Physics

• A system is an object or group of objects
• whenever anything changes in a system, energy is transferred between its stores or surroundings
• a closed system means no energy can escape to, or enter from the surroundings
• the total energy in a closed system never changes
• Energy stores: KGETCNME

  • kinetic
  • gravitational
  • elastic
  • thermal
  • chemical
  • nuclear
  • magnetic
  • electrostatic
• kinetic: energy an object has because of its movement
• gravitational: energy an object has because of its height above the ground
• elastic: energy an elastic object has when stretched or compressed
• thermal: energy an object has because of its temperature
• chemical: energy that can be transferred by chemical reactions
• nuclear: energy stored in the nucleus of an atom
• magnetic: energy a magnetic object has when near a magnet or in a magnetic feild
• electrostatic: energy a charged object has when it is near another charged object
• energy can be transferred to and from different stores by:

  • heating
  • waves
  • electricity
  • forces
• factors effecting rate of heat loss:

  • thickness of walls and roof
  • thermal conductivity of walls and roof
• lower thermal conductivity = lower rate of heat loss
• thermal conductivity of the walls and roof may be reduced by thermal insulators
• a thermal insulator has low thermal conductivity
• the rate of energy transferred through a thermal insulator is low
• the energy transfer per second through a material depends on:

  • materials thermal conductivity
  • temperature difference between the two sides of the material
  • thickness of the material
• ways of keeping heat inside a house:

  • loft insulation
  • aluminium foil between radiator and wall
  • draught excluder
  • cavity wall insulation
• gpe depends on an objects height above the ground, gravitational feild strength and mass
• gpe (J) = mass (kg) x gravitational feild strength (N/kg) x height (m)
• kinetic energy depends on an objects mass and speed
• kinetic energy (J) = 0.5 x mass (kg) x (speed)2 (m/s)
• power is how much work is done
• unit for power is watt (w)
• unit for energy is Joules (J)
• power (w) = work done (J)

  time (s)
• energy cannot be created or destroyed
• energy is never entirely transferred usefully
• all energy eventually ends up transferring to thermal
• work done causes energy to be wasted
• lubrication is a way of reducing unwanted energy transfer due to friction
• streamlining is a way of reducing energy wasted due to air resistance or drag in water
• use of thermal insulation is a way of reducing energy wasted due to heat dissipated
• efficiency is a measure of how much energy is transferred usefully
• efficiency = useful output (J)

  ——————————

  total input (J)

  efficiency = useful power output (W)

  ——————————————-

  total power input (W)
• when a car slows down energy is transferred mechanically from the kinetic store of the car to the thermal store of its breaks
• when a kettle is used to heat water the current transfers thermally from the kettle to the water