Pressure

Pressure is the force exerted by a solid, liquid, or gas on its container or another object. Pressure is typically measured in pascals (Pa) or in atmospheres (atm). A pascal is equal to one newton (1 kg·m/s²) applied over an area of one meter. One atmosphere is the amount of atmospheric pressure exerted on an object at sea level and is equal to 101,325 pascals.

The concept of pressure can be expressed in either a scalar or a vector quality. Scalar measurements only measure force, whereas vector measurements include both force and direction. Pressure is the scalar quality, whereas stress is the vector quality. The equation for pressure is P= F/A, where P is pressure, F is force, and A is the area upon which the force is applied.

Background

The earliest known experiments with pressure involved buoyancy and date back to ancient Greece. The mathematician and scientist Archimedes (ca. 287–212 BCE) was the first to write about the use of pressure in experiments, and Archimedes’s principle discusses the effects and implications of pressure displacing a liquid. This displacement effect can be used to determine the density and volume of an irregularly shaped object.

Two English scientists, astronomer Richard Towneley (1629–1707) and physician Henry Power (1626–68), mathematically determined the relationship between pressure and volume in 1660 and 1661. Drawing on their work, Irish chemist and physicist Robert Boyle conducted experiments in pressure that led to the formulation of Boyle’s law of pressure in 1662. Using mercury and air in a series of connected tubes and beakers, Boyle found that the relationship between pressure and volume is such that the two quantities will multiply to equal a constant value. Boyle’s law can be expressed as either PV = k or P₁V₁ = P₂V₂ , where P is the pressure exerted, V is the volume of the system, and k is a known constant.

Independently of Boyle, French physicist Edme Mariotte (ca. 1620–84) achieved the same results using similar experiments. Although Mariotte did not deduce Boyle’s law until 1676, the law is sometimes referred to as Boyle-Mariotte’s law.

In 1738, Swiss physicist and mathematician Daniel Bernoulli (1700–1782) calculated the relationship between pressure, gases, and the environment. This resulted in the development of Bernoulli’s principle, which is the basis of the theory of flight. The principle states that when pressure underneath the wing is higher than the pressure above the wing, lift force is generated, allowing the aircraft to take off.

In 1802, French chemist and physicist Joseph Louis Gay-Lussac (1778–1850) drew on the earlier work of fellow French scientists Jacques Alexandre César Charles (1746–1823) and Guillaume Amontons (1663–1705) in his definition of the relationship between pressure and temperature. Neither scientist had accurate enough equipment to fully explain their results, so the equation was theoretical until more precise thermometers could be produced. The law of this relationship is named after Gay-Lussac, although it is also referred to as Charles’s law. Gay-Lussac’s law is expressed by the equations P/T = k and P₁T₂ = P₂T₁ , where P is the pressure exerted, T is the temperature of the system, and k is a known constant.

Pressure Today

In the twenty-first century, pressure is an integral part of natural and industrial processes. For example, pressure is used to make both natural and synthetic diamonds and other jewels. An understanding of pressure is critical in numerous other activities and events, including scuba diving, flight, the use of explosives, and osmosis.

According to Gay-Lussac’s law, as pressure increases, so too does temperature, if mass and volume remain constant. This law is the principle behind the creation of synthetic gems such as diamonds. It is possible to turn carbon into diamonds or graphite through intense heat and pressure. Man-made forges are capable of producing the high pressure and temperatures that are needed to make this conversion. While the size and quality of such gems often make them unsuitable for jewelry or metallurgical purposes, the diamonds produced are frequently used in mining and tool production.

Scuba divers must understand the role Boyle’s law plays in diving. As a diver descends, increasing amounts of pressure will be exerted on him or her by the water. As the diver goes deeper, the air in his or her tanks will become denser, causing less air to be available for breathing. The same process also occurs in the diver’s bloodstream. If a diver swims to the surface too quickly, gases will build up in his or her blood, causing a medical condition known as decompression sickness or the bends. Decompression sickness occurs when gases in an individual’s bloodstream come out of solution as a result of depressurization. The condition is very painful and dangerous and is treated with a decompression chamber.

Flight is possible because of pressure. An aircraft is able to obtain flight during takeoff as a result of the air pressure above the wings being lower than the air pressure beneath them. This is an application of Bernoulli’s principle. While the theory was developed in the early eighteenth century, it was not until 1903 that American inventors Orville (1871–1948) and Wilbur Wright (1867–1912) successfully completed the first airplane flight in Kitty Hawk, North Carolina.

Explosives cause damage with a combination of shrapnel, heat, and pressure. Shrapnel are fragments that can cause damage to an object upon impact. The pressure comes from the waves of energy that emanate from the explosion. These waves, known as shock waves, can cause internal damage to a body as well as physical damage to structures and other surroundings.

Osmosis is a natural process by which molecules of a solvent filter through a membrane from a solution of lower solute concentration to one of higher concentration, diluting the more concentrated solution so that the two concentrations become equal. This process was observed by French physicist Jean-Antoine Nollet (1700–1770) in 1748. Reverse osmosis is a water-filtration technique developed in the mid-twentieth century that uses pressure to force impurities from the water. This technique is used throughout the world, most commonly to clean sewage water (in conjunction with chemical treatment) and for the purpose of desalination, which is the removal of salt from seawater to make it potable.

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