Energy Stores and Systems

Energy is never used up, it's just transferred between different stores and objects
Energy is transferred between stores
1. When energy is transferred to an object, the energy is stored in one of the object's energy stores
2. Energy stores
  1. Thermal (can be called internal)
  2. Kinetic
  3. Gravitational potential
  4. Elastic potential
  5. Chemical
  6. Magnetic
  7. Electrostatic
  8. Nuclear
3. Energy is transferred...
  1. Mechanically (by a force doing work)
  2. Electrically (work done by moving charges)
  3. By heating
  4. By radiation (light or sound)
When a system changes, energy is transferred
1. System: The object(s) that you are interested in
2. Can be transferred...
  1. Between different types of energy stores
  2. Between different objects
  3. Into or away from the system
3. Closed systems: systems where neither matter nor energy can enter or leave
  1. The net change in the total energy of a closed system is always zero
Energy can be transferred by...
1. Heating
  1. Boiling water in a kettle
    1. Water: System
      1. Energy is transferred to the water (from the kettle's heating element) by heating...
        ...Into the water's thermal energy store, causing the temperature of the water to rise
  2. The kettle's heating element and the water as a two-object system
    1. Energy is transferred electrically to the thermal energy source of the kettle's heating element...
      1. ...Which transfers energy by heating to the water's thermal energy store
2. Doing work
  1. Work done = energy transferred
  2. Work can be done when current flows or by a force moving an object
  3. Examples:
    1. Throwing a ball
      1. Initial force exerted by a person to throw a ball upwards does work
        Energy transfer from chemical store in person's arm...
        ...To the kinetic store of the ball and the arm
    2. A ball dropped from height
      1. Accelerated by gravity, so gravitational force does work
        Causes energy to be transferred from the ball's gravitational potential store...
        ...To it's kinetic store
    3. A car's brakes
      1. Friction between the brakes and wheels does work as it slows down
        Causes energy transfer from the wheels' kinetic store...
        ..To the thermal store of the surroundings
    4. A collision between a car and a stationary object
      1. Normal contact force between the car and the object does work
        Causes energy transfer from the car's kinetic store...
        ..To other stores
        Elastic potential store of object and car body
        Thermal store of object and car body
        Some energy might also be transferred away by sound waves

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Energy Stores and Systems

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