Pressure in Physics

Pressure is defined as the force acting normally (perpendicularly) per unit area of a surface. It is a scalar quantity and is fundamental in understanding the behavior of solids, liquids, and gases.

1. Basic Definition and Units

Mathematically, Pressure (P) = Force (F) / Area (A).
The S.I. unit of pressure is the Pascal (Pa), where 1 Pa = 1 Newton per square meter (N/m²). Other units include atmospheres (atm), mmHg, and bars.

2. Atmospheric Pressure

Atmospheric pressure is the force per unit area exerted by the weight of the air in the atmosphere of Earth. At sea level, standard atmospheric pressure is approximately 1.013 x 10⁵ Pa (or 760 mmHg).

Measuring Atmospheric Pressure

• Simple Mercury Barometer: A glass tube filled with mercury inverted into a reservoir. The height of the mercury column (approx. 760mm) indicates the pressure.
• Aneroid Barometer: A liquid-free instrument using a flexible metal box (capsule) that expands or contracts with pressure changes.
• Manometer: A U-shaped tube used to measure the pressure of a gas relative to atmospheric pressure.

Variation with Height

Atmospheric pressure decreases as altitude increases because the density of air decreases and there is less air above to exert weight. This principle allows the barometer to be used as an altimeter to measure height above sea level.

3. Pressure in Liquids

Pressure in a liquid increases with depth and density. It is calculated using the formula: P = hρg

• h: Depth below the liquid surface (m)
• ρ (rho): Density of the liquid (kg/m³)
• g: Acceleration due to gravity (approx. 9.8 or 10 m/s²)

Note: The total pressure at a depth 'h' in a liquid is P = P_atm + hρg, where P_atm is the atmospheric pressure at the surface.

4. Pascal’s Principle

Pascal's Principle states that pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and to the walls of the containing vessel.

Applications of Pascal's Principle

• Hydraulic Press: Used for lifting heavy loads. Since P1 = P2, then F1/A1 = F2/A2. A small force on a small area creates a large force on a large area.
• Hydraulic Brakes: Used in vehicles to transmit force from the brake pedal to the wheels.