Heat transfer: conduction, convection and radiation (QCE Physics Unit 1)

What this dot point is asking

QCAA wants you to identify the three modes of heat transfer, explain each in microscopic terms, and apply them to insulating systems (vacuum flasks, double glazing, roof insulation, thermal radiators).

Conduction

Conduction is heat transfer through a material by direct particle-to-particle interactions. In solids, fast-moving particles collide with slower neighbours and transfer kinetic energy through the lattice. In metals, free electrons also conduct heat (which is why metals feel cold to the touch even at room temperature: they conduct heat away from your hand quickly).

The rate of conduction depends on:

• material (high thermal conductivity for metals, low for insulators like wool or polystyrene),
• cross-sectional area (larger area = more heat per second),
• thickness (thicker = slower),
• temperature gradient (greater $\Delta T$ across a given thickness = faster heat flow).

Conduction is dominant in solids and is also present in liquids and gases (but is usually small compared to convection in fluids).

Convection

Convection is heat transfer through a fluid (gas or liquid) by bulk movement of the fluid itself. Hot fluid expands, becomes less dense, and rises. Cool fluid sinks to take its place. The result is a circulating current that transports heat from one region to another.

Natural convection is driven by density differences (a heater in a room sets up a circulating air current). Forced convection uses a fan or pump (a hairdryer, a car radiator).

Convection cannot occur in a solid (the fluid cannot move) or in a vacuum (there is no fluid).

Radiation

Radiation is the emission of electromagnetic waves (mostly infrared at terrestrial temperatures) by any body with a temperature above absolute zero. Radiation does not require a medium and is the only mode that operates across a vacuum.

The Stefan-Boltzmann law gives the power radiated per unit area:

where $\sigma = 5.67 \times 10^{-8}$ W m$^{-2}$ K$^{-4}$ and $\varepsilon$ is the emissivity (between $0$ and $1$). Black surfaces absorb and emit well ($\varepsilon$ near $1$); shiny silvered surfaces absorb and emit poorly. This is why solar collectors are painted matte black and vacuum flasks are silvered.

Radiation depends on the fourth power of temperature, so it dominates at high temperatures (the inside of a furnace, the surface of the Sun).

Application: keeping a building energy-efficient

Building insulation targets all three modes.

• Roof insulation (batts, fibreglass) reduces conduction by trapping air pockets and reduces convection by stopping the air from flowing.
• Double glazing has a gap (sometimes evacuated, often filled with argon) that cuts conduction and convection.
• Low-e coatings on glass have low emissivity, so they re-emit far less infrared back outside.

Examples in context

Example 1. A Townsville Queenslander uses reflective foil sarking under the corrugated steel roof. The foil's low emissivity (about $0.05$) cuts radiative heat transfer from the hot iron into the ceiling cavity by roughly twenty-fold compared with bare timber rafters. Conduction through the batts and ceiling plaster, governed by $Q/t = kA\Delta T / d$, is the dominant remaining mechanism, so installers add R$3.5$ glasswool to drop $k/d$. Convection is suppressed by sealing eaves and ridge ducts. The QCAA Unit 1 thermal-design context links all three mechanisms to a single residential thermal balance.

Example 2. A Bundaberg sugar mill steam line at $180^\circ \text{C}$ runs $60 \text{ m}$ to a centrifuge. Bare steel pipe would lose heat by convection to the ambient $25^\circ \text{C}$ air and by radiation from the high-emissivity painted surface at roughly $1.5 \text{ kW per metre}$. Engineers wrap with $50 \text{ mm}$ rockwool plus an aluminium cladding (emissivity $0.05$), cutting convective losses by lowering surface temperature to $35^\circ \text{C}$ and radiative losses through reflectivity. Total saving exceeds $80 \text{ kW}$, a worked example that recurs in QCAA EA Unit 1 industrial-context stems.

Try this

Q1. Distinguish between conduction and convection in one sentence each. [2 marks]

• Cue. Conduction: particle collisions in a solid; convection: bulk fluid flow driven by density differences.

Q2. A glass window of area $2.0 \text{ m}^2$ and thickness $4.0 \text{ mm}$ has interior $20^\circ \text{C}$ and exterior $5^\circ \text{C}$. Given $k = 0.80 \text{ W m}^{-1} \text{ K}^{-1}$, calculate the conduction heat flow rate. [3 marks]

• Cue. $Q/t = kA\Delta T/d = 0.80 \times 2.0 \times 15 / 0.004 = 6000 \text{ W}$.

Q3. A Townsville roof cavity reaches $60^\circ \text{C}$ on a summer day. (a) Identify which heat-transfer mechanism dominates between the roof iron and the ceiling, and justify. (b) Calculate the radiative loss per square metre if emissivity is $0.95$ and the ceiling surface is $30^\circ \text{C}$, using $P = \sigma \varepsilon A T^4$ (data given). (c) Recommend two design measures with reasoning. [2+3+3 marks; ISMG: Analysis and interpretation, Evaluation]

• Cue. (a) Radiation across the air gap dominates; (b) net $\approx 256 \text{ W m}^{-2}$; (c) foil sarking lowers emissivity, batts cut conduction.