## Pressure – Class 11 | Chapter – 10 | Physics Short Notes Series PDF for NEET & JEE

Pressure: Pressure is the amount of force per unit area that is exerted on a surface by a fluid or a solid. It is defined as the force applied perpendicularly to the surface divided by the area of the surface. In other words, pressure is the measure of the amount of force that is distributed over a given area.

The SI unit of pressure is the Pascal (Pa), which is defined as one Newton per square meter (N/m²). Other common units of pressure include pounds per square inch (psi), atmospheres (atm), and bars (bar).

Pressure plays an important role in many areas of science and engineering, particularly in fluid mechanics. In a fluid, pressure is related to the density, velocity, and viscosity of the fluid, as well as to external forces such as gravity or mechanical forces. Understanding the behavior of pressure in fluids is crucial for the design and analysis of many fluid systems, including hydraulic systems, piping networks, and aerodynamic structures.

In solids, pressure is related to the stress applied to the material, which can cause deformation or changes in the material’s mechanical properties. In materials science and engineering, the study of the effects of pressure on the behavior of materials is known as “high-pressure physics” or “high-pressure engineering”.

## Units of Pressure

There are several units of pressure that are commonly used in science and engineering. Some of the most commonly used units include:

• Pascal (Pa): The SI unit of pressure, defined as one Newton per square meter (N/m²).
• Atmosphere (atm): A unit of pressure that is equal to the average air pressure at sea level. One atmosphere is equivalent to 101,325 Pa.
• Bar (bar): A unit of pressure that is equal to 100,000 Pa.
• Pound per square inch (psi): A unit of pressure commonly used in the United States and the United Kingdom. One psi is equivalent to 6,894.76 Pa.
• Torr: A unit of pressure that is commonly used in vacuum physics. One Torr is equal to 1/760 of an atmosphere, or approximately 133.32 Pa.
• Millimeter of mercury (mmHg): A unit of pressure that is commonly used in medicine and biology. One mmHg is equivalent to the pressure exerted by a column of mercury that is one millimeter high at sea level. One mmHg is approximately equal to 133.3 Pa.

These are just a few of the many units of pressure that are used in science and engineering. It is important to be aware of the units of pressure being used in a particular application to ensure that measurements are accurate and consistent.

## Factors Affecting Pressure

Pressure is affected by several factors, including:

• Depth: In a fluid, pressure increases with depth due to the weight of the fluid above. This relationship is known as hydrostatic pressure, and is described by the equation P = ρgh, where P is the pressure, ρ is the density of the fluid, g is the acceleration due to gravity, and h is the depth of the fluid.
• Density: The pressure in a fluid is directly proportional to its density. A denser fluid will exert more pressure on a surface than a less dense fluid, all other things being equal.
• Velocity: The pressure in a fluid decreases as its velocity increases, due to Bernoulli’s principle. This relationship is described by the Bernoulli’s equation, which states that the sum of the pressure, kinetic energy, and potential energy of a fluid is constant along a streamline.
• Temperature: In a gas, pressure is directly proportional to temperature, according to the ideal gas law. An increase in temperature will result in an increase in pressure, all other things being equal.
• Volume: In a gas, pressure is inversely proportional to volume, according to Boyle’s law. If the volume of a gas is reduced while the temperature and number of particles remain constant, the pressure will increase.
• External forces: Pressure can also be affected by external forces such as gravity, friction, and mechanical forces. These forces can cause variations in pressure within a fluid, which can have important consequences for the behavior of the fluid.

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