7.2 - Energy Transfer Examples

• Conservation of energy law

  Energy cannot be created or destroyed. It can only be transferred from one form to another.
• Example 1: A coal fire

  Scenario:

  A fire burns fuel such as coal to produce heat.

  Energy transfer:

  • Coal has a large store of chemical energy (energy within chemical bonds).
  • When coal is burned the chemical energy is transferred by heating to the thermal energy store of the coal.
  • Hot coals and flames transfer energy to the surroundings by heating and radiation.
• Example 2: Car braking

  Scenario:

  A driver slows a car from 15 m/s to rest.

  Energy transfer:

  • The moving car has kinetic energy.
  • The brakes mechanically do work on the car to slow it down.
  • The kinetic energy of the car decreases and the thermal energy store of the brakes increases.
• Example 3: Archer firing a bow and arrow

  Scenario:

  An archer pulls the string of the bow back and loads an arrow. The archer then releases the string allowing the arrow to fire.

  Energy transfer:

  • As the string of the bow is pulled back chemical energy in the archers muscles is transferred mechanically to the elastic potential of the bow.
  • When the archer releases the string of the bow, the elastic potential energy of the string is transferred mechanically to the kinetic energy store of the arrow.
• What type of energy is stored in the string of a bow when it is pulled back?

  Elastic potential energy
• Which energy store increases when an object is lifted 1.5 m above the ground?

  Gravitational potential
• Which of the following shows the useful energy transfer that takes place when a battery powered toy car travels across the floor?

  chemical → kinetic
• A toy car has a spring which stores elastic potential energy. When released, the toy car moves across the floor. 

  Which of the following is the energy transfer mechanism for the toy car?

  mechanically