energy transfers

• energy is transferred...

  - mechanically

  - electrically

  - by heating

  - by radiation
• how is energy transferred mechanically?

  an object moving due to a force acting on it
• how is energy transferred electrically?

  a charge moving through a potential difference
• how is energy transferred by heating?

  from hotter objects to colder objects
• how is energy transferred by radiation?

  with electromagnetic waves
• energy is never...


  created or destroyed
• energy can be...


  transferred usefully, stored or dissipated
• when energy is transferred between stores...


  not all energy is transferred usefully
• dissipated energy


  energy which becomes unavailable and irrecoverable to a system
• power is the rate of...

  doing work OR energy transfer
• power (W) =


  work done (J) / time (s)
• power formula


  P=W/t
• what makes a machine powerful?

  if it can transfer a lot of energy is a short span of time
• conduction occurs mainly in...

  solids
• conduction


  the process where vibrating particles transfer energy to neighbouring particles
• energy transferred to an object by heating is transferred to the...

  thermal store of an object. this energy is shared across the kinetic energy stores of the particles in the object
• thermal conductivity


  a measure of how quickly energy is transferred through a material
• materials with high thermal conductivity transfer energy between their particles...


  quickly
• convection occurs only in...

  liquids and gases
• convection


  where energetic particles move away from hotter to cooler regions
• unlike in solids, the particles in liquids and gases are able to...

  move
• when a region of gas/liquid is heated...

  the particle move faster ad the space between individual particles increases - this caused the density of the heated region to decrease
• because liquids and gases can flow....

  the warmer and less dense region will rise above denser, cooler regions
• convection currents in radiators

  - heating a room with a radiator relies on creating convection currents in the air of the room

  - energy is transferred from the radiator to the nearby air particles by conduction (the air particles collide with the radiator surface)

  - the air by the radiator becomes warmer and less dense (as the particles move quicker)

  - the warm air rises and is replaced by cooler air, which is then heated by the radiator

  - at the same time, the previously heated air transfers energy to the surroundings - it cools, becomes denser and sinks

  - the cycle repeats, causing a flow of air to circulate around the room. this is a convection current
• whenever something moves, there is usually at least one...

  frictional force acting against it, which causes some energy in the system to be dissipated
• how do frictional forces cause energy to dissipate?

  work is done against friction, which causes heating of the two surfaces - so the internal thermal energy of the surfaces increases
• lubrication reduces...


  frictional forces
• insulation reduces...


  the rate of energy transfer by heating
• thicker walls have ... thermal conductivity

  lower
• thermal insulation in houses techniques

  cavity walls, cavity wall insulators, loft insulation, double glazed windows, draught excluders
• cavity walls


  made up of an inner and outer wall with an air gap in the middle - the air gap reduces the amount of energy transferred by conduction through the walls
• cavity wall example

  
• cavity wall insulator example

  
• cavity wall insulators


  the air gap is filled with foam also reduces energy transfer by convection in the wall cavity
• loft insulation example

  
• loft insulation


  reduces convection currents being created in lofts by reducing the movement of air upwards
• double glazed window example

  
• double glazed windows


  air gap between two sheets of glass that prevent energy transfer by conduction through the windows
• draught excluder example

  
• draught excluders


  reduce energy transfers by convection around doors and windows