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Conservation & Dissipation of Energy (AQA GCSE Physics)
Revision Note
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
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
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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