CIE IGCSE Physics (0625) Pressure Study Notes - New Syllabus
CIE IGCSE Physics (0625) Pressure Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Pressure
Key Concepts:
- Pressure
- Impulse
- Law of Conservation of Pressure
- Resultant Force in Terms of Pressure
Pressure
Pressure:
- Pressure is defined as the force applied per unit area on a surface.
- It measures how concentrated a force is on a particular area.
- Pressure is a scalar quantity.
Formula for Pressure:
\( \text{Pressure} = \dfrac{\text{Force}}{\text{Area}} \)
- \( P \) = pressure (Pa or N/m²)
- \( F \) = force applied (N)
- \( A \) = area (m²)
Units:
- 1 Pascal (Pa) = 1 Newton per square metre (N/m²)
- For small areas (like sharp tools), pressure can be very high even if force is small.
Key Idea: A smaller area leads to higher pressure for the same force. A larger area spreads out the force and reduces pressure.
Example:
A box exerts a force of 600 N on the ground. The area of contact with the floor is 1.5 m². What is the pressure exerted by the box?
▶️ Answer/Explanation
Use the formula: \( P = \dfrac{F}{A} \)
\( P = \dfrac{600}{1.5} = \boxed{400 \, \text{Pa}} \)
Pressure Varies with Force and Area
Pressure Varies with Force and Area:
- Pressure increases when:
- The force increases (more weight or push)
- The area decreases (more concentrated force)
- Pressure decreases when:
- The force decreases
- The area increases (spreads the force out)
\( P = \dfrac{F}{A} \) – where \( P \) is pressure, \( F \) is force, and \( A \) is area.
Example 1: Knife vs Spoon
A knife has a sharp edge (small area), while a spoon has a rounded edge (large area). With the same force applied:
▶️ Explanation
The knife exerts and cuts easily due to the small contact area. The spoon spreads the force and does not cut.
Example 2: Lying on a Bed of Nails
A person can lie safely on a bed of hundreds of nails without injury.
▶️ Explanation
The total contact area is large due to many nails, so the pressure at each point is low, and the skin isn’t pierced.
Example 3: Wide Tyres on a Tractor
Tractors have wide tyres to move over soft ground without sinking.
▶️ Explanation
The larger surface area reduces the pressure on the ground, preventing the tractor from sinking into mud or soil.
Pressure Beneath the Surface of a Liquid
Pressure Beneath the Surface of a Liquid:
- Pressure in a liquid increases with depth.
- Pressure in a liquid also increases with the density of the liquid.
- This is because the deeper you go, the more liquid there is above you, exerting weight due to gravity.
Qualitative Relationships:
- Greater depth → more weight of liquid above → higher pressure.
- Greater density → heavier liquid per unit volume → higher pressure at the same depth.
Pressure is directly proportional to:
- Depth \( h \)
- Density \( \rho \)
Mathematical relationship (used later):
\( P = \rho g h \) (where \( g \) is gravitational field strength)
Example
Divers feel much more pressure at greater depths underwater.
▶️ Explanation
At greater depths, the weight of the water column above the diver increases, so pressure on the body and suit rises significantly.
Example
Dams are built with thicker bases than tops.
▶️ Explanation
Pressure increases with depth, so the bottom part of the dam must withstand greater force due to the higher water pressure.
Example
At the same depth, mercury exerts more pressure than water.
▶️ Explanation
Mercury is much denser than water, so even at the same depth, the pressure beneath mercury is higher.
Example:
Calculate the pressure exerted by water at a depth of 5 meters. Take the density of water as \( 1000 \, \text{kg/m}^3 \) and \( g = 9.8 \, \text{m/s}^2 \).
▶️ Answer/Explanation
\( P = \rho g h \)
\( P = 1000 \times 9.8 \times 5 = 49,000 \, \text{Pa} \)
\(\boxed{49,000 \, \text{Pa}}\) or \( \boxed{49 \, \text{kPa}} \)
This is the pressure due to the water column alone (not including atmospheric pressure).