Sample Notes: Transfer of Thermal Energy

2.3.1 Conduction

Definition

• Conduction is the transfer of thermal energy through a material without the movement of the material itself.
• Occurs mainly in solids, particularly metals.

Key Concepts

• Energy is passed from particle to particle by vibrations of atoms/molecules.
• In metals, free electrons gain energy and move through the metal, transferring energy faster than in non-metals.

Experiments to Distinguish Good and Bad Conductors

• Metal Rod Experiment:
  • Fix drawing pins to different metal rods with wax.
  • Heat one end of each rod.
  • Good conductors melt the wax and make the pins fall quickly.
  • Poor conductors retain the pins longer.
• Comparison Test:
  • Use identical rods of metal and plastic.
  • Heat both ends equally and observe temperature rise using thermometers or thermal sensors.

Atomic Explanation

• In non-metals, conduction occurs via:
  • Lattice vibrations: Atoms vibrate more as they are heated and transfer this energy to neighboring atoms.
• In metals, conduction is faster because:
  • Free (delocalised) electrons absorb energy and move rapidly, transferring energy throughout the metal.

2.3.2 Convection

Definition

• Convection is the transfer of thermal energy in fluids (liquids or gases) due to the movement of particles.

Explanation

• Heated fluid expands → becomes less dense → rises.
• Cooler fluid moves in to take its place → convection current forms.
• Continual cycle transfers heat throughout the fluid.

Experiments to Show Convection

1. Coloured Water Experiment:
   • Place potassium permanganate crystals or dye at the bottom of a beaker of water.
   • Gently heat the side near the crystals.
   • Observe coloured trails moving upwards, showing convection current.
2. Convection in Air:
   • Use a smoke box: Place a candle below one chimney and smoke enters from the other.
   • Smoke rises with warm air showing the movement of convection currents.

2.3.3 Radiation

Definition

• Radiation is the transfer of thermal energy by infrared waves.
• Can occur through vacuum (no medium required), unlike conduction and convection.

Key Characteristics

• All objects emit and absorb infrared radiation.
• The amount depends on:
  • Surface colour and texture
  • Temperature
  • Surface area

Effect of Surface Colour and Texture

• Good emitters/absorbers:
  • Black, dull surfaces
• Poor emitters/absorbers:
  • White, shiny surfaces

Rate of Emission

• Increases with:
  • Higher surface temperature
  • Larger surface area

Experiments

To Distinguish Good and Bad Emitters

• Leslie Cube Experiment:
  • Use a metal cube with different surface finishes on each side (black, shiny, white, dull).
  • Fill with hot water.
  • Measure radiation using a thermal sensor or infrared detector.
  • Black/dull surfaces emit more IR radiation than shiny ones.

To Distinguish Good and Bad Absorbers

• Two Can Experiment:
  • Take two identical cans: one painted black and one shiny.
  • Fill both with equal volumes of cold water.
  • Expose to radiant heat source.
  • Measure temperature rise over time.
  • Black can absorbs more IR and heats up faster.

2.3.4 Consequences and Applications of Thermal Energy Transfer

Conduction Applications

• Cooking utensils: Metal pans conduct heat to cook food.
• Ironing rods: Metal tips heat up quickly.

Convection Applications

• Room heating: Warm air from heater rises, cold air sinks, forming a convection current.
• Refrigerators: Cold air sinks; warm air rises — helps maintain circulation inside.

Radiation Applications

• Infrared thermometers: Detect IR radiation from body/object surfaces.
• Solar panels: Absorb IR radiation to convert to electrical energy.
• Thermal insulation in homes:
  • Double-glazed windows reduce IR heat loss.
  • Curtains, wall insulation reduce both radiation and convection heat loss.

Thermal Insulation Examples

1. Vacuum flasks:
   • Silvered walls reduce radiation.
   • Vacuum reduces conduction/convection.
2. Building insulation:
   • Fibreglass traps air → reduces convection.
   • Foam boards reduce conduction.
   • Reflective foil reduces radiation.

Summary Comparison Table

Exam Tips

• Always compare materials in terms of how fast they transfer heat.
• Link every explanation to particles, density, or radiation behavior.
• Be clear about when a medium is needed (conduction & convection) and when not (radiation).
• Use labelled diagrams wherever allowed to support experimental descriptions.