Energy Stores and Systems

• System


  An object or group of objects
• Forms of energy

  • Kinetic (Ek)
  • Gravitational potential (Ep)
  • Elastic potential (strain)
  • Chemical
  • Light
  • Sound
  • Thermal (heat)
  • Electrical
  • Nuclear
• Energy transfer in common situations

  1. Energy can be transformed from one form to another

  2. Energy can be transferred from one place to another
• Energy transfer in common situations

  • When a box is placed on a table, it has gravitational potential energy
  • If it falls off the table, its gravitational potential energy is transformed to kinetic energy
  • When the box hits the floor it makes a noise and heats up the floor slightly, transforming energy into sound energy and thermal energy
  • Majority of the energy goes to the surroundings as thermal energy
• Forms of energy stored in objects

  • Chemical energy in fuels and food
  • Thermal energy in hot objects
  • Elastic potential (strain) energy in stretched objects
• Law (Principle) of Conservation of Energy

  • The total energy of a closed system is constant
  • Energy cannot be created or destroyed; it can only be transformed from one form to another form
• Energy changes for an electric train

  Electrical → Kinetic + Thermal + Sound
• Energy changes for a torch

  Chemical → Electrical → Light + Thermal
• Energy changes for a downhill skier

  Gravitational potential → Kinetic + Thermal + Sound
• Energy changes for a microwave oven

  Electrical → Thermal + Kinetic + Sound
• Energy changes for a pendulum

  Gravitational potential → Kinetic → Gravitational potential (+ Thermal)
• Energy changes for a bungee jump

  1. On the way down: Gravitational potential → Kinetic → Elastic potential

  2. On the way up: Elastic potential → Kinetic → Gravitational potential (+ Thermal)
• In the energy change examples, the total energy at the start is assumed to be 100 Joules, and wasted energy transfers due to friction, etc. are ignored so the total energy must remain constant