Fluids exert pressure under various conditions. When enclosed, an external pressure applied to a fluid is transmitted uniformly in all directions, as stated by Pascal's principle. When fluids are in motion, their velocity affects internal pressure according to Bernoulli's principle and the Venturi effect.
How fluid pressure behaves in a static enclosed system versus a flowing dynamic system.
An enclosed system is a physical system where matter cannot enter or leave the system.
When a fluid is enclosed in a container, an external pressure applied to the fluid is transmitted uniformly in all directions. This is the foundation of Pascal's principle and hydraulic systems.
The transmission of pressure exerted on a fluid (liquid or gas) in an enclosed system is uniform throughout the fluid and in all directions.
A fluid moving at a higher velocity produces a lower pressure in that region. As fluid velocity increases, pressure decreases.
The narrower the part of a Venturi tube, the lower the pressure in the fluid.
How a small input force can produce a large output force.
Pascal's equipment consists of a round-bottom flask with fine pores, fitted with a piston, and a large beaker filled with water. When the piston is pushed inward into the water-filled flask, water shoots out with equal force in all directions from every fine pore.
A hydraulic system operates on Pascal's principle. It is used to carry out heavy work by producing a large output force from a small input force to lift heavy loads.
Two cylindrical pistons of different surface areas connected together containing an incompressible fluid. The fluid used is water or oil because liquids do not have a fixed shape and cannot be compressed.
\[ \text{Pressure at small piston} = \text{Pressure at large piston} \]
\[ \dfrac{\text{Input force}}{\text{Area of small piston}} = \dfrac{\text{Output force}}{\text{Area of large piston}} \]
Input force = \(10\text{ N}\), area of small piston = \(2\text{ cm}^2\), area of large piston = \(100\text{ cm}^2\).
\[ \dfrac{10\text{ N}}{2\text{ cm}^2} = \dfrac{\text{Output force}}{100\text{ cm}^2} \]
\[ \text{Output force} = \dfrac{10\text{ N} \times 100\text{ cm}^2}{2\text{ cm}^2} = 500\text{ N} \]
Used to lift heavy loads, such as cars in workshops. It consists of a lever, small piston, liquid reservoir, air hole, valve A, valve B, release valve, large piston, and load.
Used to slow down or stop wheeled vehicles such as cars. Brake oil transmits pressure uniformly through metal pipes to the brake cylinders of both front and back wheels.
Uses a hydraulic system based on Pascal's principle to raise, lower, and tilt the patient's chair smoothly to the desired position.
The relationship between fluid velocity and pressure.
When the fluid is stationary, fluid pressure at points \(P_1\), \(P_2\), and \(P_3\) is equal; the liquid columns stand at the exact same height.
A Venturi tube is a non-uniform tube with a narrower centre section. When fluid flows through the tube:
The narrower the part of the Venturi tube, the lower the pressure in the fluid.
The curved upper surface causes air to flow at high velocity, creating low air pressure above the wing. The flatter lower surface causes air to flow at low velocity, creating high air pressure below the wing. The resulting difference in pressure generates an upward lift force.
Gas issues from a narrow nozzle at high velocity, creating a low-pressure area inside the burner barrel. Atmospheric air with low velocity and relatively higher pressure outside is sucked into the air hole to mix with the gas for complete combustion.
Rotating blades act as aerofoils, causing high-velocity airflow and low air pressure on the upper surface, and low-velocity airflow and high air pressure on the lower surface, producing lift.
When a train passes at high speed, the airflow in the narrow space between the train and a person on the platform moves at high velocity, producing low air pressure. The air behind the person has lower velocity and higher pressure. This pressure difference produces a force pushing the person toward the moving train.
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What does Pascal's principle state?
A hydraulic system has a small piston with an area of \(5\text{ cm}^2\) and a large piston with an area of \(150\text{ cm}^2\). If an input force of \(20\text{ N}\) is applied to the small piston, what is the output force produced on the large piston?
Based on Bernoulli's principle, explain why a person is not allowed to stand beyond the safety line at a railway station when a train is moving at high speed.
Pressure in a hydraulic system increases at the large piston.
Pressure (\(P\)) is the same throughout the enclosed fluid. The force (\(F\)) increases because the cross-sectional area (\(A\)) of the large piston is larger (\(F = P \times A\)).
The Venturi effect and Bernoulli's principle are two contradictory physics concepts.
The Venturi effect is a specific manifestation of Bernoulli's principle in a tube with a narrow section: the narrow region causes velocity to increase, and that increase in velocity results in a decrease in pressure.
1 Why is oil or water used as the transmission fluid in a hydraulic system instead of gas?
2 According to Bernoulli's principle, what happens to the air pressure in a region where the airflow velocity increases?
3 Pascal's principle states that the transmission of pressure exerted on a fluid in an enclosed system is throughout the fluid and in all directions.
4 An aerofoil wing produces lift because high velocity airflow above the wing produces air pressure compared to below the wing.
Drag each device to the scientific principle or component that explains its function. On a phone, tap an answer first, then tap a matching box. Tap the Answer Choices area to return a selected item.
Write the correct device next to each operating principle or effect.
Answer choices: Hydraulic jack • Aerofoil wing • Venturi tube • Bunsen burner
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