PressureCopy

Cheat Sheet: Pressure (O Level / IGCSE Physics)

1. Definition of Pressure

• Pressure is defined as force per unit area
• Equation:
  p = F / A
  where:
  p = pressure (Pa or N/m²)
  F = force (N)
  A = area (m²)

2. Pressure, Force, and Area in Context

• Pressure increases when:
  • Force increases (e.g. stiletto heels vs flat shoes)
  • Area decreases (e.g. knife edge vs blunt edge)
• Everyday Examples:
  • Snowshoes (increase area, reduce pressure on snow)
  • Drawing pins (sharp point = high pressure = easier to pierce surface)
  • Tank tracks (large area to reduce pressure on ground)

3. Pressure Acts Perpendicularly

• Pressure at a surface produces a force at right angles to the surface
• Experiment to Show This:
  • Place a sheet of paper on a table
  • Blow air through a straw vertically downward near the center
  • Paper bends downward, showing air pressure acts perpendicularly

4. Barometer and Atmospheric Pressure

• A liquid barometer (e.g. mercury barometer) uses the height of a mercury column to measure atmospheric pressure
• Principle:
  Atmospheric pressure = pressure due to mercury column
  = ρ × g × h
• As atmospheric pressure increases, the height of mercury rises
• Unit conversion:
  760 mmHg ≈ 1 atm ≈ 101,325 Pa

5. Pressure in Liquids (With Depth and Density)

• Pressure increases with depth and with density
• Deeper = more liquid weight above = more pressure
• Denser liquid = greater weight = more pressure

6. Equation for Pressure in Liquids

• Equation:
  ∆p = ρ × g × ∆h
  where:
  ∆p = change in pressure (Pa)
  ρ = density of liquid (kg/m³)
  g = gravitational field strength (9.8 N/kg)
  ∆h = change in height/depth (m)
• Applies to any fluid under gravity in a container