Thermal Energy Transfer: Question 4
Syllabus 2.3.1
Aluminium window frames conduct thermal energy far more readily than uPVC (a rigid plastic) window frames of the same thickness, which is why many homes now use uPVC frames to reduce heat loss through windows in winter.
Which statement correctly explains, at the particle level, why the uPVC frame conducts thermal energy so much more slowly than the aluminium frame?
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Worked solution
Step 1: Recall conduction in solids in general
In any solid, particles are held in fixed positions but vibrate about them. When one particle is heated, it vibrates more, and this extra vibration is passed on to neighbouring particles, gradually transferring thermal energy through the material. This vibration mechanism happens in all solids, both aluminium and uPVC.
Step 2: Recall what is different about a metal like aluminium
Aluminium is a metal, so it contains free (delocalised) electrons that are not tied to any one atom and can move freely throughout its structure. These free electrons gain kinetic energy in hotter regions and carry it rapidly to cooler regions, in addition to the vibration mechanism.
uPVC is a non-metal (a plastic), so it has no such free electrons. It can only transfer thermal energy by the much slower process of vibrations passing from particle to particle.
Step 3: Combine both mechanisms for aluminium
Aluminium therefore conducts thermal energy by two mechanisms working together:
- Lattice vibrations passing between neighbouring atoms (as in any solid, including uPVC).
- Free (delocalised) electrons moving through the metal and carrying energy much faster than vibration alone.
uPVC relies on vibration alone, which is why it conducts thermal energy so much more slowly than aluminium of the same thickness, and why uPVC window frames lose far less heat from a warm room.
Step 4: Match to the correct option
This is exactly what option C describes.
Why the other options are wrong
| Option | Error |
|---|---|
| A | uPVC’s particles do still vibrate and do transfer some thermal energy by conduction; also, convection cannot occur “within a solid material”. Convection requires a fluid that can flow. |
| B | Free (delocalised) electrons able to move through the whole structure are a feature of metals; a non-metal such as uPVC does not have them at all, regardless of speed. |
| D | Particle mass and vibration speed are not the main reason for the huge difference in conductivity; this explanation ignores the free electrons that are mainly responsible for a metal’s high conductivity. |
Final answers
- uPVC conducts thermal energy poorly because it has no free (delocalised) electrons and relies only on the slower vibration mechanism, unlike aluminium, which has both, option C.