Short Answer:

Pressure is defined as the normal force exerted per unit area on the surface of a body. It describes how much force is applied over a specific area and acts perpendicular to that surface. The SI unit of pressure is Pascal (Pa), which is equal to one newton per square meter (N/m²).

In simple words, pressure tells us how concentrated a force is over a surface. For example, the same force applied over a small area creates high pressure, while over a large area it creates low pressure. Pressure plays a major role in fluid mechanics, hydraulics, and aerodynamics.

Detailed Explanation :

Pressure

Pressure is a fundamental concept in physics and fluid mechanics that expresses the effect of a force distributed over an area. It is defined as the amount of normal force acting per unit area of a surface. The direction of pressure is always perpendicular (normal) to the surface on which it acts, regardless of the orientation of that surface.

Mathematically,

where,

• P = Pressure (N/m² or Pascal)
• F = Normal force acting on the surface (N)
• A = Area on which the force acts (m²)

Pressure is a scalar quantity, meaning it has magnitude but no direction, though it always acts normal to the surface.

Units and Dimensions

1. SI Unit: Pascal (Pa) = 1 N/m²
2. Other Common Units:
   • Bar (1 bar = 10⁵ Pa)
   • Atmosphere (1 atm = 1.013 × 10⁵ Pa)
   • Millimeters of mercury (1 mmHg = 133.3 Pa)
3. Dimensional Formula: [M¹L⁻¹T⁻²]

For example, if a force of 100 N acts uniformly on an area of 2 m², then

Concept of Pressure in Fluids

In a fluid at rest, pressure acts equally in all directions at a point, as stated by Pascal’s Law. This uniform distribution of pressure is the reason why fluids can transmit forces efficiently. The pressure at a point in a fluid depends on its depth, density, and acceleration due to gravity (g).

The relation between pressure and depth is given by:

where,

• P₀ = pressure at the fluid surface (N/m²)
• ρ = fluid density (kg/m³)
• g = acceleration due to gravity (9.81 m/s²)
• h = depth of the point below the surface (m)

This shows that pressure increases with depth in a fluid because of the increasing weight of the fluid above.

Types of Pressure

1. Absolute Pressure:
   This is the total pressure measured from an absolute vacuum (zero pressure).

1. Gauge Pressure:
   It is the pressure measured relative to the atmospheric pressure. It can be positive or negative.
   Example: The pressure in a car tire is usually given as gauge pressure.
2. Atmospheric Pressure:
   The pressure exerted by the weight of the Earth’s atmosphere on the surface is called atmospheric pressure. At sea level, it is approximately 101.3 kPa or 1.013 × 10⁵ Pa.
3. Vacuum Pressure:
   When the pressure inside a system is less than the atmospheric pressure, the difference is known as vacuum pressure.

Variation of Pressure in Fluids

Pressure in fluids increases with depth due to the weight of the fluid above.
At depth h in a fluid of density ρ, the pressure is:

This concept is used to determine pressure in tanks, dams, and underwater structures. For instance, the pressure at 10 meters depth in water (ρ = 1000 kg/m³) is:

This means the pressure at 10 meters underwater is almost equal to atmospheric pressure at sea level.

Measurement of Pressure

Pressure can be measured using different instruments depending on the type of fluid and range of pressure:

1. Manometer:
   Used for measuring small pressure differences, often using a column of liquid such as mercury or water.
2. Barometer:
   Used for measuring atmospheric pressure. It consists of a mercury column that balances the weight of the air above.
3. Bourdon Gauge:
   Used for measuring high pressures, such as in gas cylinders and hydraulic systems.
4. Piezometer:
   A simple tube used to measure the pressure of liquids in tanks or pipelines.

Applications of Pressure

1. Hydraulic Systems:
   Pressure is used to transmit force in hydraulic lifts, brakes, and presses based on Pascal’s Law.
2. Fluid Flow:
   Pressure differences drive fluid motion in pipes, pumps, and turbines.
3. Aviation and Meteorology:
   Atmospheric pressure helps in predicting weather conditions and controlling aircraft altitude.
4. Underwater Engineering:
   Pressure calculations are essential for designing dams, submarines, and diving equipment.
5. Everyday Uses:
   Air pressure in tires, water pressure in plumbing systems, and blood pressure in the human body are all applications of pressure in daily life.

Factors Affecting Pressure

1. Force:
   Higher force applied on the same area increases pressure.
   Example: A sharp knife cuts better because the same force acts on a smaller area, creating higher pressure.
2. Area:
   Pressure decreases when the same force is applied over a larger area.
   Example: Wide tires reduce pressure on the road surface, improving stability.
3. Depth (in fluids):
   Pressure increases linearly with depth due to the weight of the fluid above.
4. Density:
   Fluids with higher density exert more pressure at a given depth.

Importance of Pressure in Engineering

Pressure plays a crucial role in various mechanical and fluid systems:

• In hydraulic machines, pressure transmission enables lifting and pressing actions.
• In thermodynamic systems, pressure controls gas expansion and compression.
• In fluid transport systems, maintaining pressure ensures steady and reliable flow.
• In aerospace and marine engineering, pressure differences are used for lift generation and buoyancy analysis.

Conclusion

In conclusion, pressure is the normal force acting per unit area within a fluid or on a solid surface. It is one of the most fundamental quantities in fluid mechanics and engineering, determining how forces are distributed in fluids and on structures. Pressure varies with depth, density, and external conditions, and its understanding is essential for designing safe and efficient systems such as hydraulic machines, pipelines, aircraft, and underwater structures.