Conservation & Dissipation of Energy (Oxford AQA IGCSE Physics)
Revision Note
Conservation of Energy
What is the conservation of energy?
The law of conservation of energy states that
Energy can be transferred usefully, stored, or dissipated, but cannot be created or destroyed
This means the total amount of energy in a closed system remains constant
The total energy transferred into an open system must be equal to the total energy transferred out of the system
Example 1: a bat hitting a ball
A bat striking a ball
The moving bat has energy in its kinetic store
Some of that energy is usefully transferred mechanically to the kinetic store of the ball
Some of that energy is transferred from the kinetic store of the bat to the thermal store of the ball mechanically due to the impact of the bat on the ball
Some of that energy is dissipated by heating to the thermal store of the bat, the ball, and the surroundings
Energy flow diagram for a bat striking a ball
Example 2: boiling water in a kettle
Energy transfers as a kettle boils water
When an electric kettle boils water, energy is transferred electrically from the mains supply to the thermal store of the heating element inside the kettle
As the heating element gets hotter, energy is transferred by heating to the thermal store of the water
Some of the energy is transferred to the thermal store of the plastic kettle
Some energy is dissipated to the thermal store of the surroundings due to the air around the kettle being heated
Example 3: trampoline
A person bouncing on a 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 as the fabric is stretched
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
Worked Example
The diagram shows a rollercoaster going down a track.
The rollercoaster takes the path 1 → 2 → 3 → 4.
Which statement about the energy changes that occur for the rollercoaster down this track is true?
A. EK → EP → EP → EK
B. EK → EP → EK → EP
C. EP → EK → EK → EP
D. EP → EK → EP → EK
Answer: D
At point 1:
The rollercoaster is raised above the ground, therefore it has energy in its gravitational potential store
As it travels down the track, energy is transferred mechanically to its kinetic store
At point 2:
Energy is transferred mechanically from the kinetic store to the gravitational potential store
As the kinetic energy store empties, the gravitational potential energy store fills
At point 3:
Energy is transferred mechanically from the gravitational potential store to the kinetic store
At point 4:
The flat terrain means there is no change in the amount of energy in its gravitational potential store, the rollercoaster only has energy in its kinetic store
The kinetic energy store is full
In reality, some energy will also be transferred to the thermal energy store of the tracks due to friction, and to the thermal energy store of the surroundings due to sound
We say this energy is dissipated to the surroundings
The total amount of energy in the system will be constant
Total energy in = total energy out
Examiner Tip
It is helpful to think of energy stores as beakers and the total energy in the system as water. The water can be poured from one beaker into another as energy is transferred between stores.
You may not always be given the energy transfers that happen in a system in exam questions. By familiarising yourself with the energy stores and transfer pathways, you can relate these to the question situation. For example, a ball rolling down a hill transfers energy mechanically from the ball's gravitational potential energy store to its kinetic energy store, whilst a spring transfers energy mechanically from its elastic potential energy store to its kinetic energy store.
Dissipated Energy
When energy is transferred only part of it may be usefully transferred
Unintended or wasted energy transfers are inevitable
There is no such thing as a perfect energy transfer
Most wasted energy transfers result in the heating of objects and their surroundings
We say this energy is dissipated (spread out) to the thermal store of the surroundings
An electric kettle boiling water
Useful energy transfer of a kettle
Energy is transferred by heating from the thermal store of the heating element in the kettle to the thermal store of the water
Wasted energy transfers of a kettle
Energy is transferred from the thermal store of the water to the thermal store of the kettle casing
Energy is transferred from the thermal store of the kettle casing to the thermal store of the surroundings and the temperature of the air in the room will increase slightly
Energy is transferred from the thermal store of the water to the thermal store of the surroundings as water evaporates
Energy transfers are deemed useful if they have a purpose or contribute to a process
The purpose of a kettle is to heat water so the energy transfer to the thermal store of the water is useful
Energy transfers are deemed wasted if the energy can no longer be used for that purpose
The energy transferred to the air surrounding the kettle does not heat the water, therefore this energy is wasted
Energy & Friction
Work done against air resistance, frictional forces, and resistance in wires all result in heating
Energy is transferred to the thermal store of the surroundings increasing the temperature of the air particles and surrounding objects
Once energy is in the thermal store of the surroundings, it can not be 'gathered' for any specific use
Therefore, it is referred to as wasted energy
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