thermal processes

Thermal equilibrium

  • describes a state in which two or more objects have the same temperature

    • there is no net transfer of energy between them

  • energy is transferred from a region of higher temperature (hotter object) to a region of lower temperature (cooler object) until both regions have the same temperature

Conduction

  • transfer of heat through a medium without the physical movement of the medium

  • metals are good conductors of heat

  • most non-metals such as glass, rubber and wood are poor conductors of heat

    • water and air are poor conductors of heat

  • how does conduction in solids work

    • particles at the hotter end gain kinetic energy and vibrate more vigorously

    • these particles collide with neighbouring particles, transferring kinetic energy from one particle to another

    • metals have free electrons that move to cooler regions and transfer kinetic energy upon collisions with other particles

Radiation

  • transfer of heat using infrared radiation without the use of a medium

  • factors affecting rate of radiation

    • colour, texture

      • black/dull surfaces are better emitters/absorbers of radiation compared to silver/shiny surfaces

    • surface temperature

      • the higher the temperature of object relative to the surroundings, the higher rate of radiation

    • surface area

      • for objects with the same size and mass, the object with larger surface area will have a higher rate of radiation

Convection

  • transfer of heat from one place to another by the physical movement of medium

  • occurs in fluids (liquid and gas) by convection currents due to a difference in density

  • heating

    • liquid near the heat source gets heated, expands, becomes less dense and rises

    • cooler, denser liquid at the top sinks to take its place

    • this sets up a convection current which heats up the whole liquid

  • cooling

    • air near the air-con gets cooled, contracts, becomes denser and sinks

    • warmer, less dense air at the bottom rises to take its place

    • this sets up a convection current which cools down the whole room

Answering technique

  • material

    • conduction

      • air spaces/sponge lagging

      • Material (metal, plastic)

    • radiation

      • black/dull/silver/shiny surface

  • increases/reduces the rate of heat gain/loss from to by conduction/convection/radiation/evaporation

  • reason

    • conduction

      • __ (feature) is a good/poor conductor of heat

    • convection

      • limits the convection current that can be set up

      • allows/reduces air flow that is needed for convection

      • prevents hot air from escaping

    • radiation

      • black/dull surfaces are better emitters/absorbers of radiation

      • silver/shiny surfaces are poor emitters/absorbers of radiation