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GCSE Physics 08 — Particle Model of Matter

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Topics include Density and Measurement, The Particle Model: Solids, Liquids, and Gases, Changes of State, Internal Energy, Specific Heat Capacity (Theory), Required Practical: Specific Heat Capacity, Specific Latent Heat, and Heating and Cooling Curves.

Physics EN
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Density and Measurement

Density is a measure of the mass per unit volume of a substance, defined by the equation ρ=mV\rho = \frac{m}{V}. It depends on the material's particle arrangement and spacing.

Key points

  • Equation: ρ=mV\rho = \frac{m}{V} where ρ\rho is density (kg/m3\text{kg/m}^3), mm is mass (kg), and VV is volume (m3\text{m}^3).
  • Unit Conversion: 1 g/cm3=1000 kg/m31 \text{ g/cm}^3 = 1000 \text{ kg/m}^3.
  • States: Solids are generally densest (particles closely packed); gases are least dense (particles far apart). Water is an anomaly (ice is less dense than liquid water).
  • Floating: Objects float if their density is lower than the fluid's density.

Worked example

Question

A student measures the mass of an irregular stone as 60 g. Using a displacement can, they collect 24 cm³ of displaced water. Calculate the density of the stone in kg/m³.

Solution

1. Calculate density in g/cm³:
ρ=mV=6024=2.5 g/cm3\rho = \frac{m}{V} = \frac{60}{24} = 2.5 \text{ g/cm}^3
2. Convert to kg/m³:
Multiply by 1000: 2.5×1000=2500 kg/m32.5 \times 1000 = 2500 \text{ kg/m}^3.
Interpretation: The stone is significantly denser than water (1000 kg/m³), so it will sink.

Common pitfalls

  • Forgetting to subtract the mass of the measuring cylinder/beaker when measuring liquids (taring).
  • Assuming solids are always denser than liquids (e.g., wood vs water, ice vs water).
  • Parallax error when reading measuring cylinders (read at eye level to the bottom of the meniscus).

Prerequisites

  • Basic algebra (rearranging equations)
  • Volume formulae for regular shapes
Further resources