Pressure

Definition of Pressure

Pressure is defined as the force applied perpendicular to the surface of an object per unit area over which that force is distributed.

Formula:

$$\text{Pressure} = \frac{\text{Force}}{\text{Area}}$$

where pressure is measured in pascals (Pa), force in newtons (N), and area in square metres (m²).

1 pascal (Pa) is equal to 1 newton per square metre (1 Pa = 1 N/m²).

For example, if a force of 50 N is applied over an area of 0.5 m², the pressure is:

$$\text{Pressure} = \frac{50}{0.5} = 100 \text{ Pa}$$

For instance, a force of 10 N applied over an area of 0.1 m² produces a pressure of 100 Pa.

Pressure in Fluids

Fluids include liquids and gases. Pressure in a fluid acts equally in all directions at a given depth.

As you go deeper into a fluid, the pressure increases because the weight of the fluid above increases.

The pressure at a certain depth in a fluid can be calculated using:

$$\text{Pressure} = \text{density} \times \text{gravity} \times \text{height}$$

where:

• density ($\rho$) is in kilograms per cubic metre (kg/m³),
• gravity (g) is approximately 9.8 m/s² on Earth,
• height (h) is the depth below the surface in metres (m).

This formula gives the pressure due to the fluid at depth $h$, often called the hydrostatic pressure.

For example, the pressure 5 m below the surface of water (density = 1000 kg/m³) is:

$$\text{Pressure} = 1000 \times 9.8 \times 5 = 49000 \text{ Pa}$$

Pressure in fluids acts in all directions, which is why a balloon inflates evenly and why hydraulic systems work.

Atmospheric Pressure

Atmospheric pressure is the pressure exerted by the weight of the air in the Earth's atmosphere.

It acts on everything at the Earth's surface and decreases with increasing altitude because the air becomes less dense.

Atmospheric pressure is measured using a barometer, which often contains mercury or uses electronic sensors.

Typical atmospheric pressure at sea level is about 101,000 Pa (or 101 kPa).

For example, on a mountain top, atmospheric pressure is lower than at sea level, which is why water boils at a lower temperature there.

Pressure Differences and Fluid Movement

Fluids move from regions of higher pressure to regions of lower pressure. This pressure difference causes fluid flow.

This principle explains many natural and technological phenomena:

• Hydraulics: Systems like car brakes use pressure differences to multiply force using fluids.
• Weather systems: Air moves from high-pressure areas to low-pressure areas, creating wind.

The greater the pressure difference, the faster the fluid flows.