Conservation & Dissipation of Energy (AQA GCSE Physics)

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Conservation of Energy

  • The law 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 
  • Energy can be transferred from store to store usefully (to do work)
  • Or energy can be dissipated to the thermal store of the surroundings 

Examples of Energy Conservation

  • Conservation of energy applies to all energy transfers

Example 1: A bat hitting a ball

  • The moving bat has energy in its kinetic store
  • Some of that energy is transferred usefully to the kinetic store of the ball
  • Some of that energy is dissipated  by heating to the thermal store of the bat, the ball, and the surroundings
    • The impact of the bat and the ball cause the particles of the bat and ball to vibrate 
    • The sound wave causes the air particles to vibrate

bat-and-ball-energy-flow-diagram-new

bat-and-ball-energy-transfer

Conservation of energy: a bat hitting a ball

Example 2: An electric heater

  • Energy is transferred electrically from the mains supply to the thermal store of the heating element
  • Some of that energy is usefully transferred to the thermal store of the surroundings by heating the air particles in the room
  • Some of that energy is dissipated to the thermal store of the surroundings by radiation (light)

new-heater-energy-flow-diagram 

heater-energy-transfers

Conservation of energy: electric heater

Example 3: Rollercoasters

  • The roller coaster has energy in its gravitational potential store when it is on an elevated piece of track
  • Energy is transferred usefully to the kinetic store as the rollercoaster gains speed as it descends
  • Energy is transferred from the kinetic store to the gravitational store as the rollercoaster climbs again
  • And energy is transferred usefully to the kinetic store as descends again
  • Energy is dissipated to the thermal store of the surroundings by heating due to friction heating the wheels and track, and due to sound waves vibrating the air particles

WE - Energy transfers question image, downloadable AS & A Level Physics revision notes

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

gravitational potential store → kinetic store → gravitational potential store → kinetic store

  • This is sometimes also described as

GPE ➝ KE ➝ GPE ➝ KE

Example 4: Trampoline

  • Whilst jumping, the person has energy in their kinetic store
  • When the person lands on the trampoline, most of that energy is transferred to the elastic potential store of the trampoline
  • That energy is transferred usefully back to the kinetic store of the person as they bounce upwards
  • Energy is transferred from the kinetic store of the person to the gravitational potential store of the person as they gain height 
  • Some of the energy is dissipated by heating to the thermal store of the surroundings (the person, the trampoline and the air)

 

  • The useful energy transfers taking place are:

elastic potential energy ➝ kinetic energy ➝ gravitational potential energy

conservation-of-energy-trampoline-new

Useful energy transfers: person on a trampoline

Worked example

Describe the energy transfers in the following scenarios:

a) A falling object

b) A battery powering a torch

c) A mass on a spring

Part (a)

    • For a falling object:

Energy is transferred mechanically from the gravitational potential store of the object to the kinetic store of the object

Part (b)

    • For a battery powering a torch:

Energy is transferred electrically from the chemical store of the cell to the thermal store of the bulb

Part (c)

    • For a mass on a spring:

Energy is transferred mechanically from the elastic potential store of the spring to the kinetic store of the mass

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Leander

Author: Leander

Expertise: Physics

Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.