2.3.4 Consequences of Thermal Energy Transfer
Learning objective
Explain the everyday applications and consequences of the three modes of heat transfer – conduction, convection and radiation – and recognise situations where more than one mode operates simultaneously (as required by the Cambridge IGCSE 0625 syllabus).
2.3.4.1 Conduction
What it is
Conduction is the transfer of thermal energy through a material without the material itself moving. Energy is passed from high‑energy (hot) particles to neighbouring low‑energy (cool) particles.
Key ideas for the syllabus
- Good conductors vs. insulators: Metals (e.g. copper, aluminium) have high thermal conductivity \$k\$ and transfer heat quickly; wood, plastic and air have low \$k\$ and act as insulators.
- Simple classroom experiment: Place a metal rod and a wooden rod in contact with a hot water bath at one end and a cold water bath at the other. Observe that the metal rod becomes hot much faster – a visual demonstration of the difference in conductivity.
Everyday application – heating a kitchen pan (syllabus example of conduction)
- The stove (gas flame or electric coil) supplies heat at a high temperature.
- Heat is conducted through the metal of the pan to its bottom surface and then into the food.
- Metals such as aluminium and copper have large \$k\$ values, so they heat quickly and spread temperature evenly.
- Handles are made from low‑\$k\$ materials (wood, plastic, silicone) to minimise heat flow to the hand.
Optional “extra‑credit” formula
Fourier’s law (rate of heat flow through a uniform rod)
\$\dot Q = -k\,A\,\frac{dT}{dx}\$
- \$k\$ – thermal conductivity (W m⁻¹ K⁻¹)
- \$A\$ – cross‑sectional area (m²)
- \$\dfrac{dT}{dx}\$ – temperature gradient (K m⁻¹)
Note: the formula is not required for the core syllabus but is useful for extended learners.
2.3.4.2 Convection
What it is
Convection involves the bulk movement of a fluid (liquid or gas) that carries thermal energy with it. When a fluid is heated it becomes less dense, rises, and cooler fluid descends to replace it, creating a circulating current.
Key ideas for the syllabus
- Simple classroom experiment: Heat water in a clear beaker, sprinkle a few drops of food colouring on the surface and watch the coloured streams rise as warm water circulates – a visual illustration of convection currents.
Everyday application – heating a room by convection (syllabus example of convection)
- A radiator or electric heater warms the air in immediate contact with its surface.
- Warm air becomes less dense and rises toward the ceiling.
- Cooler air near the floor moves toward the heater, completing a continuous circulation.
- This convective current distributes heat throughout the room, raising the overall temperature.
Factors that affect the rate of heating (syllabus‑relevant)
- Surface area (\$A\$): Larger radiators transfer more heat.
- Convective heat‑transfer coefficient (\$h\$): Fans, open windows or natural drafts increase \$h\$.
- Room geometry: High ceilings or obstructed floor plans can slow the upward flow of warm air.
Optional “extra‑credit” formula
Heat transferred by convection from a surface
\$Q = h\,A\,(Ts - T\infty)\,t\$
- \$h\$ – convective heat‑transfer coefficient (W m⁻² K⁻¹)
- \$A\$ – surface area of the heated object (m²)
- \$T_s\$ – surface temperature (K)
- \$T_\infty\$ – temperature of the fluid far from the surface (K)
- \$t\$ – time of exposure (s)
Note: this equation is beyond the core requirement but helps deepen understanding.
2.3.4.3 Radiation
What it is
Radiation is the transfer of energy by electromagnetic waves. All bodies emit radiation; the amount and wavelength depend on temperature.
Key ideas for the syllabus
- Effect of colour and texture: Dark, matte surfaces have high emissivity and absorb/emit radiation efficiently; light, shiny surfaces reflect most radiation and have low emissivity.
- Simple classroom experiment: Place a black sheet of paper and a white sheet of paper under an incandescent lamp. After a few minutes, feel that the black paper is noticeably warmer – a demonstration of differing emissivity.
Everyday relevance (syllabus example of radiation)
- Sunlight heating a room through windows (radiation from the Sun, no medium required).
- Heat loss from a poorly insulated wall – low‑emissivity (reflective) coatings reduce radiative loss.
- Infra‑red cookers that heat food directly by radiation.
Optional “extra‑credit” formula (Stefan‑Boltzmann law)
\$P = \varepsilon \,\sigma \,A\, T^{4}\$
- \$P\$ – radiated power (W)
- \$\varepsilon\$ – emissivity of the surface (0 – 1)
- \$\sigma = 5.67\times10^{-8}\ \text{W m}^{-2}\text{K}^{-4}\$ – Stefan‑Boltzmann constant
- \$A\$ – emitting area (m²)
- \$T\$ – absolute temperature (K)
Beyond the core syllabus; useful for students who wish to explore quantitative aspects.
2.3.4.4 Combined Modes – Situations Where More Than One Mode Is Important
Many everyday processes involve a mixture of conduction, convection and radiation. The syllabus asks for at least one example; two are given below.
- Fire (burning wood or coal) – conduction through the solid fuel, convection of hot gases and fresh air, and radiation from the glowing flames.
- Cooking a stew on a stove – conduction through the metal pot, convection within the liquid as hot pockets rise and cooler liquid descends, and radiation from the burner or flame onto the pot’s outer surface.
2.3.4.5 Comparison of the Three Modes
| Aspect | Conduction | Convection | Radiation |
|---|
| Medium required | Solid (or stationary fluid) | Moving fluid (liquid or gas) | None – works in vacuum |
| Typical speed of transfer | Slow to moderate (depends on \$k\$) | Moderate to fast (depends on \$h\$ and fluid motion) | Very fast (speed of light) |
| Key controlling factor | Thermal conductivity \$k\$ | Convective coefficient \$h\$ and density differences | Temperature to the fourth power \$T^{4}\$ and emissivity \$\varepsilon\$ |
| Everyday example (syllabus) | Heating a metal pan on a stove | Room heating by a radiator | Sun warming a room through a window |
2.3.4.6 Summary
Understanding how thermal energy moves by conduction, convection and radiation enables us to design efficient cooking equipment, heating systems, and building insulation. Recognising when several modes act together (e.g., fire, cooking a stew) helps predict real‑world behaviour and choose appropriate materials, shapes, and placements for the desired thermal outcome.