2.1.5Thermal Physics

Gases and Absolute Temperature — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 10 parts Lesson 11 of 45 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision

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Lesson summary

2.1.5 Gases and Absolute Temperature — IGCSE PHYSICS notes: Use the Kelvin scale, relate gas pressure to absolute temperature and calculate with p₁/T₁ = p₂/T₂.Cooling a gas at constant volume lowers its pressure; extending a pressure–Celsius graph backwards reaches zero pressure …

Absolute zero
Conversion
Explanation
Constant pressure
Constant temperature
Combining them gives pV ÷ T

Part 1 of 8Absolute zero and the Kelvin scale

Cooling a gas at constant volume lowers its pressure; extending a pressure–Celsius graph backwards reaches zero pressure at about −273 °C for any gas.

Absolute zero

Absolute zero the lowest possible temperature, 0 K or −273 °C, where particles have minimum kinetic energy.

The Kelvin (absolute) scale starts at absolute zero, so it has no negative values.

Conversion

T (in K) = θ (in °C) + 273, and θ = T − 273.
0 °C = 273 K and 100 °C = 373 K.

A change of 1 K is the same size as a change of 1 °C.

The kelvin is the SI unit of temperature, written K and not °K.

Part 2 of 8Temperature and kinetic energy

The Kelvin temperature is directly proportional to the average kinetic energy of the particles of a gas.

Doubling the absolute temperature doubles the average kinetic energy.

A rise from 20 °C to 40 °C is only from 293 K to 313 K, about 7%, so it is not a doubling.

Absolute zero can be approached but never reached.

Part 3 of 8Pressure and absolute temperature

For a fixed mass of gas at constant volume, pressure is directly proportional to absolute temperature (p ∝ T).

Equation

Equation p₁/T₁ = p₂/T₂.

Temperatures must be in kelvin; pressures can be in any unit if both are the same.

Explanation

Explanation higher temperature gives greater average speed, so particles hit the walls more often and harder.

Graphs:

Pressure against Kelvin temperature is a straight line through the origin.
Pressure against Celsius temperature is a straight line with an intercept at −273 °C.

Part 4 of 8Method for calculations

Convert all temperatures to kelvin by adding 273.

Check that the mass of gas and the volume are constant.

Write p₁/T₁ = p₂/T₂ and substitute.

Rearrange: p₂ = p₁ × (T₂ ÷ T₁).

Give the answer with the units of pressure used.

Using Celsius values gives a wrong answer and could suggest zero pressure at 0 °C.

Part 5 of 8Other gas relationships

Other gas relationships

Constant pressure

Constant pressure volume is proportional to absolute temperature (V ∝ T), so a gas expands on heating and hot-air balloons rise.

Constant temperature

Boyle's law, pV = constant.

Combining them gives pV ÷ T

Combining them gives pV ÷ T constant for a fixed mass of gas.

Part 6 of 8Demonstrating the pressure law

Demonstrating the pressure law

Place a sealed flask of air connected to a pressure gauge in a water bath.

Measure the pressure at several temperatures.

Plot pressure against temperature and extrapolate the line back to find absolute zero.

Part 7 of 8Celsius and Kelvin compared

	Celsius	Kelvin
Zero point	Freezing point of water	Absolute zero
Size of one degree	Same as one kelvin	Same as one degree Celsius
Use in physics equations	Not suitable	Required

A rise of 10 K equals a rise of 10 °C, but a temperature of 10 K is far colder than 10 °C.

Part 8 of 8More from the specification

Thermal expansion of gases versus solids

Thermal expansion of gases versus solids gases expand substantially with rising temperature at constant pressure, exhibiting greater volume changes than dense solids.

Kinetic molecular theory of gas pressure

Gas pressure results from the rate of change of momentum of gas molecules colliding elastically with the container walls (p = F / A).
What the symbols mean: p = pressure (Pa = N m⁻²) · F = force acting at right angles to the surface (N) · A = area (m²)

Absolute temperature on the Kelvin scale

Absolute zero is the theoretical temperature where molecular translational kinetic energy reaches minimum.
- (0 K = -273.15 °C)

Pressure-temperature relationship

For a fixed mass of gas at constant volume, gas pressure is directly proportional to absolute thermodynamic temperature: p1 / T1 = p2 / T2.

Gas pressure in a container

Gas pressure in a container gas molecules in continuous random motion collide with container walls, exerting an outward force per unit area.

Brownian motion observations

Brownian motion observations microscopic smoke particles exhibit erratic zig-zag motion when bombarded unevenly by invisible surrounding air molecules.

States of matter shape and flow

States of matter shape and flow unlike solid matter with fixed shape, liquid particles slide past each other while occupying variable container shapes.

Solid

State with a fixed shape and a fixed volume.

Brownian motion

Random jiggling of small visible particles hit by fluid molecules.

Particle model

Representing an object as a single point.

Random motion

Movement in no specific path with sudden changes at collisions.

Inversely proportional

One quantity falls as the other rises in the same ratio.

Molecule

Tiny particle of the fluid, too small to be seen.

Liquid

State with a fixed volume that takes the shape of its container.

Random

Without a pattern in speed or direction.

Collision

Short interaction with large internal forces.

Force

A push or pull measured in newtons.

p1V1

p1V1 p2V2

What the symbols mean: p₁, V₁ = pressure and volume before · p₂, V₂ = after; only for a fixed mass of gas at constant temperature

Key terms

Term	Meaning
Absolute zero	Lowest possible temperature, 0 K or −273 °C.
Kelvin	SI unit of temperature on the absolute scale.
Absolute temperature	Temperature measured in kelvin.
Pressure law	At constant volume, p is proportional to T.
Fixed mass of gas	Same number of particles throughout.

Avoid these mistakes

Using Celsius temperatures in p/T equations — convert to kelvin first.

Writing °K instead of K.

Thinking doubling the Celsius temperature doubles the pressure.

Applying p₁/T₁ = p₂/T₂ when the volume changes.

Adding 237 or 373 instead of 273.

Last updated: 18 October 2026