AQA A Level Physics复习笔记4.6.4 Conservation of Energy

• The Principle of Conservation of Energy states that:

Energy cannot be created or destroyed, it can only be transferred 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

Types of Energy

Energy types can be separated into transfers or 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

   

• 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

   

Energy transfers whilst jumping on a trampoline

• There may also be work done against resistive forces such as friction
• For example, if an object travels up a rough inclined surface, then

Loss in kinetic energy = Gain in gravitational potential energy + Work done against friction

Energy Equations

• The most common equations used in the conservation of energy calculations are: