energy

• an energy store is an object that has a certain type of energy, eg food, balloons, hot coffee, magnets.
• an energy transfer is when energy is changed from one form to one or more different forms. some energy is always wasted, as it is dissipated to the surroundings, eg convection, conduction, radiation
• work is the force applied to an object over a distance. work done (J) = force (N) x distance (M)
• power is the rate at which work is done or energy is transferred. power (W) = energy transferred (J) / time (s)
• efficiency is the ratio of useful energy/power out, to the total energy/power in.

  efficiency =useful energy out/total energy in

  it is a ratio so has no units - it can be given as a decimal less than 1, or a percentage less than 100%
• sankey diagrams show energy transfers that take place, and whether the energy is useful or not.

  A) total energy in

  B) useful energy out

  C) wasted energy out

  
• kinetic energy is the energy that an object has as a result of its motion. kinetic energy (J) = 1/2 mass (kg) x velocity² (m/s)
• kinetic energy and gravitational potential energy practical

  • a ball is released on a slope from a measured height
  • due to the conservation of energy, this decrease in gravitational potential energy store is matched by an equal increase in its kinetic energy store
  • the ball is timed using light gates over a measured distance between x and y after the slope
  • change in gravitational potential energy store = mass x gravitational field strength (9.8N/Kg) x change in height
• temperature and energy

  • temperature represents the average kinetic energy of its particles
  • thermal energy represents the total kinetic energy of its particles
  • internal energy is the total of the kinetic and potential energy of a substance
  • there is potential energy between the particles due to their bonds - the further the particles, the greater the potential energy between them
• conduction is when heat energy is transmitted through collisions between neighbouring atoms or molecules. it only occurs in solid metals as the particles are close together and neatly arranged. not all metals conduct evenly. order of conductivity:

  1. aluminium
  2. copper
  3. brass
  4. iron

  plastic and wood don't conduct heat.
• convection is the process by which heat is transferred by movement of a heated fluid such as air or water.
• convection in a radiator

  1. air above radiator is heated and becomes less dense
  2. this causes the hot air to rise
  3. the hot air is no longer near the radiator so cools down and becomes denser
  4. denser/cooler air sinks
  5. process starts again

  
• convection in a fridge/freezer -

  1. warm air rises as it is less dense
  2. the vent removes heat from the fridge to cool it down
  3. denser/cooler air sinks
  4. air is far from the vent so gets warmer and rises
• radiation happens with no relevance to particles. it occurs in a vacuum.
• emitters give out heat. black is the best emitter, white is the worst emitter.
• absorber takes in heat. white is the best absorber, black is the worst absorber.
• variables

  • independent - what you change
  • dependent - what you measure
  • control - what you keep the same
• specific heat capacity is the amount of energy required to raise 1 kilogram of a substance through 1 degree Celsius.

  energy (J) = mass (kg) x SHC (J/kg/degree celcius)x change in temperature
• The specific latent heat of a substance is the amount of energy needed to change the state of 1 kg of the substance without changing its temperature. energy = mass x specific latent heat