Thermal Physics

Summary: Covers the kinetic particle model of matter (solids, liquids, gases), Brownian motion, evaporation vs boiling, thermal expansion, specific heat capacity, specific latent heat, the gas laws (Boyle’s, Charles’, Pressure Law), and absolute zero (Kelvin scale). This topic connects microscopic particle behaviour to macroscopic thermal properties. Tags: igcse physics thermal heat kinetic-theory gas-laws Created: 2026-07-16 Last Updated: 2026-07-16

The Kinetic Particle Model of Matter

All matter consists of tiny particles (atoms, molecules, or ions) in constant random motion. The amount of motion depends on temperature — higher temperature = faster average particle speed.

Forces between particles:

• Solids: strong forces hold particles in fixed positions
• Liquids: weaker forces allow particles to move past each other but stay close
• Gases: negligible forces — particles are effectively free (except during collisions)

Pressure of a gas is caused by particles colliding with the walls of the container. Each collision exerts a tiny force; the combined effect of billions of collisions per second creates pressure.

Brownian Motion

Brownian motion is the random, jerky movement of small particles (e.g., smoke particles in air, pollen grains in water) when observed under a microscope.

Explanation: The visible particles (smoke/pollen) are being bombarded unevenly by much smaller, invisible particles (air/water molecules) moving randomly. When more molecules hit one side than the other, the particle moves in that direction.

Significance: Brownian motion provided crucial evidence for the existence of atoms/molecules and the particle theory of matter — it shows that invisible particles are in constant random motion.

Evaporation vs Boiling

Both change a liquid to a gas, but they are different processes:

The cooling effect of evaporation:

• The fastest-moving (most energetic) particles at the surface escape from the liquid
• The average kinetic energy of the remaining particles decreases
• Since temperature depends on average KE, the temperature of the remaining liquid decreases
• This is why sweating cools the body

Factors affecting the rate of evaporation:

1. Temperature: higher temperature → faster evaporation (more particles have enough KE to escape)
2. Surface area: larger surface area → faster evaporation (more particles near the surface)
3. Air movement (draught): more air movement → faster evaporation (escaped particles are carried away, preventing them from returning)

Thermal Expansion

Most substances expand when heated and contract when cooled.

Why? When heated, particles gain kinetic energy, vibrate more vigorously, and take up more space (average separation increases).

Relative expansion:

• Solids expand least
• Liquids expand more than solids
• Gases expand most

Important exception: Water expands on freezing (ice is less dense than liquid water — this is why ice floats, and why pipes burst when water freezes inside them).

Practical Applications of Thermal Expansion

Specific Heat Capacity

Definition: The specific heat capacity (c) of a substance is the energy required to raise the temperature of 1 kg of the substance by 1°C (or 1 K).

Energy transferred = mass × specific heat capacity × temperature change
E = mcΔθ

• E = energy (J), m = mass (kg), c = specific heat capacity (J/kg°C), Δθ = temperature change (°C or K)

Key values:

• Water: c = 4200 J/kg°C (very high — water is excellent for thermal storage and cooling)
• Aluminium: c ≈ 900 J/kg°C
• Copper: c ≈ 385 J/kg°C

Example 1: How much energy is needed to heat 2 kg of water from 20°C to 80°C? (c = 4200 J/kg°C)

E = mcΔθ = 2 × 4200 × (80 − 20) = 2 × 4200 × 60 = 504,000 J = 504 kJ

Example 2: A 500 W electric heater heats 0.5 kg of oil for 2 minutes. The temperature rises by 48°C. Calculate the specific heat capacity of the oil.

Energy supplied = Pt = 500 × (2 × 60) = 500 × 120 = 60,000 J
c = E / (mΔθ) = 60,000 / (0.5 × 48) = 60,000 / 24 = 2500 J/kg°C

Experiment: Measuring Specific Heat Capacity (Electrical Method)

1. Measure the mass (m) of the solid block (e.g., aluminium) using a balance
2. Insert an electric heater and a thermometer into holes in the block
3. Wrap the block in insulation to minimise heat loss
4. Record the initial temperature (θ₁)
5. Switch on the heater and start a stopwatch simultaneously
6. Record the current (I), potential difference (V), and time (t) — so energy supplied = IVt
7. Record the final temperature (θ₂) after a suitable rise
8. Calculate c = IVt / m(θ₂ − θ₁)

Sources of error: heat loss to surroundings, not all heat transferred from heater to block. Use insulation and allow for cooling correction to improve accuracy.

Specific Latent Heat

Definition: The specific latent heat (L) of a substance is the energy required to change the state of 1 kg of the substance without changing its temperature.

Energy transferred = mass × specific latent heat
E = mL

• E = energy (J), m = mass (kg), L = specific latent heat (J/kg)

There are two types:

• Specific latent heat of fusion (L_f) : solid ⇌ liquid (melting/freezing)
• Specific latent heat of vaporisation (L_v) : liquid ⇌ gas (boiling/condensing)

Key concept: During a state change, the temperature remains constant — all the energy supplied goes into breaking bonds between particles (increasing potential energy), not increasing their kinetic energy. The flat sections on a heating/cooling curve indicate state changes.

Important: L_v is always much larger than L_f. It takes far more energy to vaporise water than to melt ice because in vaporisation, particles must be separated completely (overcoming all attractive forces).

Example 3: The specific latent heat of fusion of ice is 334,000 J/kg. How much energy is needed to melt 2 kg of ice at 0°C without changing its temperature?

E = mL = 2 × 334,000 = 668,000 J = 668 kJ

Example 4: A 50 W heater is used to boil water. In 10 minutes, 0.013 kg of water is turned into steam. Calculate the specific latent heat of vaporisation of water.

Energy supplied = Pt = 50 × (10 × 60) = 50 × 600 = 30,000 J
L_v = E / m = 30,000 / 0.013 ≈ 2,310,000 J/kg ≈ 2.3 × 10⁶ J/kg

The Gas Laws

The behaviour of a fixed mass of an ideal gas is described by three gas laws. Temperature (T) must always be in Kelvin (K) for gas law calculations.

Boyle’s Law (constant temperature)

For a fixed mass of gas at constant temperature:

p₁V₁ = p₂V₂   or   pV = constant

• Pressure and volume are inversely proportional: as volume increases, pressure decreases
• Graph: p vs 1/V is a straight line through origin

Charles’ Law (constant pressure)

For a fixed mass of gas at constant pressure:

V₁ / T₁ = V₂ / T₂   or   V / T = constant

• Volume and Kelvin temperature are directly proportional
• Graph: V vs T(K) is a straight line through origin

Pressure Law (constant volume)

For a fixed mass of gas at constant volume:

p₁ / T₁ = p₂ / T₂   or   p / T = constant

• Pressure and Kelvin temperature are directly proportional
• Graph: p vs T(K) is a straight line through origin

Example 5 (Boyle’s Law): A gas occupies 2.0 m³ at a pressure of 100 kPa. The temperature stays constant while the volume changes to 0.5 m³. Find the new pressure.

p₁V₁ = p₂V₂ → 100 × 2.0 = p₂ × 0.5 → p₂ = 200 / 0.5 = 400 kPa

Example 6 (Charles’ Law): A gas occupies 300 cm³ at 27°C. At constant pressure, what volume does it occupy at 127°C?

T₁ = 27 + 273 = 300 K,  T₂ = 127 + 273 = 400 K
V₁/T₁ = V₂/T₂ → 300/300 = V₂/400 → V₂ = 400 cm³

Absolute Zero and the Kelvin Scale

Absolute zero is the lowest possible temperature: −273°C = 0 K (zero kelvin).

At absolute zero:

• Particles have the minimum possible energy (for IGCSE purposes, effectively zero kinetic energy)
• The pressure and volume of an ideal gas would theoretically be zero

Conversion:

T(K) = T(°C) + 273

• The Kelvin scale does not use the degree symbol — it is simply “kelvin” (K), not “degrees Kelvin”
• A temperature change of 1 K is exactly the same as a change of 1°C

Sources

• BBC Bitesize GCSE Physics — Particle model topic, BBC (free educational resource)
• OpenStax College Physics — Heat chapter, Rice University (free, CC BY 4.0)
• Cambridge IGCSE Physics 0625 — Thermal physics section, Cambridge Assessment International Education
• CK-12 Physics for High School — Thermal Physics chapter, CK-12 Foundation (free, CC BY-NC 3.0)

Related Notes

• Forces and Motion — Kinetic energy of particles relates to temperature
• Energy Resources and Transfer — Thermal energy transfer by conduction, convection, radiation
• Waves — Sound waves travel through media by particle vibration
• IGCSE-Phys-Index — Full IGCSE Physics index

Keywords

kinetic model, Brownian motion, evaporation, boiling, thermal expansion, bimetallic strip, specific heat capacity, specific latent heat, latent heat of fusion, latent heat of vaporisation, Boyle's Law, Charles' Law, Pressure Law, absolute zero, Kelvin scale, temperature, pressure, state change

Common Misconceptions