Pressure

What is Pressure?

Pressure is defined as the force applied per unit area on the surface of an object. It is commonly measured in units of pascals (Pa) or pounds per square inch (psi). Pressure indicates how much force is exerted over a specific area and is a key concept in both physics and engineering.

Pressure Formula

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

Where:

• P is the pressure,
• F is the force applied,
• A is the area over which the force is distributed.

Unit of Pressure

The unit of pressure in the International System of Units (SI) is the pascal (Pa), which is defined as one newton per square meter (N/m²).

How to Calculate Pressure

1. Identify the Force: Determine the force acting on the surface. This force should be measured in newtons (N).
2. Measure the Area: Measure or calculate the area over which the force is distributed. This area should be in square meters (m²).
3. Use the Pressure Formula: Apply the formula:
   Pressure (P) = Force (F) / Area (A)
   Where:
   • P is the pressure,
   • F is the force in newtons,
   • A is the area in square meters.
4. Calculate: Plug the values of the force and the area into the formula to find the pressure.
5. Express the Pressure in Appropriate Units: The result will be in pascals (Pa) if you use newtons for force and square meters for area. You can convert this into other units like psi (pounds per square inch) or bar, depending on your requirements.

Types of Pressure

Atmospheric Pressure

This is the pressure exerted by the weight of the atmosphere above us. It decreases with altitude and is often measured using a barometer.

Gauge Pressure

Gauge pressure is the pressure measured relative to the atmospheric pressure. For instance, the pressure in a car tire is typically given as gauge pressure, which does not account for atmospheric pressure. A gauge pressure of zero means it is equal to the surrounding atmospheric pressure.

Absolute Pressure

Absolute pressure is the total pressure exerted on a system, including atmospheric pressure. It is gauge pressure plus atmospheric pressure and is crucial in calculations where vacuum or true total pressure is considered.

Differential Pressure

Differential pressure is the difference in pressure between two points. It is used in various applications, such as in measuring fluid flow or the level of a liquid.

Hydrostatic Pressure

Hydrostatic pressure is the pressure exerted by a fluid at equilibrium due to the force of gravity. It increases with depth and is a critical factor in engineering and hydrodynamics.

Dynamic Pressure

Dynamic pressure is associated with the fluid flow. It is the kinetic energy per unit volume of a fluid particle. It plays a key role in fluid dynamics and aerodynamics.

Pressure Examples

1. Atmospheric Pressure
   • Atmospheric pressure acts on everything at the Earth’s surface. It is why a barometer, an instrument that measures atmospheric pressure, is important in weather forecasting. Lower atmospheric pressure often indicates stormy weather.
2. Hydraulic Systems
   • In hydraulic systems, pressure applied to a fluid in a closed system is transmitted undiminished throughout the fluid. This principle is used in hydraulic lifts and brakes, where small forces applied over smaller areas are converted into larger forces over larger areas.
3. Blood Pressure
   • Blood pressure is the force exerted by circulating blood on the walls of blood vessels. It is a critical physiological measurement for human health.
4. Tire Pressure
   • The pressure of the air inside a tire supports the weight of a vehicle. Proper tire pressure ensures optimal performance and safety.
5. Boyle’s Law in Gases
   • According to Boyle’s Law, at constant temperature, the pressure of a gas increases as the volume decreases. This is observable in a syringe or in scuba diving as divers go deeper and the pressure increases.
6. Artesian Wells
   • The hydrostatic pressure of the water in the ground can cause water to flow up naturally in artesian wells, without the need for pumping.
7. Aerodynamics
   • Dynamic pressure is used in aerodynamics to describe the pressure variations on airplane wings which generate lift and allow the plane to fly.
8. Pressure Cookers
   • Pressure cookers utilize steam pressure. By sealing the steam inside, the pressure and temperature rapidly increase, allowing food to cook faster.

Factors Affecting Pressure

1. Area
   • Pressure is inversely proportional to the area over which a force is distributed. Smaller areas under a constant force will experience higher pressure. This is why sharp knives cut better than dull ones; the force is concentrated over a smaller surface area, increasing the pressure.
2. Force
   • The amount of force applied directly affects the pressure. More force on a given area results in higher pressure. This relationship is straightforward and is the basis for hydraulic systems, where pressure is used to multiply force.
3. Fluid Depth
   • In fluids, pressure increases with depth due to the weight of the fluid above. This is why the pressure at the bottom of a filled container is greater than at the top. Hydrostatic pressure, which depends on the density of the fluid and the acceleration due to gravity, also plays a role here.
4. Temperature
   • In scenarios involving gases, temperature can significantly affect pressure. According to the ideal gas law, at a constant volume, an increase in temperature will increase the pressure. This is because higher temperatures increase the kinetic energy of the molecules, which in turn increases the frequency and force of collisions against container walls.
5. Fluid Density
   • The density of a fluid has a direct effect on pressure, especially in fluid columns and in the context of buoyancy. Heavier fluids exert more pressure at a given depth compared to lighter fluids due to greater mass in the same volume.
6. Altitude or Elevation
   • Atmospheric pressure decreases with an increase in altitude. Higher altitudes have less air above a given area, thus exerting less pressure. This is noticeable in activities such as hiking or flying, where reduced air pressure can affect breathing and the boiling point of liquids.
7. State of the Fluid
   • The state of the fluid (whether it’s a liquid or a gas) affects how it responds to changes in pressure. Gases are compressible, meaning their pressure changes significantly with volume changes. Liquids, however, are nearly incompressible, and their pressure is more influenced by changes in depth or external forces.

What’s the difference between absolute pressure and gauge pressure?

Aspect	Absolute Pressure	Gauge Pressure
Definition	The total pressure exerted by a fluid, including atmospheric pressure.	The pressure of a fluid relative to the ambient atmospheric pressure.
Reference Point	Measured relative to a perfect vacuum (zero reference point).	Measured relative to the local atmospheric pressure.
Indication	Always positive, as it measures the total pressure including the atmospheric component.	Can be positive or negative; negative gauge pressure indicates a vacuum compared to the ambient.
Measurement	Includes the atmospheric pressure plus the pressure exerted by the fluid.	Excludes atmospheric pressure; it is the difference between the absolute pressure and the atmospheric pressure.
Typical Use	Used when it is important to have a true measurement of pressure, such as in scientific research and in conditions where vacuums are measured.	Common in everyday applications, such as tire pressure, HVAC systems, and blood pressure measurements.
Unit	Often still measured in pascals (Pa), bars, or psi.	Also measured in pascals (Pa), bars, or psi, but referenced as gauge (e.g., psi gauge).

FAQs

What is pressure in physics units?

Pressure in physics is measured in pascals (Pa), where one pascal equals one newton per square meter (N/m²). Other units include atmospheres and psi.

How is pressure measured in physics?

Pressure is measured using devices like barometers, manometers, and pressure sensors, which indicate pressure by measuring the force exerted over a known area.

How does pressure relate to force?

Pressure is the result of force distributed over an area. It increases as the force increases or as the area over which the force is applied decreases.

What force creates pressure?

Any force exerted over an area creates pressure. This can be due to gravity, mechanical forces, or fluid dynamics in gases and liquids.

Why is pressure important in physics?

Pressure is crucial for understanding fluid dynamics, weather patterns, engineering applications, and many physical phenomena like buoyancy and the behavior of gases.