Knowledge Organiser: Heat Transfer
Key Terms
- Conduction: particle vibrations transfer energy (solids/metals)
- Convection: fluid movement transfers energy (fluids only)
- Radiation: infrared waves transfer energy (all media + vacuum)
- Free electrons: responsible for metal's high conductivity
Key Facts
- Conduction: particles don't move, only vibrate
- Convection: hot fluid rises (less dense), cool sinks
- Radiation: works in vacuum; black surfaces best absorbers/emitters
- Metals conduct well due to free electrons
What Works Where
- Solids: conduction only
- Fluids: convection (and conduction)
- Vacuum: radiation only
- Thermos flask: vacuum blocks conduction + convection; silvered walls reduce radiation
Exam Tips
- Convection needs density change — mention this in explanations
- Metals: free electrons make them better conductors
- Black surfaces: best absorbers AND best emitters
- Trapped air = good insulator (poor conduction, no convection)
Key Equations
- ΔE = m × c × ΔT (thermal energy change)
- Rate of transfer ∝ surface area (larger area → faster transfer)
- Rate of transfer ∝ temperature difference (bigger ΔT → faster transfer)
- Efficiency = useful energy out ÷ total energy in × 100%
Common Mistakes
- Saying convection occurs in solids: Convection requires a fluid (liquid or gas) — particles in solids cannot move freely to form currents
- Forgetting density change in convection explanations: Must state that hot fluid expands, becomes less dense, and rises — not just "hot air rises"
- Confusing black and shiny surfaces: Black/matt surfaces are best emitters AND absorbers; shiny/silver surfaces are poor emitters AND poor absorbers
- Saying radiation needs a medium: Infrared radiation travels through a vacuum — it does not need particles
- Confusing insulation with zero transfer: Insulation slows heat transfer — it does not stop it completely
Memory Aid
- Remember the three methods with: CCR — Conduction, Convection, Radiation
- To remember what each one needs: "Conduction needs Contact (particle to particle), Convection needs Currents (fluid moves), Radiation needs Nothing (works in vacuum)"
- Black body memory: "Black absorbs, Black emits" — both absorption and emission are maximised by black, matt surfaces.