Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Linked: Energy Transfers and Sankey Diagrams, Efficiency | Linked practical: RP 2: Thermal Insulation
Notes
Energy can’t be created or destroyed, but in every system some energy is dissipated (wasted). It is usually stored in less useful ways, often by heating the surroundings.
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- Unwanted energy transfers can be reduced by lubrication (reduces friction between moving parts) and thermal insulation (reduces heating of the surroundings).
- The higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction across it.
- The rate of cooling of a building depends on the thickness and thermal conductivity of its walls. Thicker walls with lower thermal conductivity mean a slower rate of cooling.
Insulating a home
- Loft insulation: a thick layer of material that traps air (a poor conductor) to reduce energy transfer through the roof.
- Cavity wall insulation: fills the gap between the two layers of a wall, reducing convection in the gap.
- Double glazing: a layer of air or gas between two panes of glass reduces conduction.
- Draught excluders: reduce energy lost by warm air escaping.
See the lesson Cut the Bills – Insulating a Home for more.
Questions
Q1 (F) What does it mean when energy is ‘dissipated’?
Show answer
It is transferred to less useful stores, usually by heating the surroundings, so it is wasted.
Q2 (F) How does lubrication reduce wasted energy in a machine?
Show answer
It reduces friction between moving parts, so less energy is wasted heating the parts and surroundings.
Q3 (F) Name two ways to reduce energy loss from a house.
Show answer
Any two from: loft insulation, cavity wall insulation, double glazing, draught excluders, thicker walls.
Q4 (F/H) What does a high thermal conductivity mean?
Show answer
Energy is transferred through the material by conduction at a high rate.
Q5 (F/H) Two houses are identical except that house A has thicker walls made of a material with a lower thermal conductivity. Which house cools more slowly? Explain.
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House A. Thicker walls and a lower thermal conductivity both reduce the rate of energy transfer through the walls.
Q6 (H) Explain why loft insulation made of fibres is a good insulator.
Show answer
The fibres trap air, which has a very low thermal conductivity. Trapped air also can’t move to transfer energy by convection, so the rate of energy transfer through the roof is reduced.
Q7 (H) A motor wastes 300 J for every 1000 J supplied. Suggest one way to improve its efficiency and explain why it works.
Show answer
Lubricate the moving parts. This reduces friction, so less energy is dissipated by heating and more of the input energy is transferred usefully. (Its efficiency is currently 700 ÷ 1000 = 70%.)
