Density & PressureEdexcel IGCSE Physics: Revision notes
Section 1
Density
Density is a measure of how much mass is packed into a given volume of a substance:
ρ = m / V
where ρ (rho) is density (kg/m³), m is mass (kg) and V is volume (m³). A material with a high density has a lot of mass packed into a small volume; a low-density material has less mass for the same volume.
Density can be investigated practically by directly measuring the mass (using a balance) and volume (using a measuring cylinder for liquids or irregular solids by displacement, or by calculation for regular solids) of a sample, then applying the equation.
A block has a mass of 540 g (0.54 kg) and a volume of 200 cm³ (0.0002 m³). ρ = 0.54 / 0.0002 = 2700 kg/m³.
Always check units are consistent — convert cm³ to m³ (divide by 1,000,000) and g to kg (divide by 1000) before substituting.
Section 2
Pressure
Pressure is the force acting per unit area:
p = F / A
where p is pressure (Pa), F is force (N) and A is area (m²). For the same force, a smaller area produces a larger pressure, and a larger area produces a smaller pressure — this is why sharp blades cut easily and snowshoes prevent sinking into snow.
A 60 N force acts over an area of 0.02 m². p = 60 / 0.02 = 3000 Pa.
Section 3
Pressure in fluids
The pressure at a point within a fluid (liquid or gas) at rest acts equally in all directions. This is because the fluid's molecules move randomly and collide with surfaces from every direction.
Pressure in a liquid also increases with depth, because there is more liquid (and therefore more weight) above a deeper point. This is given by:
p = h × ρ × g
where p is the pressure difference (Pa), h is the depth below the surface (m), ρ is the density of the liquid (kg/m³) and g is gravitational field strength (N/kg).
Find the pressure at a depth of 5 m in water (ρ = 1000 kg/m³, g = 10 N/kg). p = 5 × 1000 × 10 = 50,000 Pa.
This equation gives the pressure due to the liquid above a point, which is added to atmospheric pressure for total pressure at that depth — but the p = hρg formula itself is only the liquid's contribution.
Must Know
- density, ρ = m / V, measured in kg/m³
- pressure, p = F / A, measured in pascals (Pa); smaller area gives larger pressure for the same force
- pressure at a point in a fluid at rest acts equally in all directions
- pressure in a liquid increases with depth: p = h × ρ × g
- density can be measured practically using mass (balance) and volume (measuring cylinder or displacement)
- always convert units (cm³→m³, g→kg) before substituting into equations
That's the notes covered.
Carry on to the next subtopic.