Learning outcomes
By the end of this lesson, students should be able to:
- analyse systems using conduction, convection and radiation together
- explain material choices in cooking equipment
- explain room heating and non-contact temperature measurement
- explain how vacuum flasks and building insulation reduce energy transfer
- evaluate insulation measures using cost, effectiveness and practical limitations
12.1 Choosing the dominant pathway
Real situations normally involve all three transfer pathways. The task is to identify which are possible and which dominate. Conduction occurs through materials in contact, convection occurs through moving liquids or gases, and infrared radiation can cross empty space or transparent media.
A good answer follows energy from a hotter region to a colder region. Name the pathway, describe the physical mechanism and connect it to the design feature. Avoid merely listing “conduction, convection and radiation” without explaining where each occurs.
12.2 Kitchen pans and heating objects
A pan base is often metal because metal conducts energy rapidly from the hotplate or flame to the food. A thick, high-conductivity base spreads energy more evenly and reduces hot spots. The handle is made of a poor conductor and may be separated from the hot body by a narrow connection.
Convection circulates water or oil within the pan, while infrared radiation and hot gases transfer energy from a flame to the pan. A dark base can absorb infrared well, though durability and cleanliness also influence design.
Figure 25. Original KG2UNI diagram.
12.3 Vacuum flask
A vacuum flask has double walls separated by a vacuum. With almost no particles in the gap, conduction and convection across it are greatly reduced. Silvered or shiny surfaces reflect infrared and are poor emitters and absorbers. An insulating stopper reduces conduction and prevents convection and evaporation at the neck.
Small supports and a narrow neck reduce remaining solid-conduction paths. The flask does not stop all energy transfer; it reduces the rate. The same design keeps cold liquids cold by reducing energy transfer from warmer surroundings into the liquid.
12.4 Building insulation
Loft insulation contains trapped air and reduces conduction through the roof while suppressing convection. Cavity-wall insulation fills spaces that might otherwise allow air circulation. Double glazing traps a layer of air or gas between panes, reducing conduction and convection. Low-emissivity coatings reduce infrared transfer.
Draught proofing prevents bulk movement of warm air out and cold air in. Thick curtains reduce conduction, convection near windows and radiation. Shiny foil behind a radiator reflects infrared into the room and reduces absorption by the wall.
Insulation decisions involve installation cost, energy-price savings, lifetime, climate, ventilation and moisture control. Airtight buildings still need safe ventilation. Evaluation questions reward balanced reasoning, not the claim that the thickest insulation is always best.
Figure 26. Original KG2UNI diagram.
12.5 Infrared thermometers
An infrared thermometer measures radiation emitted by a surface and estimates its temperature. It is useful for very hot objects, moving machinery, electrical components or situations where contamination must be avoided. It responds quickly and does not need thermal contact.
Limitations include uncertain emissivity, reflected radiation from shiny surfaces, restricted measurement spot size and inability to measure internal temperature directly. A shiny metal surface can give a misleading reading unless the device is adjusted or a suitable high-emissivity patch is used.
12.6 Writing design explanations
Use a feature-mechanism-result structure. Example: “The walls are silvered (feature), so they are poor infrared emitters and reflectors of radiation (mechanism), reducing thermal energy transfer across the gap (result).” This is more precise than saying “silver keeps heat in.”
When comparing designs, consider every pathway. A vacuum is excellent against conduction and convection but not radiation, so silvering is added. Trapped air is effective only if the pockets are small enough to prevent significant circulation.
Worked examples
Vacuum flask wall
The vacuum contains almost no particles, so conduction and convection across the gap are greatly reduced. Silvered surfaces reduce infrared transfer.
Double glazing
The trapped gas layer is a poor conductor, and the narrow sealed gap prevents large convection currents. A low-emissivity coating also reduces radiation.
Evaluating loft insulation
Thicker insulation reduces transfer further but costs more and gives diminishing financial returns. The best choice balances installation cost, expected energy savings and service life.
Practical focus
Investigation
Compare cooling in identical containers wrapped with different insulating materials. Use equal water masses and starting temperatures, identical lids and the same time interval. Calculate temperature drop. A fair test needs equal material thickness or, if comparing realistic products, the thickness must be recorded and included in the evaluation.
Examination guidance
- Use feature-mechanism-result sentences.
- A vacuum reduces conduction and convection, not radiation.
- Write “reduces the rate of energy transfer,” not “stops heat.”
Check your understanding
- Why is metal used for a pan base?
- How does a vacuum reduce energy transfer?
- Why are flask surfaces silvered?
- Give two ways double glazing reduces transfer.
- State one limitation of an infrared thermometer.
Answers
- It is a good conductor and transfers energy rapidly to the food.
- There are almost no particles for conduction, and no fluid for convection.
- Shiny surfaces are poor emitters and absorbers and reflect infrared.
- The trapped gas conducts poorly and the narrow sealed gap prevents convection; a coating may reduce radiation.
- Its reading depends on surface emissivity or it measures surface rather than internal temperature.
