📖 In-Depth Theory
Thermal Conductivity
THERMAL CONDUCTIVITY measures how well a material transfers thermal energy by conduction.
Good THERMAL CONDUCTORS: metals (copper, aluminium, iron). Transfer energy quickly. Free electrons carry thermal energy through the material.
Good THERMAL INSULATORS: air, wood, fibreglass, polystyrene, wool. Transfer energy slowly. No free electrons; energy transferred only by vibration between tightly packed or sparse particles.
Unit of thermal conductivity: W/m·K (watts per metre per kelvin).
Higher thermal conductivity → energy transferred faster for the same temperature difference and thickness.
Examples:
Copper: ~400 W/m·K — excellent conductor.
Glass: ~1 W/m·K — poor conductor.
Air: ~0.025 W/m·K — excellent insulator.
Reducing Unwanted Energy Transfers
All energy transfers involve some unwanted dissipation — energy transferred to the thermal store of the surroundings.
METHODS TO REDUCE THERMAL ENERGY LOSS:
1. INSULATION:
Surround objects with poor conductors.
Cavity wall insulation (fibreglass or foam) — reduces conduction through walls.
Loft insulation — reduces conduction through roof.
Double-glazing — air gap between panes of glass reduces conduction.
Foam lagging on hot water pipes — reduces heat loss to surroundings.
2. LUBRICATION:
Reduces friction between moving surfaces.
Less friction → less thermal energy dissipated.
Machine oil, grease used in engines, bearings, chains.
3. STREAMLINING:
Reduces air resistance on moving vehicles.
Less drag → less energy wasted overcoming resistance.
4. ELECTROMAGNETIC SHIELDING:
Reduces energy loss from electrical components.
THICKNESS AND THERMAL CONDUCTIVITY:
Thicker insulation → less energy transferred per second (for same temperature difference).
More insulating material (lower conductivity) → less energy transferred.
Energy lost per second ∝ thermal conductivity × area × (temperature difference) ÷ thickness.
Required Practical — Thermal Insulation
REQUIRED PRACTICAL (RP2 — physics only):
Investigate the effectiveness of different materials as thermal insulators.
METHOD:
Wrap beakers of hot water in different materials (wool, bubble wrap, newspaper, foil).
Measure temperature of water at regular time intervals.
Plot temperature-time graphs for each material.
Compare rate of cooling — steeper gradient = less effective insulator.
VARIABLES:
Independent: type of insulating material.
Dependent: rate of cooling (temperature change per unit time).
Controlled: initial temperature, volume of water, thickness of insulation, surface area.
CONCLUSION:
Material with lowest thermal conductivity → slowest cooling → best insulator.
Air is often the best insulator — sealed air pockets in fibreglass work well.
APPLICATIONS:
Building insulation — reduces heating bills and carbon footprint.
Refrigeration — insulated walls slow thermal energy entering the cold space.
Cryogenics — extreme insulation to maintain very low temperatures.
⚠️ Common Mistake
INSULATORS do not stop heat transfer — they slow it down. A perfect insulator doesn't exist. Trapped air is one of the best insulators because air is a poor conductor and convection is reduced when it is trapped in small spaces.