Physics of Fluids and Pressure: Key Concepts

Forces and Their Deforming Effects

A force can produce a deformation in a body. Experience shows that the distorting effect of a force depends on its intensity and the area on which it acts.

Understanding Fluids and Their Properties

Effects of Forces on Fluids

Substances called fluids are characterized by their ability to flow, even passing through small orifices. They do not have a definite form but conform to that of the container they occupy. Both liquids and gases are classified as fluids. A key distinction is that liquids are generally difficult to compress, while gases are highly compressible.

Principles of Pressure in Fluids

Pascal's Principle

Pascal's Principle states that the pressure exerted on any point of a confined incompressible fluid is transmitted undiminished throughout the fluid in all directions.

Communicating Vessels

In communicating vessels, which are two or more interconnected containers holding a liquid, the liquid level remains the same in all of them when at rest, regardless of their shape or volume, provided they are open to the same atmospheric pressure.

Hydraulic Systems

Pascal's Principle is the fundamental basis for hydraulic systems, such as the hydraulic elevator. A hydraulic elevator typically consists of two interconnected cylindrical containers of different cross-sectional areas, filled with a fluid. Each cylinder is equipped with a movable piston.

Atmospheric Pressure and Its Measurement

Defining Atmospheric Pressure

Atmospheric pressure is defined as the force per unit area exerted by the Earth's atmosphere on all bodies located within it.

Experimental Evidence: Torricelli's Experiment

The Italian physicist Evangelista Torricelli conducted a pivotal experiment to demonstrate the existence of atmospheric pressure. He filled a glass tube, closed at one end, with mercury and inverted it into a basin of mercury. The mercury in the tube descended some centimeters but then remained in equilibrium, forming a column approximately 76 cm in height. The pressure exerted by this mercury column was precisely balanced by the pressure exerted by the surrounding air, thus proving the existence of atmospheric pressure.

Value of Atmospheric Pressure

The average value of atmospheric pressure at sea level is approximately 101,300 Pascals (Pa).

Barometers: Measuring Atmospheric Pressure

Apparatus used to measure atmospheric pressure are called barometers. Two common types include:

• Aneroid Barometers (Metal Barometers): These consist of a sealed metal box from which most of the air has been evacuated, creating a partial vacuum. The box deforms slightly in response to changes in atmospheric pressure. A needle, attached to the box, indicates the pressure value on a graduated scale.
• Mercury Barometers: These typically consist of a glass tube filled with mercury, similar to Torricelli's setup. The height of the mercury column directly indicates the value of the atmospheric pressure.

Archimedes' Principle: The Buoyant Force

Understanding Buoyancy

Bodies immersed in water or any other fluid experience an upward-pushing force known as the buoyant force.

Calculating Buoyant Force

Archimedes' Principle states that the magnitude of the buoyant force on a body immersed in a fluid is equal to the weight of the fluid displaced by the body.