Edexcel IGCSE Physics 复习笔记 4.1.2 Conservation of Energy

• The principle of conservation of energy states that:

Energy cannot be created or destroyed, it can only be transferred from one store to another

• This means the total amount of energy in a closed system remains constant but can change where, and how, it is stored
• Energy can therefore not necessarily be 'lost', but just transferred into wasted energy (often heat)
• Conservation of energy applies to all energy systems

Example 1: A Television

• The energy changes in televisions are:

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

Useful and wasted energy conversions for a television

Example 2: Heaters

• The energy changes in an electrical heater: 

  electrical energy ➝ thermal energy + sound energy + light energy

   

• In a gas cooker, the energy transfers are similar but the initial source of energy is different:

chemical energy ➝ thermal energy + sound energy + light energy

• In both these cases, thermal energy is useful, whereas sound and light are not

Useful and wasted energy conversions in an electric heater and gas cooker

Example 3: Rollercoasters

• A rollercoaster increases its speed as it travels down a steep section of track and gains kinetic energy (KE)
• When the rollercoaster travels up a steep section of track, this kinetic energy is transferred into gravitational potential energy (GPE)
• These energy changes happen several times during a rollercoaster ride as the track goes up and down and around

• As the rollercoaster in the diagram travels from A to D, the energy changes that take place are:

GPE ➝ KE ➝ GPE ➝ KE

• Some energy is transferred to the surroundings as heat and sound energy too
• 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 track means the rollercoaster only has KE

   

Example 4: Trampoline

• When jumping on a trampoline the energy changes are:

elastic potential energy ➝ kinetic energy ➝ gravitational potential energy

• Some energy is transferred to the surroundings as heat and sound energy too

Energy conversions for someone jumping on a trampoline

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