Theme B — The Particulate Nature of Matter

B.1.10 Thermal Conduction — IB Physics HL Revision Notes & Topical Questions

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B.1.10 Thermal Conduction — IB PHYSICS SL notes: Explain conduction in solids in terms of particles and use thermal conductivity in the form Q/t = kAΔT/Δx.Methods — thermal energy moves from a hotter to a cooler region by conduction, convection or radiation.Cooling — objects lose…

Mug of tea
Main method
Liquids and gases
Insulation by air
Lattice vibrations
Free electrons

Part 1 of 9Three transfer mechanisms

Methods

Methods thermal energy moves from a hotter to a cooler region by conduction, convection or radiation.

Cooling

Cooling objects lose heat until they reach thermal equilibrium with their surroundings.

Mug of tea

Mug of tea it loses energy by all three methods at once.

Part 2 of 9Conduction in solids

Conduction

Conduction the transfer of thermal energy through a material without bulk movement of the material.

Main method

Main method it is the main method of transfer in solids, and it requires a temperature difference.

Contact

Contact when two solids at different temperatures touch, energy passes from the hotter to the cooler.

Direction

Direction energy flows until thermal equilibrium is reached.

Liquids and gases

Liquids and gases they are poorer conductors because the particles are further apart, and gases are the poorest.

Insulation by air

Insulation by air trapped air in wool or foam is an excellent insulator because it conducts poorly and its small pockets prevent convection.

Part 3 of 9Two particle mechanisms

Two particle mechanisms

Lattice vibrations

Lattice vibrations the atoms at the hot end vibrate more and collide with neighbours, passing energy from atom to atom.
It occurs in all solids, metals and non-metals alike.

Free electrons

Free electrons metals have many delocalised electrons that collide with atoms and speed up the transfer.
Metals reach thermal equilibrium faster than non-metals.

Good conductors

Good conductors metals, which are good conductors of both heat and electricity.

Poor conductors

Poor conductors plastic and glass, which are also poor electrical conductors, suggesting a link between the two mechanisms.

More free electrons

More free electrons more delocalised electrons per unit volume means better thermal conduction.
- (diamond is an exception)

Part 4 of 9Thermal conductivity

Thermal conductivity k

Thermal conductivity k the ability of a substance to transfer heat by conduction, in W m⁻¹ K⁻¹.

Excellent conductors

Excellent conductors a high k, a fast transfer rate and usually many delocalised electrons.

Insulators

Insulators a low k, a slow transfer rate and few delocalised electrons.

Substance	k / W m⁻¹ K⁻¹
Air	0.024
Rubber	0.13
Water	0.6
Ice	1.6
Iron	80
Copper	400
Silver	429
Diamond	1600

Part 5 of 9Rate of energy transfer

Equation

Q/t = kAΔT/Δx.
Q/t is the flow of thermal energy per second in W.
k is the thermal conductivity in W m⁻¹ K⁻¹.
A is the cross-sectional area in m².
ΔT is the temperature difference in K or °C.
Δx is the thickness in m.

Proportions

Q/t is proportional to A and ΔT and inversely proportional to the thickness.

Temperature gradient

ΔT/Δx; energy flows down the gradient.

Steady state

Steady state in a lagged uniform rod the temperature falls linearly and the energy flow is the same through every cross-section.

Before steady state

Before steady state energy warms each portion of the rod, so the temperature profile curves and changes with time.

Part 6 of 9Layers in series

Same power

Same power if the area is constant, the same rate of energy flow passes through every layer.

Analogy

Analogy it is like a constant current through components of different resistance in a series circuit.

Equation

Equation k₁ΔT₁/Δx₁ = k₂ΔT₂/Δx₂ = k₃ΔT₃/Δx₃.

Equal rods

Equal rods with identical dimensions k₁ΔT₁ = k₂ΔT₂, so the poorer conductor has the larger temperature drop.

Method

Method write the temperature differences of the layers in terms of the junction temperatures and solve the simultaneous equations.

Double glazing

Double glazing the poor conductor carries most of the temperature drop.
- (a layer of air or inert gas)

Part 7 of 9Applications and explanations

Applications and explanations

Saucepans

Saucepans metal for rapid transfer, plastic handles to prevent burns.

Houses

Houses loft insulation and cavity walls reduce the rate of loss.

Metal feels colder than wood

Metal feels colder than wood at the same temperature, the better conductor removes energy from the hand faster.

Insulation

Insulation use a low k, trapped air, a thick layer and a small area.

Question cue

Question cue if a question mentions thermal energy transfer in metals, the answer is likely about conduction.

Part 8 of 9Comparing conductors experimentally

Wax method

Wax method rods of different metals are coated with wax and heated at one end;
- the distance the wax melts in a given time shows the relative conductivity.

Better method

Better method thermometers or sensors at fixed distances measure temperature gradients in the steady state.

Part 9 of 9More from the specification

Terrestrial geothermal conduction through rock

Terrestrial geothermal conduction through rock geothermal heat flows outward from Earth's molten core via conduction through solid crustal lithosphere rocks.

Thermal conduction

Thermal conduction energy passes through a material from particle vibrations and, in metals, by diffusion of free electrons.

Rate of conduction

Rate of conduction depends on cross-sectional area, temperature gradient and thermal conductivity.

Specific heat capacity

Specific heat capacity energy needed to change the temperature of a unit mass by one kelvin.

Latent heat of vaporisation

Latent heat of vaporisation energy to change liquid to gas without temperature change;
- molecular kinetic energy is unchanged and potential energy rises.

Phase change

Phase change particles gain potential energy as their spacing changes,
- which affects density and luminosity discussions of stars through energy transfer.

Fusion

Joining of light nuclei into a heavier one.

Intensity

Power per unit area perpendicular to the wave direction.

Kinetic energy

K = ½mv², energy of motion.

Apparent brightness

The power received per unit area at the observer

Phase change

Change of matter from one state to another

Internal energy

Total kinetic and potential energy of the particles in a system

Surface temperature

Effective temperature of the star's radiating surface.

Potential energy

Energy stored by the configuration of a system.

Molecule

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

Luminosity

The total power of radiation emitted by a star.

Capacity

Charge a cell can supply, in A h or C.

Density

Mass per unit volume.

Wavelength

Distance between a point on one wave and the same point on the next wave

Key terms

Term	Meaning
Conduction	Transfer of thermal energy through a material by particle vibrations and collisions
Thermal conductivity	Ability of a substance to conduct heat, k in W m⁻¹ K⁻¹
Temperature gradient	Change in temperature per unit distance, ΔT/Δx
Delocalised electrons	Free electrons in a metal that move through the lattice and carry energy
Insulator	Material with a low thermal conductivity
Steady state	State in which temperatures along a rod no longer change with time

Avoid these mistakes

Explaining that a metal feels colder because it is at a lower temperature — it conducts energy away faster.

Saying conduction occurs in a vacuum — it needs particles.

Forgetting that the thickness lowers the rate — Q/t is inversely proportional to Δx.

Thinking only metals conduct — all solids conduct through lattice vibrations.

Using different power through layers in series — the same power flows through each.

Last updated: 15 October 2026