P7

• Energy


  It can only be transferred, never made, created or destroyed
• Object thrown upwards


  1. Kinetic energy transfers to gravitational potential energy (GPE)

  2. GPE transfers back to kinetic energy (KE) as it falls back down
• Object projected up a slope

  1. Kinetic energy transferred to GPE

  2. Reverse when rolling back down
• Moving object hitting a wall

  Kinetic energy transferred to elastic potential energy (EPE) when it gets squashed
• Object accelerated by a constant force


  1. The force does work on the object

  2. Transferring energy from whatever applies the force to KE in the object
• Vehicle slowing down


  Kinetic energy of the vehicle transferred to heat energy from friction between brakes and wheels and the wheels and road
• Kettle boiling water


  Electrical energy transferred to thermal energy in water
• Heating


  • When heat energy is transferred to an object, the particles gain more kinetic energy
  • The temperature of the object increases as the energy supplied is used to overcome the specific heat capacity
• Work done


  • Energy from whatever is applying the force is transferred to the object
  • The force itself does not give the object energy but the force being applied over a distance requires energy
• When current flows


  Work is done against the resistance, so kinetic energy from flowing charge is transferred to heat due to energy loss from resistance
• Equations commonly needed


  • Work done = force x distance
  • Voltage = current x resistance
  • Power = current^2 x resistance
  • Energy = power x time
  • Kinetic energy = 1/2 mv^2
  • Gravitational potential energy = mgh
  • Elastic potential energy = 1/2 kx^2
• Energy loss


  • Energy is lost when it is transferred into other forms, instead of the desired form
  • It is not actually lost, just transferred into something wasteful
• Energy loss examples


  • Heat dissipated by wires heating up
  • Sound when a ball hits a wall
  • Light such as a spark
• Electricity energy loss


  • Energy is typically lost in electrical systems due to heating of wires
  • Motors heat up when moving, losing some energy due to inefficient contacts between the wires and spinning coil
• Power ratings


  • Electrical appliances have power ratings, showing the power and voltage
  • This can be used to find current and shows how much energy will be used for the item
  • The larger the power, the more energy used per second, therefore it indicates efficiency
• Efficiency


  Useful energy output / Total energy input
• Increasing efficiency


  • Decreasing current will increase efficiency, as power loss in heating wires is due to current and resistance
  • Recycling waste energy, or preventing it from dissipating, so the energy can be reused
  • Lubricating mechanical systems reduces friction and heat loss
  • Thermal insulation ensures all heat energy is used, and not lost to surroundings
• Insulation


  • Without insulation, convection currents form, losing energy
  • With insulation, convection currents cannot form and the material weakly conducts heat so the rate of cooling is low
  • Thicker walls also result in a lower rate of cooling