Unit of Pressure - Unit, Types, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 04:42 PM IST

Pressure:

Pressure is defined as the force applied perpendicular to an object's surface per unit area across which that force is diffused.
P=F/A
Where, the pressure is P, the normal force is F, and the area of the surface on contact is A.
The pressure gauge in relation to the ambient pressure is known as gauge pressure. Pressure is measured in a variety of ways. Some of these are derived from a unit of force divided by a unit of area; for example, the SI unit of pressure, the pascal(Pa), is one newton per square metre(N/m2); similarly, the imperial and US customary systems' conventional unit of pressure is the pound-force per square inch (psi).

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Pressure:
Unit of pressure:
Types of pressure:

Unit of Pressure - Unit, Types, FAQs

The amount of force delivered at right angles to an object's surface per unit area is known as pressure. P is the symbol for it. The IUPAC recommends a lower-case p for pressure. Upper-case P, on the other hand, is extensively used. The use of P vs p is determined by the field in which one is employed, the presence of additional symbols denoting quantities such as power and momentum in the direct proximity, and writing style.

The quantity of pressure is a scalar quantity. It connects the normal force operating on the surface to the vector area element. The scalar proportionality constant that connects the two normal vectors is called pressure.

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Unit of pressure:

The unknown quantities are compared to a unit, which is a standard quantity. A specified magnitude of a physical quantity that has been adopted by law or convention is defined. For example, pascal (Pa) is the unit for pressure.

The pascal (Pa) is a SI unit or MKS unit for pressure, equivalent to one newton per square metre (N/m2, or kgms2). Pressure in SI was repeatedly expressed in newton per square metre. Other pressure units are frequently used, such as pounds per square inch (lbf/in2) and a bar.

The CGS unit of pressure is equal to 1 dyn.cm-2 or 0.1 Pa. However, with SI, using the term kilogram, gram, kilogram-force, or gram-force (or their symbols) as units of force is specifically prohibited.

The system which is a logical set of units based on the fundamental units of centimetre, gram, and second for distance, mass, and time is the CGS system. The MKS system, which was based on the fundamental units of metre for distance, kilogram for mass, and second for time, subsequently substituted it.

As a result, it is associated with energy density and can be expressed in joules per cubic metre. Pressure is a measure of potential energy conserved per unit volume because a system under pressure has the ability to work on its surroundings.

Atmospheric pressure:

The pressure within Earth's atmosphere, often known as atmospheric pressure or barometric pressure. The hydrostatic pressure induced by the weight of air above the measurement location roughly approximates atmospheric pressure. Because there is less overlaying atmospheric mass as height rises, atmospheric pressure falls as elevation rises.

The force exerted on the atmospheric gases above the surface by the planet's gravitational influence is a function of the planet's mass, the radius of the surface, the volume and composition of the gases, as well as their vertical distribution in the atmosphere.The planetary rotation, as well as local elements such as wind velocity, temperature-induced density variations, and composition changes, have an impact on it.

The force produced by the weight of the air molecules is referred to as air pressure. Even the tiniest air molecules have mass, and the huge amounts of air molecules that make up the layers of our atmosphere have a lot of mass together.

Temperature affects air pressure because as the air warms, the molecules begin to move around more, causing them to collide more frequently and create higher pressure. However, air pressure has an impact on temperature because the more those molecules collide, the more heat is generated. The application of atmospheric pressure is that, when a man sucks fluid from a straw, for example, the pressure within is moderately low, and the atmospheric pressure outside drives up the fluid into the straw. When a vacuum cleaner is powered on, the pressure inside plunges off due to air inside.

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Unit of atmospheric pressure:

The pascal (Pa) is the SI unit of pressure, which is defined as a force of one Newton per square metre. The calculation is as follows for converting between atm, Pa, and torr, 101325 Pa =760 torr=1 atm. It's sometimes referred to as a reference or standard pressure. It is almost equal to the atmospheric pressure at sea level on Earth.

Measurement of the pressure and the atmospheric pressure:

Pressure metres, pressure gauges, and vacuum gauges are instruments that measure and show pressure in an integral unit. A manometer is a good example, as it measures and indicates pressure by using the surface area and weight of a column of liquid. A barometer, typically known as barometric pressure, is a scientific device used to measure atmospheric pressure.

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Types of pressure:

Fluid pressure:

The compressive tension at a point within a fluid is commonly referred to as fluid pressure.

Fluid pressure can occur in one of two situations: open channel flow, such as in the atmosphere, or closed channel flow, such as in the ocean.

In most cases, pressure in open conditions can be approximated by pressure in "static" or non-moving conditions, since the motions cause relatively minor variations in pressure. Such circumstances are consistent with fluid statics principles. The hydrostatic pressure is the pressure in a non-moving (static) fluid at any given place. Closed bodies of fluid are either "static" or "dynamic," depending on whether the fluid may move, as in a pipe or by squeezing an air gap in a closed container. The pressure in closed situations is consistent with fluid dynamics principles.

Vapour pressure:

Vapour pressure is a measurement of a vapour in thermal equilibrium with its condensed phases in a closed environment.. All liquids and solids have a potential to evaporate into gas, and all vapours have a potential to condense back into their liquid or solid state. The temperature at which the vapour pressure equals the ambient air pressure is known as the normal boiling point of a liquid which is the boiling point of a liquid under atmospheric pressure. Partial vapour pressure is the surface tension that a single component in a mixture contributes to the ambient pressure in the system.

Liquid pressure:

Water pressure acts on a person's eardrums while he or she dives under the water. The stronger the pressure, the deeper the swimmer swims. The weight of the water above the individual is causing the pressure. As a swimmer swims deeper, there is more water above them, resulting in increased pressure. The pressure exerted by a liquid is proportional to its depth. The density of the liquid also affects liquid pressure. The pressure would be higher if someone was submerged in a liquid that was denser than water. As a result, we may state that depth, density, and liquid pressure are all proportional.

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Frequently Asked Questions (FAQs)

1. Define pressure and its formula.

Pressure is defined as the force applied perpendicular to an object's surface per unit area across which that force is diffused.

P=F/A

Where, the pressure is P, the normal force is F, and the area of the surface on contact is A.

2. What is the unit of pressure?

The pascal (Pa) is a SI unit or MKS unit for pressure, equivalent to one newton per square metre (N/m2, or kgms2).
The CGS unit of pressure is equal to 1 dyn.cm-2 or 0.1 Pa.

3. Define atmospheric pressure.

The pressure within Earth's atmosphere, often known as atmospheric pressure or barometric pressure. The gravitational attraction of the planet on the atmospheric gases above the surface causes atmospheric pressure.

4. What is the application of atmospheric pressure?

The applications of atmospheric pressure is that,

When a man sucks fluid from a straw, for example, the pressure within is moderately low, and the atmospheric pressure outside drives up the fluid into the straw.
When a vacuum cleaner is powered on, the pressure inside plunges off due to air inside.

5. What is the unit of the atmospheric pressure?

The pascal (Pa) is the SI unit of pressure, which is defined as a force of one Newton per square metre.
The calculation is as follows for converting between atm, Pa, and torr, 101325 Pa =760 torr=1 atm.
It's sometimes referred to as a reference or standard pressure.

6. What is pressure and how is it defined in physics?

Pressure is the force applied perpendicular to a surface per unit area. It is defined as the ratio of force to the area over which that force is distributed. In simpler terms, pressure measures how concentrated a force is over a given area.

7. How does pressure differ from force?

While force is a vector quantity that has both magnitude and direction, pressure is a scalar quantity that only has magnitude. Pressure is the force applied per unit area, whereas force is simply a push or pull acting on an object.

8. What is the relationship between pressure, force, and area?

Pressure is directly proportional to force and inversely proportional to area. The mathematical relationship is expressed as P = F/A, where P is pressure, F is force, and A is area.

9. Why does a sharp knife cut more easily than a blunt one?

A sharp knife cuts more easily because it concentrates the applied force over a smaller area, resulting in higher pressure. The higher pressure allows the knife to overcome the material's resistance more effectively, making cutting easier.

10. Why do our ears "pop" when we change altitude quickly?

Our ears "pop" due to the pressure difference between the air inside our middle ear and the outside air. As we change altitude, the external pressure changes, but the pressure inside our ears doesn't change as quickly. The "pop" occurs when this pressure equalizes, often by opening the Eustachian tubes.

11. What is the difference between gauge pressure and absolute pressure?

Gauge pressure is the pressure measured relative to atmospheric pressure, while absolute pressure is the total pressure including atmospheric pressure. Absolute pressure is always positive, while gauge pressure can be positive, negative, or zero.

12. What is negative pressure, and how is it possible?

Negative pressure refers to a pressure that is below the reference pressure (usually atmospheric pressure). It's possible when the measured pressure is less than the reference pressure. For example, when you suck on a straw, you create negative pressure in your mouth relative to atmospheric pressure.

13. What is atmospheric pressure?

Atmospheric pressure is the force per unit area exerted by the weight of the atmosphere. It varies with altitude and weather conditions but is approximately 101,325 Pa (or 1 atm) at sea level under standard conditions.

14. How does atmospheric pressure change with altitude?

Atmospheric pressure decreases with increasing altitude. This is because there is less air above you as you go higher, resulting in less weight of the atmosphere pressing down.

15. How does a barometer work?

A barometer measures atmospheric pressure by using a column of liquid (usually mercury) in a glass tube. The height of the liquid column changes with atmospheric pressure, allowing for precise measurements. As atmospheric pressure increases, it pushes down on the reservoir, causing the liquid to rise in the tube.

16. What is the SI unit of pressure?

The SI unit of pressure is the Pascal (Pa). One Pascal is equal to one Newton of force applied over one square meter of area (1 Pa = 1 N/m²).

17. Why do we need different units to measure pressure?

Different units are used to measure pressure because of the wide range of pressures encountered in various fields and applications. Some units are more convenient for specific contexts, like atmospheric pressure or blood pressure, while others are better suited for scientific or engineering purposes.

18. What is Pascal's principle?

Pascal's principle states that pressure applied to an enclosed fluid is transmitted equally in all directions throughout the fluid. This principle is the basis for hydraulic systems and explains why you can lift a car with a small force applied to a hydraulic jack.

19. What is the relationship between pressure and depth in a fluid?

Pressure in a fluid increases linearly with depth. This is because the deeper you go, the more fluid is above you, exerting weight. The relationship is expressed as P = ρgh, where P is pressure, ρ (rho) is fluid density, g is gravitational acceleration, and h is depth.

20. What is Boyle's law, and how does it relate to pressure?

Boyle's law states that for a fixed amount of an ideal gas at constant temperature, pressure and volume are inversely proportional. As pressure increases, volume decreases, and vice versa. This relationship is expressed as P₁V₁ = P₂V₂, where P is pressure and V is volume.

21. How does a Bourdon gauge measure pressure?

A Bourdon gauge measures pressure using a curved, flexible tube called a Bourdon tube. As pressure inside the tube increases, it tends to straighten out. This movement is mechanically linked to a pointer on a calibrated scale. The amount of straightening is proportional to the pressure, allowing for accurate pressure readings.

22. What is the principle behind a hydraulic ram pump?

A hydraulic ram pump uses the water hammer effect to pump water without external power. It works by using the kinetic energy of flowing water to create a pressure surge when a valve suddenly closes. This pressure surge forces a small amount of water up to a higher elevation. The process repeats automatically, allowing continuous pumping using only the energy of the flowing water.

23. What is the relationship between pressure and volume in an adiabatic process?

In an adiabatic process, where no heat is exchanged with the surroundings, the relationship between pressure and volume is described by the equation PVᵞ = constant, where γ (gamma) is the heat capacity ratio. This shows that as volume decreases, pressure increases more rapidly than in an isothermal process, due to the additional effect of temperature change.

24. Why do deep-sea creatures often look strange or distorted?

Deep-sea creatures often look strange or distorted because they have adapted to the extremely high pressures in the deep ocean. Their bodies are designed to withstand these pressures, often with features like compressible organs or gelatinous bodies that help equalize internal and external pressures.

25. How does tire pressure affect a vehicle's performance?

Tire pressure significantly affects a vehicle's performance. Properly inflated tires improve fuel efficiency, handling, and safety. Underinflated tires increase rolling resistance, reducing fuel efficiency and potentially causing overheating. Overinflated tires can lead to reduced traction and a harsher ride.

26. What is the difference between hydrostatic pressure and hydrodynamic pressure?

Hydrostatic pressure is the pressure exerted by a fluid at rest due to its weight, while hydrodynamic pressure is the pressure exerted by a moving fluid. Hydrostatic pressure depends on fluid density and depth, whereas hydrodynamic pressure also depends on fluid velocity and flow characteristics.

27. How does a pressure cooker work?

A pressure cooker works by creating a sealed environment where steam pressure builds up. This increased pressure raises the boiling point of water, allowing food to cook at higher temperatures. The higher temperature and pressure reduce cooking time and can make food more tender.

28. What is vapor pressure, and how does it relate to boiling?

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid phase at a given temperature. Boiling occurs when the vapor pressure of a liquid equals the atmospheric pressure. This is why water boils at lower temperatures at high altitudes where atmospheric pressure is lower.

29. How do pressure suits protect astronauts in space?

Pressure suits protect astronauts by maintaining a pressurized environment around their bodies. In the vacuum of space, where there's no atmospheric pressure, these suits prevent bodily fluids from boiling and tissues from expanding. They also provide oxygen for breathing and regulate temperature.

30. What is the principle behind a hydraulic press?

A hydraulic press works on Pascal's principle. When pressure is applied to a small piston in a closed system, that pressure is transmitted equally throughout the fluid to a larger piston. This allows a small force on the small piston to create a much larger force on the larger piston, amplifying the input force.

31. How does blood pressure relate to the physics of pressure?

Blood pressure is a measure of the force exerted by blood against the walls of blood vessels. It follows the principles of fluid dynamics and pressure. The pressure varies throughout the circulatory system, with higher pressure in arteries and lower pressure in veins, driven by the heart's pumping action.

32. What is the difference between static pressure and dynamic pressure in fluid flow?

Static pressure is the pressure exerted by a fluid at rest or in motion, perpendicular to the flow direction. Dynamic pressure is the pressure resulting from the fluid's motion, parallel to the flow. The sum of static and dynamic pressure is the total pressure in a moving fluid.

33. How does a manometer measure pressure?

A manometer measures pressure differences using the height of a column of liquid. It consists of a U-shaped tube partially filled with a liquid (often mercury or water). The pressure difference between the two ends of the tube causes the liquid levels to differ, and this difference in height is used to calculate the pressure difference.

34. What is the concept of pressure head in fluid mechanics?

Pressure head is a way to express pressure in terms of the height of a column of fluid. It's calculated by dividing pressure by the product of fluid density and gravitational acceleration (h = P / ρg). This concept is useful in understanding fluid flow in pipes and channels.

35. How does the shape of an airplane wing create lift?

An airplane wing creates lift due to pressure differences. The wing's shape causes air to move faster over the top surface than the bottom. According to Bernoulli's principle, faster-moving air has lower pressure. This pressure difference between the top and bottom of the wing generates an upward force called lift.

36. What is cavitation, and how is it related to pressure?

Cavitation is the formation of vapor cavities in a liquid when local pressure drops below the liquid's vapor pressure. These cavities collapse rapidly, potentially causing damage to surfaces. It often occurs in pumps or propellers where local pressure drops significantly due to high-speed fluid flow.

37. How does a siphon work, and what role does atmospheric pressure play?

A siphon works by using the force of gravity and atmospheric pressure to move liquid from a higher level to a lower level over a barrier. Atmospheric pressure pushes on the liquid surface, helping to maintain the flow. The siphon continues to work as long as the outlet is lower than the inlet and the tube remains filled with liquid.

38. What is the relationship between pressure and temperature in an ideal gas?

For an ideal gas at constant volume, pressure is directly proportional to temperature. This relationship is described by Gay-Lussac's law, which states that P₁/T₁ = P₂/T₂, where P is pressure and T is absolute temperature. As temperature increases, the gas molecules move faster, colliding more frequently with the container walls and increasing pressure.

39. How does a vacuum pump create low pressure?

A vacuum pump creates low pressure by removing gas molecules from a sealed chamber. It works by expanding a cavity, allowing gas to flow in from the chamber, then sealing off the cavity and compressing the gas to expel it. Repeating this process progressively reduces the number of gas molecules in the chamber, lowering the pressure.

40. What is the principle behind a Venturi meter?

A Venturi meter works on the principle that as the velocity of a fluid increases through a constriction, its pressure decreases. By measuring the pressure difference between the wider and narrower sections of the meter, the flow rate can be calculated. This application of Bernoulli's principle is used to measure fluid flow in pipes.

41. How does pressure affect the boiling point of liquids?

Pressure directly affects the boiling point of liquids. As pressure increases, the boiling point rises because more energy is required for the vapor pressure to overcome the external pressure. Conversely, at lower pressures (like at high altitudes), liquids boil at lower temperatures because there's less atmospheric pressure to overcome.

42. What is the difference between compressibility and incompressibility in fluids?

Compressibility refers to a fluid's ability to change volume when subjected to pressure. Liquids are generally considered incompressible because they resist volume changes under normal pressures. Gases, on the other hand, are highly compressible and can significantly change volume with pressure changes.

43. How does a hydraulic brake system work?

A hydraulic brake system works based on Pascal's principle. When the brake pedal is pressed, it applies force to a small piston in the master cylinder. This force is transmitted equally through the brake fluid to larger pistons in the wheel cylinders. The larger area of these pistons amplifies the force, providing the necessary pressure to apply the brakes effectively.

44. What is the relationship between pressure and density in a fluid at rest?

In a fluid at rest, pressure increases linearly with depth due to the weight of the fluid above. This relationship is described by the hydrostatic pressure equation: P = ρgh, where P is pressure, ρ (rho) is fluid density, g is gravitational acceleration, and h is depth. This shows that pressure is directly proportional to density for a given depth.

45. What is the concept of partial pressure in gas mixtures?

Partial pressure is the pressure exerted by a particular gas in a mixture of gases. According to Dalton's law of partial pressures, the total pressure of a gas mixture is equal to the sum of the partial pressures of its component gases. Each gas in the mixture behaves as if it alone occupies the entire volume at that temperature.

46. How does pressure affect the solubility of gases in liquids?

Pressure directly affects the solubility of gases in liquids. According to Henry's law, the amount of dissolved gas is proportional to its partial pressure above the liquid. As pressure increases, more gas molecules are forced into the liquid, increasing solubility. This is why carbonated drinks fizz when opened, as the dissolved CO2 comes out of solution when pressure is released.

47. How does pressure affect the melting point of solids?

Pressure generally increases the melting point of solids. This is because higher pressure favors the more compact solid state over the less dense liquid state. However, there are exceptions like water, where increased pressure can lower the melting point due to its unique molecular structure.

48. What is the working principle of a pressure relief valve?

A pressure relief valve is designed to open at a predetermined pressure to protect a system from overpressure. When the system pressure exceeds the set point, the valve opens, allowing fluid to escape and reducing pressure. Once the pressure drops below the set point, the valve closes again. This automatic operation helps prevent equipment damage or failure due to excessive pressure.

49. How does the concept of pressure relate to the buoyant force?

The buoyant force is directly related to pressure differences in a fluid. According to Archimedes' principle, the buoyant force on an object is equal to the weight of the fluid displaced by the object. This force results from the difference in pressure between the top and bottom of the submerged object, with higher pressure at the bottom pushing the object upward.

50. How does a pitot tube measure fluid velocity using pressure?

A pitot tube measures fluid velocity by comparing static pressure and total pressure in a moving fluid. It has two openings: one facing the flow (measuring total pressure) and one perpendicular to the flow (measuring static pressure). The difference between these pressures is the dynamic pressure, which is related to fluid velocity by Bernoulli's equation.

51. What is the concept of critical pressure in thermodynamics?

Critical pressure is the pressure required to liquefy a gas at its critical temperature. Above the critical pressure and temperature, the distinction between liquid and gas phases disappears, and the substance exists as a supercritical fluid. This concept is important in understanding phase transitions and in various industrial processes.

52. How does pressure affect the speed of sound in a medium?

In gases, the speed of sound increases with pressure. This is because higher pressure results in more frequent molecular collisions, allowing sound waves to propagate faster. In liquids and solids, the effect of pressure on sound speed is generally less pronounced but still present, often increasing with pressure due to increased molecular proximity.

53. What is the principle behind a deadweight tester for pressure calibration?

A deadweight tester is a primary standard for pressure calibration. It works by balancing the pressure to be measured against the weight of precisely calibrated masses on a piston of known area. The pressure is calculated as P = F/A, where F is the force exerted by the weights and A is the piston area. This method provides highly accurate pressure measurements.

54. How does the concept of pressure apply to the formation of weather systems?

Pressure plays a crucial role in weather systems. Areas of high and low pressure create pressure gradients that drive wind patterns. Low-pressure systems often bring cloudy, rainy weather as air rises and cools, while high-pressure systems typically bring clear skies as air descends and warms. The movement of these pressure systems is fundamental to weather forecasting.

55. What is the relationship between pressure and the boiling point of a mixture (e.g., in distillation)?

In a mixture, the total vapor pressure is the sum of the partial pressures of each component. Boiling occurs when this total vapor pressure equals the external pressure. As pressure changes, the boiling point of the mixture changes, affecting the composition of