One atmosphere of pressure is equal to 760 mm Hg; 29.92 inches of mercury; 1013.25 millibars; or 101.3 kilopascals.

Atmospheric pressure depends on a number of factors, including altitude and weather conditions. In general, the higher one goes in the atmosphere, the lower the atmospheric pressure. Also, the greater the humidity (amount of moisture in the air), the lower the atmospheric pressure.

pres·sure / ˈpreshər/ • n. 1. the continuous physical force exerted on or against an object by something in contact with it: the slight extra pressure he applied to her hand. ∎  the force exerted per unit area: gas can be fed to the turbines at a pressure of around 250 psi.2. the use of persuasion, influence, or intimidation to make someone do something: the proposals put pressure on Britain to drop its demand| the many pressures on girls to worry about their looks. ∎  the influence or effect of someone or something: oil prices came under some downward pressure. ∎  the feeling of stressful urgency caused by the necessity of doing or achieving something, esp. with limited time: you need to be able to work under pressure and not get flustered | some offenders might find prison a refuge against the pressures of the outside world. • v. [tr.] attempt to persuade or coerce (someone) into doing something: it might now be possible to pressure him into resigning | [tr.] she pressured her son to accept a job offer from the bank.