Transfer of Thermal EnergyCopy

Cheat Sheet: Thermal Energy Transfer – Conduction, Convection, Radiation (O Level / IGCSE Physics)

2.3.1 Conduction

1. Experiments to Identify Good and Bad Conductors

• Metal rod test:
  • Attach pins to different material rods using wax
  • Heat one end of each rod
  • Observe which pin falls off first
  • Faster fall = better conductor (e.g. metals); slower = poor conductor (e.g. wood, plastic)

2. Explanation of Conduction in Solids

• Non-metals:
  • Heat causes lattice vibrations
  • Vibrations pass from atom to atom → slow energy transfer
• Metals:
  • Same lattice vibration transfer plus:
  • Free electrons gain energy, move rapidly, transfer energy efficiently
  • Metals are better conductors due to free (delocalised) electrons

2.3.2 Convection

1. Explanation of Convection

• Occurs in liquids and gases only
• Heated fluid → expands → becomes less dense
• Rises, cooler fluid sinks to replace it
• This sets up a convection current

2. Experiments to Show Convection

• Coloured crystal in water:
  • Heat bottom of beaker with potassium permanganate crystal
  • Coloured streak shows upward flow of heated water
• Smoke box:
  • Candle under chimney of box
  • Smoke flows in through one side, rises with warm air

2.3.3 Radiation

1. Infrared Radiation

• Transfer of thermal energy via electromagnetic waves (infrared)
• Does not require a medium — can occur in vacuum
  • e.g. energy from Sun to Earth

2. Effect of Surface Colour & Texture

3. Factors Affecting Radiation Rate

• Surface temperature: hotter = more radiation emitted
• Surface area: larger area = more radiation
• Material and surface type also matter (black, dull = better emitter)

4. Experiment: Good vs Bad Emitters

• Use two metal cans, same size:
  • One painted dull black, one shiny silver
  • Fill with hot water, insert thermometer
  • Black can cools faster = better emitter

5. Experiment: Good vs Bad Absorbers

• Two identical metal plates:
  • One black, one shiny
  • Place equal distance from radiant heat source (lamp)
  • Measure temperature rise over time
  • Black plate heats faster = better absorber

2.3.4 Everyday Applications of Thermal Transfer

(a) Heating Kitchen Pans (Conduction)

• Metal base conducts heat efficiently
• Plastic/wooden handles prevent heat transfer to hands

(b) Heating a Room (Convection)

• Heater warms air, hot air rises, cold air sinks
• Creates a convection cycle distributing warm air

(c) Infrared Thermometer (Radiation)

• Detects infrared radiation from skin/object
• Converts into temperature reading — useful for contactless measurement

(d) Thermal Insulation

• Vacuum flasks: prevent conduction, convection (vacuum), radiation (shiny inner surface)
• Building insulation: foam, double-glazing, cavity walls — trap air, reduce heat loss
• Clothing: traps air; wool/fleece = poor conductor
• Reflective foils in roofs reflect radiation away