Conduction (Oxford AQA IGCSE Combined Science Double Award)

Conduction

• Thermal conduction is the process of energy transfer by vibrating particles in a substance

  • It is the main method of thermal energy transfer in solids

  • The direction of energy transfer is always from hot to cold

• Conduction occurs when:

Two objects, or substances, of different temperatures come in contact with one another, thermal energy is transferred from the hotter object to the cooler object

Conduction in solids

• When a substance is heated, the particles, start to move around (vibrate) more

  • The atoms at the hotter end of the solid will vibrate more than the atoms at the cooler end

  • The vibrating particles transfer energy from their kinetic store to the kinetic store of neighbouring particles as they collide

  • Eventually, thermal equilibrium is achieved throughout the substance

  • This occurs in all solids, metals and non-metals alike

Conduction in metals

• Metals are the best thermal conductors

  • This is because they have a high number of free electrons

  • When a metal is heated, the free electrons gain energy and pass through the structure of the metal

  • They collide with other electrons and metal ions that make up the metal, transferring energy with each collision

  • This speeds up the transfer of energy throughout the metal

Free electrons in a metal

Conduction in liquids and gases

• Both types of fluid (liquid or gas) are poor conductors of heat

• For thermal conduction to occur in a fluid the particles need to be close together so that when they vibrate the vibrations are passed along

• In liquids, particles are close but slide past each other

  • Therefore, liquids are generally better conductors than gases

• In gases, particles have much more space in between them

  • Therefore, gases are generally poor conductors

Conductors to insulators

Insulators

• There are many situations where energy transfers are unwanted:

  • Keeping a house warm

  • Keeping a hot drink hot or cold

  • Dressing to stay warm in cold weather

Insulated mug

• An insulator is a substance that is a poor thermal conductor

  • Examples include wool, plastic, wood

• Insulators are used to reduce energy transfers, for example, to keep a house warm or build a soundproof room

  • This is why in cold weather, a woollen jumper is worn to retain body heat and keep warm

• Good insulators which keep the energy transfer through them as low as possible have:

  • A low thermal conductivity

  • Layers that are as thick as possible