Pressure & Pressure Differences in Fluids (AQA GCSE Physics): Revision Notes
Pressure
What is pressure?
Pressure measures how much force is spread over a certain area. When you push on something, the pressure depends on two things:
- How hard you push (the force)
- How big the area is that you're pushing on
Pressure always acts straight down on surfaces (at right angles). This means fluids like liquids and gases push directly onto the surfaces they touch.
Fluids (both liquids and gases) create pressure that always acts at right angles to any surface they touch. This is why water pressure pushes straight into the sides of a swimming pool, not at an angle.
The pressure equation
You can calculate pressure using this simple equation:
Pressure = Force ÷ Area
Written as a formula:
Where:
- = pressure (measured in pascals)
- = force (measured in newtons)
- = area (measured in square metres)
Remember that the force must be measured perpendicular to the surface - this means the force acting at right angles to the area, not at an angle.
Units of pressure
Pressure is measured in pascals (Pa).
- 1 pascal = 1 newton per square metre
- Written as:
This means 1 pascal is the pressure when 1 newton of force acts on 1 square metre of area.
Atmospheric pressure
The air around us creates pressure because it has weight. Think of the atmosphere as a very tall column of air sitting above your body.
- Atmospheric pressure is about 100,000 Pa (or 100 kPa)
- This pressure acts on every surface on Earth
- The higher you go, the less air is above you, so pressure gets lower
- The air also becomes less dense as you go higher up
The massive weight of our atmosphere means that every square metre of your body has about 100,000 newtons of force pushing down on it! That's equivalent to about 10 tonnes of weight per square metre.
How pressure changes
The pressure equation shows us two important rules:
Higher force = Higher pressure
- If you push harder on the same area, pressure increases
Smaller area = Higher pressure
- If you use the same force on a smaller area, pressure increases
- This is why stiletto heels sink into soft ground but flat shoes don't
Key relationship to remember:
- Same force, smaller area → much higher pressure
- Same area, bigger force → higher pressure
- This explains why sharp objects (small contact area) can cut through materials that blunt objects (large contact area) cannot.
Simple example
Worked Example: Calculating Pressure
A wooden block pushes down with 100 N of force on an area of 0.1 m².
Step 1: Use the pressure formula
Step 2: Substitute the values
Step 3: What happens if we turn the block on its end? If the area becomes 0.01 m²:
Result: The pressure is 10 times higher because the area is 10 times smaller!
Remember!
Key Points to Remember:
- Pressure = Force ÷ Area ()
- Pressure is measured in pascals (Pa)
- Atmospheric pressure is about 100,000 Pa
- Smaller area means higher pressure for the same force
- Pressure from fluids acts at right angles to surfaces