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Pressure: the story on HearLore | HearLore
Pressure
The invisible weight of the atmosphere presses down on every square inch of your body with the force of a small car, yet you feel nothing because your internal pressure pushes back with equal strength. This constant, crushing force is what scientists call pressure, defined simply as the force applied perpendicular to a surface divided by the area over which that force is distributed. While the concept seems abstract, it governs everything from the air you breathe to the blood coursing through your veins. The symbol for this physical quantity is typically a lower-case p, though upper-case P is widely used in engineering and thermodynamics depending on the field of study. The International Union of Pure and Applied Chemistry recommends the lower-case p, but the choice often depends on nearby symbols like power or momentum to avoid confusion. Pressure is a scalar quantity, meaning it has magnitude but no direction, a property that distinguishes it from vector forces. When a gas molecule collides with the wall of a container, it exerts a force, but the pressure itself is the scalar proportionality constant that relates the vector area element to the normal force acting on it. This distinction is crucial because while the force has a specific direction, the pressure remains the same regardless of how the surface is oriented. The minus sign in the mathematical equation arises from the convention that the force is considered towards the surface element, while the normal vector points outward. This fundamental parameter in thermodynamics is conjugate to volume and is defined as the derivative of the internal energy of a system at fixed entropy and particle number. Without understanding this invisible weight, the behavior of fluids, the stability of structures, and the very atmosphere surrounding the planet would remain a mystery.
The Battle of Units
The battle of units for measuring pressure raged for centuries before the pascal became the standard, a struggle that still echoes in the diverse systems used today. The SI unit for pressure is the pascal, named after Blaise Pascal, and it was officially added in 1971 to replace the less specific newton per square metre. One pascal is equal to one newton per square metre, or one kilogram per meter per second squared, but this unit is often too small for practical use in engineering. Consequently, the bar and the atmosphere remain popular, with the standard atmosphere defined as approximately 101,325 pascals, representing the typical air pressure at Earth's mean sea level. Before 1982, the value for the standard atmosphere was often used as 101,325 pascals, but the International Union of Pure and Applied Chemistry now recommends 100,000 pascals for the standard atmosphere, creating a subtle divergence in scientific definitions. In the imperial and US customary systems, the pound-force per square inch, or psi, remains the traditional unit, often written as lbf/in2. Meteorologists prefer the hectopascal, which is equivalent to the older unit millibar, while oceanographers measure underwater pressure in decibars because pressure increases by approximately one decibar per metre of depth. The CGS unit of pressure is the barye, equal to one dyne per square centimetre, or 0.1 pascals, though it is rarely used outside of historical contexts. Manometric units such as the centimetre of water, millimetre of mercury, and inch of mercury are still used to express pressures in terms of the height of a column of a particular fluid in a manometer. The most common choices are mercury and water, with mercury's high density allowing a shorter column to be used to measure a given pressure. When millimetres of mercury are quoted today, these units are not based on a physical column of mercury but have been given precise definitions that can be expressed in terms of SI units. One millimetre of mercury is approximately equal to one torr, a unit named after Evangelista Torricelli. The technical atmosphere, symbolized as at, is 1 kilogram-force per square centimetre, which equals 98.0665 kilopascals or 14.223 psi. The confusion is compounded by the fact that using the names kilogram, gram, kilogram-force, or gram-force as units of force is deprecated in SI, yet these terms persist in technical work. The US National Institute of Standards and Technology recommends that modifiers be applied to the quantity being measured rather than the unit of measure, suggesting that 100 kPa (gauge) is preferred over 100 kPag. Despite these efforts, the inch of mercury is still used in the United States, and units like the metre sea water are essential for underwater divers to measure pressure exposure in diving chambers and personal decompression computers.
Pressure is defined as the force applied perpendicular to a surface divided by the area over which that force is distributed. It is a scalar quantity that has magnitude but no direction, distinguishing it from vector forces.
When was the pascal officially adopted as the SI unit for pressure?
The pascal was officially added to the SI system in 1971 to replace the less specific newton per square metre. One pascal equals one newton per square metre or one kilogram per meter per second squared.
Who discovered Pascal's Law and when did this discovery occur?
Blaise Pascal discovered Pascal's Law in the 17th century. This principle states that when a liquid is confined, the pressure at any given point is the same in all directions.
What is the standard atmosphere pressure value recommended by the International Union of Pure and Applied Chemistry?
The International Union of Pure and Applied Chemistry now recommends 100,000 pascals for the standard atmosphere. Before 1982, the value was often used as 101,325 pascals, creating a subtle divergence in scientific definitions.
How does negative pressure function in the context of liquid mercury and plant sap?
Liquid mercury has been observed to sustain up to negative 140 megapascals in clean glass containers. Negative liquid pressures are thought to be involved in the ascent of sap in plants taller than 10 metres.
The fluid's secret lies in its ability to transmit pressure equally in all directions, a phenomenon that allows a small force to lift a heavy car or drive a massive hydraulic press. This principle, known as Pascal's Law, was discovered by Blaise Pascal in the 17th century and forms the foundation of fluid statics. When a liquid is confined, the pressure at any given point is the same in all directions, meaning that if you submerge your head in water, you feel the same pressure on your ears regardless of how you tilt your head. This is because a liquid can flow, and the pressure is not only downward but also acts sideways and upward. The pressure exerted by a column of liquid of height h and density rho is given by the hydrostatic pressure equation, where g is the gravitational acceleration. 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. The pressure in a liquid of uniform density is represented by the formula p equals rho times g times h, where p is liquid pressure, rho is density of the liquid, g is the acceleration due to gravity, and h is depth within the liquid. The pressure does not depend on the amount of liquid present; volume is not the important factor, depth is. A wide but shallow lake with a depth of 10 metres exerts only half the average pressure that a small 20-metre deep pond does. The average water pressure acting against a dam depends on the average depth of the water and not on the volume of water held back. If four interconnected vases contain different amounts of water but are all filled to equal depths, then a fish with its head dunked a few centimetres under the surface will be acted on by water pressure that is the same in any of the vases. If the fish swims a few centimetres deeper, the pressure on the fish will increase with depth and be the same no matter which vase the fish is in. If water pressure at the bottom of a vase were greater than water pressure at the bottom of a neighboring vase, the greater pressure would force water sideways and then up the neighboring vase to a higher level until the pressures at the bottom were equalized. This is why water seeks its own level. The pressure does not depend on the amount of liquid present, but rather on the depth and density of the liquid. The energy per unit volume in an ideal, incompressible liquid is constant throughout its vessel, and the two energy components change linearly with the depth so the sum of pressure and gravitational potential energy per unit volume is constant throughout the volume of the fluid. This principle is described by Bernoulli's equation, where velocity head is zero and comparisons per unit volume in the vessel are made. The pressure in a liquid also depends on the density of the liquid. If someone was submerged in a liquid more dense than water, the pressure would be correspondingly greater. Thus, we can say that the depth, density, and liquid pressure are directly proportionate. Liquids are practically incompressible, meaning their volume can hardly be changed by pressure. Water volume decreases by only 50 millionths of its original volume for each atmospheric increase in pressure. The pressure in a liquid of uniform density is represented by the formula p equals rho times g times h, where p is liquid pressure, rho is density of the liquid, g is the acceleration due to gravity, and h is depth within the liquid. The pressure does not depend on the amount of liquid present, but rather on the depth and density of the liquid. The energy per unit volume in an ideal, incompressible liquid is constant throughout its vessel, and the two energy components change linearly with the depth so the sum of pressure and gravitational potential energy per unit volume is constant throughout the volume of the fluid. This principle is described by Bernoulli's equation, where velocity head is zero and comparisons per unit volume in the vessel are made.
The Negative Force
The negative force, or tension, is a rare and fragile state where bulk solids and liquids are pulled apart, creating a pressure that is effectively negative. While pressures are generally positive, there are several situations in which negative pressures may be encountered, particularly when dealing in relative gauge pressures. An absolute pressure of 80 kilopascals may be described as a gauge pressure of negative 21 kilopascals, which is 21 kilopascals below an atmospheric pressure of 101.325 kilopascals. Negative absolute pressures are possible and are effectively tension, and both bulk solids and bulk liquids can be put under negative absolute pressure by pulling on them. Microscopically, the molecules in solids and liquids have attractive interactions that overpower the thermal kinetic energy, so some tension can be sustained. Thermodynamically, however, a bulk material under negative pressure is in a metastable state, and it is especially fragile in the case of liquids where the negative pressure state is similar to superheating and is easily susceptible to cavitation. In certain situations, the cavitation can be avoided and negative pressures sustained indefinitely, for example, liquid mercury has been observed to sustain up to negative 140 megapascals in clean glass containers. Negative liquid pressures are thought to be involved in the ascent of sap in plants taller than 10 metres, where the atmospheric pressure head of water is insufficient to explain the height. The Casimir effect can create a small attractive force due to interactions with vacuum energy, and this force is sometimes termed vacuum pressure, though it is not to be confused with the negative gauge pressure of a vacuum. For non-isotropic stresses in rigid bodies, depending on how the orientation of a surface is chosen, the same distribution of forces may have a component of positive stress along one surface normal, with a component of negative stress acting along another surface normal. The pressure is then defined as the average of the three principal stresses. The stresses in an electromagnetic field are generally non-isotropic, with the stress normal to one surface element being negative, and positive for surface elements perpendicular to this. In cosmology, dark energy creates a very small yet cosmically significant amount of negative pressure, which accelerates the expansion of the universe. This negative pressure is a key component of the stress-energy tensor in general relativity, where pressure increases the strength of a gravitational field and so adds to the mass-energy cause of gravity. This effect is unnoticeable at everyday pressures but is significant in neutron stars, although it has not been experimentally tested. The existence of negative pressure challenges our intuitive understanding of force and suggests that the universe is driven by forces that are not immediately apparent in our daily lives. The study of negative pressure has led to new insights into the behavior of materials and the fundamental forces that govern the cosmos.
The Pressure of Life
The pressure of life is measured in millimetres of mercury, a unit that has persisted for centuries despite the advent of the pascal, and it is the standard for measuring blood pressure in most of the world. Blood pressure is measured in millimetres of mercury in most of the world, and lung pressures in centimetres of water are still common. The pressure exerted by a column of liquid of height h and density rho is given by the hydrostatic pressure equation, where g is the gravitational acceleration. 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 are quoted today, these units are not based on a physical column of mercury but have been given precise definitions that can be expressed in terms of SI units. One millimetre of mercury is approximately equal to one torr, a unit named after Evangelista Torricelli. The pressure in a liquid of uniform density is represented by the formula p equals rho times g times h, where p is liquid pressure, rho is density of the liquid, g is the acceleration due to gravity, and h is depth within the liquid. The pressure does not depend on the amount of liquid present, but rather on the depth and density of the liquid. The energy per unit volume in an ideal, incompressible liquid is constant throughout its vessel, and the two energy components change linearly with the depth so the sum of pressure and gravitational potential energy per unit volume is constant throughout the volume of the fluid. This principle is described by Bernoulli's equation, where velocity head is zero and comparisons per unit volume in the vessel are made. The pressure in a liquid also depends on the density of the liquid. If someone was submerged in a liquid more dense than water, the pressure would be correspondingly greater. Thus, we can say that the depth, density, and liquid pressure are directly proportionate. Liquids are practically incompressible, meaning their volume can hardly be changed by pressure. Water volume decreases by only 50 millionths of its original volume for each atmospheric increase in pressure. The pressure in a liquid of uniform density is represented by the formula p equals rho times g times h, where p is liquid pressure, rho is density of the liquid, g is the acceleration due to gravity, and h is depth within the liquid. The pressure does not depend on the amount of liquid present, but rather on the depth and density of the liquid. The energy per unit volume in an ideal, incompressible liquid is constant throughout its vessel, and the two energy components change linearly with the depth so the sum of pressure and gravitational potential energy per unit volume is constant throughout the volume of the fluid. This principle is described by Bernoulli's equation, where velocity head is zero and comparisons per unit volume in the vessel are made. The pressure in a liquid also depends on the density of the liquid. If someone was submerged in a liquid more dense than water, the pressure would be correspondingly greater. Thus, we can say that the depth, density, and liquid pressure are directly proportionate. Liquids are practically incompressible, meaning their volume can hardly be changed by pressure. Water volume decreases by only 50 millionths of its original volume for each atmospheric increase in pressure.