Energy Stores & Transfers (Edexcel GCSE Physics: Combined Science)

• Energy is stored in objects in different energy stores

Energy Stores Table

Energy Transfer Pathways

• Energy is transferred between stores by different energy transfer pathways
• The energy transfer pathways are:
  • Mechanical
  • Electrical
  • Heating
  • Radiation

• These are described in the table below:

Energy Transfer Pathway Table

• An example of an energy transfer by heating is a hot coffee heating up cold hands

Energy is transferred by heating from the hot coffee to the mug to the cold hands

Energy Transfer Diagrams

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) 

Sankey Diagrams

• Sankey diagrams can be used to represent energy transfers
  • Sankey diagrams are characterised by the splitting arrows that 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

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

Sankey diagram for modern vs. old filament light bulb

 Systems

• In physics, a system is defined as:

An object or group of objects

• An apple sitting on a table can be defined as a system
• Defining the system in physics is a way of narrowing the parameters to focus only on what is relevant to the situation being observed

• When a system is in equilibrium, nothing changes and so nothing happens
• When there is a change in a system, things happen, and when things happen energy is transferred

• If the table is removed, the apple will fall
• As the apple falls, energy is transferred

• Energy is measured in units of joules (J)

• A thermodynamic system, for example, can be isolated, closed or open
  • An open system allows the exchange of energy and matter to or from its surroundings
  • A closed system can exchange energy but not matter to or from its surroundings
  • An isolated system does not allow the transfer of matter or energy to or from its surroundings

A system can be open, closed or isolated

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

• This means the total amount of energy in a closed system remains constant 
• The total energy transferred into a system must be equal to the total energy transferred out of the system

• Therefore, energy is never '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, and can then be described as wasted 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
• Some of that energy is dissipated by heating to the thermal store of the bat, the ball, and the surroundings

Energy transfers taking place when a bat hits 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 (wasted energy transfer)
• And some energy is dissipated to the thermal store of the surroundings due to the air around the kettle being heated (wasted energy transfer)

Energy transfer taking place as a kettle boils water