Energy transfers

• Energy Stores:

  Chemical – e.g. the food we eat

  Kinetic – movement energy

  Gravitational – objects that are lifted up

  Elastic – e.g. from springs

  Thermal – from hot objects

  Magnetic – objects in magnetic fields

  Electrostatic – charged objects

  Nuclear –  stored within a nucleus
• Energy is never created or destroyed, only transferred from one source to another
• Energy transfers:

  • mechanical work - a force moving an object through a distance
  • electrical work - charges moving due to a potential difference
  • heating - due to temperature difference caused electrically or by chemical reaction
  • radiation - energy transferred as a wave, eg light and infrared - light radiation and infrared radiation are emitted from the sun
• efficiency = useful energy output/total energy output x 100
• In Sankey diagrams, the energy flow is shown by arrows whose width is proportional to the amount of energy involved. The wasted and useful energy outputs are shown by different arrows.
• Conduction is the transfer of thermal energy through a substance by the vibration of the atoms within the substance. Metals are good conductors because they have free electrons that can move easily through the metal, making the transfer of energy happen faster.
• Convection occurs in a liquid or gas. These expand when heated because the particles move faster and take up more volume – the particles remain the same size but become further apart. The hot liquid or gas is less dense, so it rises into colder areas. The denser, colder liquid or gas falls into the warm areas. In this way, convection currents are set up which transfer heat from place to place.
• Thermal radiation is the transfer of energy by infrared (IR) waves. These travel very quickly in straight lines.
• Convection in everyday phenomena

  • Air close to the radiator is heated.
  • It expands, becomes less dense and rises.
  • It is replaced by the cooler, denser air which surrounds it.
  • This air is in turn heated, expands becomes less dense and rises.
  • The process continues, a convection current is set up and heat is transferred through the air and hence through the room.
• What type of surfaces are good reflectors of infrared (IR) radiation?

  Light, shiny surfaces
• Why are dark, matt surfaces effective at absorbing infrared radiation?


  They are poor reflectors of IR
• If a dark object is placed next to a heat source, what will happen compared to a light object?

  The dark object will heat up faster
• How do dark matt surfaces compare to light shiny surfaces in terms of IR emission at the same temperature?

  Dark matt surfaces emit more IR
• What happens to the amount of IR emitted as the temperature of an object increases?

  Hotter objects emit more IR per second
• How does the type of electromagnetic (EM) wave emitted change with temperature?

  Higher temperature means higher frequency of EM wave
• What are the properties of surfaces in relation to infrared radiation absorption and emission?

  • Light, shiny surfaces: good reflectors, poor absorbers
  • Dark, matt surfaces: poor reflectors, good absorbers
  • Dark matt surfaces: best emitters of IR
• • Place purple potassium permanganate crystals in a beaker of cold water, to the side
  • Using a bunsen burner, heat the beaker on the side of the crystals
  • As the temperature of the water around the crystals increases, they dissolve, forming a purple solution
  • The purple solution is carried through the water by convection and traces the path of the convection currents through the water
• A good insulating material is a poor conductor that contains trapped air, e.g. foam, feathers, glass fibre. Being a poor conductor (non-metal) prevents heat transfer by conduction and the trapped air prevents convection currents.