Energy Conservation & Dissipation (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

The priciple of 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)

  • Stored in a different energy store (e.g. gravitational potential, elastic potential)

  • 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 causes the particles of the bat and ball to vibrate 

  • The sound wave causes the air particles to vibrate

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)

Conservation of energy: electric heater

Example 3: Rollercoasters

• When the rollercoaster is on an elevated piece of track, it has energy in its gravitational potential store

• When the rollercoaster descends to a lower piece of track, energy is transferred usefully to the kinetic store as the rollercoaster gains speed

• When the rollercoaster ascends again, energy is transferred from the kinetic store to the gravitational store as the rollercoaster loses speed

• Energy is dissipated to the thermal store of the surroundings by heating due to

  • friction heating the wheels and track

  • sound waves vibrating the air particles

• 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 person's kinetic store as they bounce upwards

• Energy is transferred from the person's kinetic store to their gravitational potential store 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 store → kinetic store → gravitational potential store

Useful energy transfers: person on a trampoline

Wasted Energy

• In practice, most systems tend to be open systems

• When energy transfers occur that are not useful, these are described as energy being dissipated to the surroundings 

  • Dissipated just means spread out

  • Dissipated energy is stored in less useful ways, typically in the thermal store of the surroundings

  • This is considered to be wasted energy

  • These less useful energy transfers often involve heating, light and sound

• When energy is transferred to the thermal stores of the objects, the temperature of the objects increases

  • The particles that make up the objects vibrate more, hence the transfer pathway is by heating (of the particles)

• Visible light is electromagnetic radiation

  • Therefore, when light is produced, energy is transferred by radiation

• When sound is produced, the sound waves make the air particles vibrate as the wave carries energy away 

  • This increases the energy in the thermal store of the air, hence the transfer pathway is by heating (of the particles)

• Useful energy can be defined as:

  The energy that is transferred from store to store and used for an intended purpose

• Wasted energy can be defined as:

  The energy that is not useful for the intended purpose and is dissipated to the surroundings