Energy Transfers

• SI units for gravitational potential energy


  Energy (J), Mass(kg), Gravitational Field Strength (N/kg), Height (m)
• Kinetic energy equation


  ½ m v2
• SI units for kinetic energy


  Energy (J), Mass(kg), Velocity(m/s)
• Conservation of energy


  • The total amount of energy in a closed system remains constant
  • Energy cannot be made or destroyed but can change from one form to another
• Energy changes when a ball is thrown upwards and returns to starting position


  1. Upwards: KE is converted to GPE

  2. Peak: Maximum GPE, zero KE

  3. Downwards: GPE is converted to KE
• The total energy of a kicked ball remains constant due to conservation of energy
• Energy changes in a filament light-bulb


  1. Electrical energy is transferred into light and heat energy

  2. Light is a useful energy form, heat is waste energy
• Energy transfers for a bungee jumper


  1. When falling, the GPE is converted to KE of jumper

  2. As the cord tightens, KE is converted and stored as Elastic Potential Energy (EPE)

  3. At lowest point, the jumper's initial GPE equals the EPE stored in the cord
• Bungee jumper slows down once the cord begins to stretch


  • Kinetic energy decreases since it is converted to elastic potential energy
  • Since KE is proportional to (velocity)², as KE decreases, so does velocity
• Waste energy


  The energy that is not used by the device for its desired purpose, or not converted into the desired form
• Equations to calculate efficiency


  • Useful Output Energy / Total Input Energy
  • Useful Power Output/Total Power Output
• Consequence of high thermal conductivity

  The rate of energy transfer through the material is higher than for a material with a lower thermal conductivity
• Double-glazed windows have a lower thermal conductivity than single-glazed windows
• Methods to reduce heat loss in a building


  • Double glazing
  • Loft and wall insulation
  • Thicker walls
• Reducing unwanted energy transfer in a mechanical system

  Reduce friction by lubricating moving parts
• How to increase efficiency of a system


  • Reducing waste output (by lubrication, thermal insulation etc.)
  • Recycling waste output (eg. recycling thermal waste energy as input energy)