Liquid Pressure Virtual Lab Guide
PhysicsIntermediateReading time: 3 min
Overview
Why must deep-sea divers wear heavy pressure-resistant suits? Why are dams built wider at the bottom? The answers lie deep within the liquid. This experiment uses a micro-manometer (U-tube manometer) to take you underwater to intuitively measure and explore the 'three key variables' that determine liquid pressure.
Background
The study of liquid pressure is inseparable from the French scientist Blaise Pascal (1623–1662). In 1648, Pascal performed the famous 'Barrel Experiment': he inserted a long, thin tube into the top of a sealed wooden barrel filled with water. When a small amount of water was poured into the thin tube from a height, the barrel actually burst due to the immense internal pressure! This experiment vividly proved that liquid pressure depends only on depth, not the total amount of liquid. Pascal also discovered 'Pascal's Principle': pressure applied to a confined liquid is transmitted undiminished in all directions, a principle that remains the basis for hydraulic presses and jacks today. To honor his contributions, the international unit of pressure is named the 'Pascal' (Pa).
Key Concepts
Manometer
An instrument for measuring liquid pressure. The rubber membrane of the probe senses pressure, causing a height difference in the liquid levels on the two sides of the U-tube. The greater the height difference, the greater the pressure.
Liquid Pressure
The pressure exerted by a liquid on its interior and the container walls due to gravity and its ability to flow.
Formulas & Derivation
Liquid Pressure Formula
Pressure depends on the liquid's density , gravitational acceleration , and depth . Note: It is independent of the liquid's mass, volume, and container shape.
Experiment Steps
- 1
Check Airtightness
Before the experiment, gently press the rubber membrane of the probe. If a clear height difference appears in the U-tube liquid levels and restores after releasing, the device is airtight. - 2
Explore Depth vs Pressure
Place the probe vertically into clear water and change the 'Probe Depth'. Observe how the U-tube height difference changes as depth increases. - 3
Verify Directional Equality
Keep at the same depth and switch the 'Probe Direction' (Up, Down, Left, Right). Observe and confirm whether the height difference remains constant. - 4
Compare Different Liquids
At the same depth, switch the liquid from 'Clear Water' to 'Salt Water' or 'Kerosene'. Compare the effect of different densities on pressure.
Learning Outcomes
- Confirm that pressure at the same point underwater is equal in all directions
- Master the law that pressure increases linearly with depth
- Verify that at the same depth, the greater the liquid density, the greater the pressure
- Understand the application of the 'Communicating Vessels' principle in the measuring process
Real-world Applications
- Three Gorges Dam: Designed to be extremely wide at the bottom to resist the huge horizontal pressure at deep water levels
- Diving Limits: The design differences between ordinary divers and deep-sea submersibles are precisely to cope with different hydrostatic pressures
- Water Towers: Utilize height difference to generate pressure to deliver water to residents on higher floors
Common Misconceptions
Misconception
The pressure at the bottom of a large bucket of water is always greater than the pressure at the bottom of a small cup of water
Correct
Not necessarily. Pressure depends only on depth and density. If the water level in a partially filled large bucket is lower than that in a cup, the pressure at the bottom of the bucket is actually smaller.
Misconception
Liquid pressure only acts downwards
Correct
Incorrect. Because liquids can flow, they exert pressure on side walls and, due to the principle of upward buoyancy, even upward pressure on the bottom of objects.
Further Reading
Ready to start?
Now that you understand the basics, start the interactive experiment!