CIE A Level Physics复习笔记5.1.2 The Principle of Conservation of Energy

• The Principle of Conservation of Energy states that:
  • Energy cannot be created or destroyed, it can only change from one form to another
• This means the total amount of energy in a closed system remains constant, although how much of each form there is may change
• Common examples of energy transfers are:
  • A falling object (in a vacuum): gravitational potential energy ➝ kinetic energy
  • A battery: chemical energy ➝ electrical energy ➝ light energy (if connected to a bulb)
  • Horizontal mass on a spring: elastic potential energy ➝ kinetic energy

Types of energy

Diagram showing the forms of energy transfers and stores

Energy dissipation

• When energy is transferred from one form to another, not all the energy will end up in the desired form (or place)
• Dissipation is used to describe ways in which energy is wasted
• Any energy not transferred to useful energy stores is wasted because it is lost to the surroundings
• These are commonly in the form of thermal (heat), light or sound energy
• What counts as wasted energy depends on the system
• For example, in a television:

electrical energy ➝ light energy + sound energy + thermal energy

• • Light and sound energy are useful energy transfers whereas thermal energy (from the heating up of wires) is wasted

• Another example, in a heater:

electrical energy ➝ thermal energy + sound energy

• • The thermal energy is useful, whereas sound is not

ANSWER: D

• • At point A:
    • The rollercoaster is raised above the ground, therefore it has GPE
    • As it travels down the track, GPE is converted to KE and the roller coaster speeds up
  • At point B:
    • KE is converted to GPE as the rollercoaster rises up the loop
  • At point C:
    • This GPE is converted back into KE as the rollercoaster travels back down the loop
  • At point D:
    • The flat terrain means the rollercoaster only has KE