Conservation & Dissipation of Energy (AQA GCSE Physics: Combined Science)

• 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 

• 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

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

• 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

• 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

Useful energy transfers: person on a trampoline

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