Describe and distinguish between conduction, convection and radiation as mechanisms of heat transfer, with reference to everyday and industrial applications

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.

Common traps

Calling convection "heat rising". Heat is not a thing that rises. Hot fluid rises because it is less dense. Convection is the resulting current.

Saying radiation needs a medium. Radiation passes through vacuum (this is how the Sun warms the Earth).

Confusing absorption with emission. Good absorbers are good emitters (Kirchhoff's radiation law). Matte black is both. Shiny silver is neither.

Treating the wind as conduction. Wind cooling a person is forced convection, not conduction.

In one sentence

Heat transfer occurs by conduction (particle collisions in a solid or fluid, no bulk motion), convection (bulk fluid motion driven by density differences in a gas or liquid) and radiation (emission of electromagnetic waves with $P/A = \varepsilon \sigma T^4$, the only mode that crosses a vacuum), and good insulators reduce all three by trapping still air, using vacuum gaps, and adding low-emissivity surfaces.