Conservation of Energy (Cambridge (CIE) IGCSE Physics)

Conservation of energy

• The principle of conservation of energy states that:

Energy cannot be created or destroyed, it can only be transferred from one store to another

• The principle of conservation of energy means that for a closed system, the total amount of energy is constant

• The total amount of energy transferred into the system must be equal to the total amount of energy transferred away from the system

• Therefore, energy cannot be ‘lost’, but it can be transferred to the surroundings

  • Energy can be dissipated (spread out) to the surroundings by heating and radiation

  • Dissipated energy transfers are often not useful, in which case they can be described as wasted energy

Examples of the principle of conservation of energy

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 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

  • This energy transfer is not useful; the energy is wasted

• Some of that energy is dissipated by heating to the thermal store of the bat, the ball, and the surroundings

  • This energy transfer is not useful; the energy is wasted

• The total amount of energy transferred into the system is equal to the total amount of energy transferred away from the system

Conservation of energy: a bat hitting a ball

The principle of conservation of energy applied to a bat hitting a ball

Example 2: Boiling Water in a Kettle

• 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

  • This energy transfer is not useful; the energy is wasted

• And some energy is dissipated to the thermal store of the surroundings due to the air around the kettle being heated

  • This energy transfer is not useful; the energy is wasted

• The total amount of energy transferred into the system is equal to the total amount of energy transferred away from the system

Conservation of energy: a kettle boiling water

The principle of conservation of energy applied to a kettle boiling water

Energy flow diagrams

• Energy stores and transfers can be represented using a flow diagram

  • This shows both the stores and the transfers taking place within a system

Energy flow diagram showing energy stores and transfers in a nuclear power plant.

Note the colour difference of the labels (stores) and the arrows (transfer pathways) 

• In an energy flow diagram, energy is always conserved

total energy in = total energy out

Sankey diagrams

Extended tier only

• Sankey diagrams can be used to represent energy transfers

  • Sankey diagrams are characterised by arrows that split to show the proportions of the energy transfers taking place

• The different parts of the arrow in a Sankey diagram represent the different energy transfers:

  • The left-hand side of the arrow (the flat end) represents the energy transferred into the system

  • The straight arrow pointing to the right represents the energy that ends up in the desired store; this is the useful energy output

  • The arrows that bend away represent the wasted energy

Features of a Sankey diagram

Total energy in, wasted energy and useful energy out shown on a Sankey diagram

• The width of each arrow is proportional to the amount of energy being transferred

• As a result of the conversation of energy:

total energy in = total energy out

total energy in = useful energy out + wasted energy

• A Sankey diagram for a modern efficient light bulb will look very different from that for an old filament light bulb

• A more efficient light bulb has less wasted energy

  • This is shown by the smaller arrow downwards representing the heat energy

Sankey diagrams for an energy efficient bulb and a filament bulb

Filament bulbs have a much greater proportion of wasted energy than modern energy efficient bulbs