Consequences of Thermal Energy Transfer (Cambridge (CIE) IGCSE Physics)

Simple consequences of energy transfer

• Conduction, convection and radiation have many everyday applications and consequences

Examples of conduction

• Good conductors help transfer thermal energy quickly

• Examples include:

  • Metal pans to heat food quickly

  • Metal radiators to transfer heat from water inside to the surrounding air quickly

• Bad conductors (insulators) help retain thermal energy as they transfer heat slowly

• Examples include:

  • Plastic handles of saucepans to slow thermal energy transferred to hands

  • Air spaces in the walls or windows of some houses help to retain heat, as air is a poor conductor

Double-glazed windows

Double-glazed windows use a layer of air to act as an insulator and slow the transfer of thermal energy out of the house

Examples of convection

• Common applications of convection are:

  • heating a room with a radiator

  • steam rising which cools a hot liquid

• Radiators use convection to raise the temperature of a room in a building:

  1. The metal radiator is hot and transfers thermal energy to air nearby

  2. The particles of this hot air spread out, making it less dense

  3. The spread-out air is less dense than the air above it, so this hot air rises

  4. The radiator heats the cold air which replaces the hot air

  5. The newly heated air also rises, cools and sinks (as it contracts and increases in density)

Convection in steam from coffee

Thermal energy is transferred from the hot coffee to the air by convection currents rising from the surface

Complex consequences of energy transfer

 Extended tier only

Multiple paths of energy transfer

• In real situations there is very rarely only one form of energy transfer

  • Usually two or three happen at once

  • These are sometimes called 'complex' applications of energy transfer

Tea cup example

• In the diagram below a more complex - and more 'real' - version of the situation above is shown

• Thermal energy is transferred from hotter areas (the tea) to cooler areas (the cup, hands and air) by the processes of:

  • Conduction; by direct contact between the tea and the solid sides of the cup and also by direct contact from the cup to the surface it is sitting on 

  • Convection; from the surface of the coffee to the air directly above it

  • Radiation; from the sides of the hot cup in all directions to the surrounding air

In this example, heat is lost via conduction, convection and radiation

Wood fire example

• A wood (or coal) fire in a room heats it through radiation and convection

• As the fuel is so hot in a wood fire, it transfers a lot of thermal energy to the room through radiation

  • The fire transfers a much greater amount of thermal energy to nearby objects via radiation

• Air surrounding the fire is heated and rises, forming a convection current

  • This transfers thermal energy throughout the whole room

Convection currents are set up in a room with a wood or coal fire

Car radiator example

• A car radiator transfers heat away from the engine, which reaches high temperatures

• A liquid travels between the radiator and the engine

  • When the liquid passes over the engine, it absorbs energy from the engine through conduction

  • This liquid then travels back to the radiator and transfers heat to the radiator, again by conduction

  • The radiator then transfers thermal energy to the surrounding air through radiation

• A car radiator is a dark colour, which allows it to emit more radiation

  • A large surface area also helps

• Once the radiator has absorbed thermal energy from the liquid, the liquid is cooler and the cycle begins again