What is Pressure? (Formula, Unit, Types of Pressure)

Pressure is all around us. From the air we breathe to the tyres on your car, it is everywhere. But what exactly is pressure, and why does it matter?

In simple terms, pressure is the force applied over a surface. It plays a huge role in science, engineering, and everyday life.

In this guide, we will explore the what is pressure, its formula, units, and different types. By the end, you’ll see how understanding pressure makes everything clearer, from weather forecasts to your daily gadgets. Let’s get started!

What is Pressure?

“Pressure is the force applied per unit area on a surface.“

SI unit of pressure,

Pressure is measured in pascals (Pa) in the SI system.

Formula of pressure,

The formula of pressure is:

Pressure (P) = Force (F) / Area (A)

Example,

When a person stands on snow wearing boots, they sink slightly. However, using snowshoes, which distribute their weight over a larger area, they don’t sink as much. This happens because pressure decreases when the area increases.

Types of Pressure

These are the types of pressure:

• Atmospheric Pressure
• Gauge Pressure
• Absolute Pressure
• Hydrostatic Pressure
• Dynamic Pressure

Atmospheric Pressure

Atmospheric pressure is the force exerted by the weight of air above a surface.

Unit:
The SI unit is Pascal (Pa). It is also measured in atmospheres (atm) or millimetres of mercury (mmHg).

Formula:
P=ρ⋅g⋅h

Where:

• P = Pressure
• ρ = Air density
• g = Gravitational acceleration
• h = Height of the air column

Example:
At sea level, the atmospheric pressure is approximately 101,325 Pa (or 1 atm).

Gauge Pressure

Gauge pressure is the pressure measured relative to the atmospheric pressure. It excludes atmospheric pressure from its value.

Unit:
The SI unit is Pascal (Pa), but it can also be measured in bars or psi (pounds per square inch).

Formula:
P_g= P − Patm
Where:

• P_g = Gauge pressure
• P = Absolute pressure
• P_atm​ = Atmospheric pressure

Example:
If the absolute pressure inside a tyre is 250,000 Pa and atmospheric pressure is 101,325 Pa, the gauge pressure is:

P_g = 250,000 − 101,325 = 148,675 Pa

Absolute Pressure

Absolute pressure is the total pressure measured relative to a perfect vacuum. It includes atmospheric pressure.

Unit:
The SI unit is Pascal (Pa), but it can also be expressed in bars or psi.

Formula:

P_a = P_g + P_atm

Where:

• P_a = Absolute pressure
• P_g​ = Gauge pressure
• P_atm​ = Atmospheric pressure

Example:
If a gauge pressure in a car tyre is 200,000 Pa and atmospheric pressure is 101,325 Pa, the absolute pressure is:

P_a= 200,000 + 101,325=301,325 Pa

Hydrostatic Pressure

Hydrostatic pressure is the pressure exerted by a fluid at rest due to the force of gravity. It increases proportionally with the depth of the fluid.

Unit:
The SI unit of hydrostatic pressure is the Pascal (Pa), where 1 Pa = 1 N/m². Other common units include atmospheres (atm), bars, or pounds per square inch (psi).

Formula:

P=ρgh

Where:

• P = Hydrostatic pressure (Pa)
• ρ = Density of the fluid (kg/m³)
• g = Acceleration due to gravity (9.81 m/s²)
• h = Height (depth) of the fluid column (m)

Example:
Calculate the hydrostatic pressure at a depth of 10 m in water (ρ=1000 kg/m³)

P= ρgh = (1000)(9.81)(10) = 98,100Pa (or 98.1kPa)

Dynamic Pressure

Dynamic pressure is the pressure associated with the kinetic energy of a fluid in motion. It represents the energy per unit volume of fluid flow.

Unit:
The SI unit of dynamic pressure is also the Pascal (Pa).

Formula:

q = 1​/2ρv²

Where:

• q = Dynamic pressure (Pa)
• ρ = Density of the fluid (kg/m³)
• v = Velocity of the fluid (m/s)

Example:
Calculate the dynamic pressure of air (ρ=1.225 kg/m³) moving at a velocity of 50 m/s.

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