Conservation of Energy (Edexcel GCSE Physics)

Revision Note

Test yourself
Leander

Author

Leander

Last updated

Closed Systems

    • In physics, a system is defined as:

    An object or group of objects

 

  • Defining a 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, energy is transferred

  • An apple sitting on a table can be defined as a system
  • If nothing changes, the apple will just sit there
  • If the table is removed, the apple will fall
  • As the apple falls, energy is transferred

apple-table-system

A change in a system causes a transfer of energy from the apple's gravitational potential store to its kinetic store

 

  • Energy is measured in units of joules (J)

 

 

  • closed system is defined as:

A system where there is no net change to the total energy in that system

 

  • As a result, the total amount of energy within that system must remain constant
    • This is due to the conservation of energy

Did this video help you?

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

Energy Stores

 

  • Energy is stored in objects in different energy stores

Energy Stores Table

1-1-1-energy-stores-table-new

 

Energy Transfer Pathways

  • Energy is transferred between stores via transfer pathways
  • Examples of these are:
    • Mechanically
    • Electrically
    • By heating
    • By radiation

  • These are described in the table below:

Energy Transfer Pathway Table

1-1-1-energy-transfer-mechanisms-table-new

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

1--thermal-energy-transfer--new

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

Representing Energy Transfers

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

1-7-4-conservation-of-energy-flow-diagram

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

8-1-2-sankey-diagram-demonstration_sl-physics-rn

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 = 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

cie-igcse-1-7-4-phy-rn-sankey-comparison-new

Sankey diagram for modern vs. old filament light bulb

Worked example

An electric motor is used to lift a weight. The diagram represents the energy transfers in the system.

  

WE Sankey Question image, downloadable IGCSE & GCSE Physics revision notes

  

Calculate the amount of wasted energy.

 

Step 1: State the conservation of energy

    • Energy cannot be created or destroyed, it can only be transferred from one store to another
    • This means that:

Total energy in = Useful energy out + Wasted energy

Step 2: Rearrange the equation for the wasted energy

Wasted energy = Total energy in – Useful energy out

Step 3: Substitute the values from the diagram

500 – 120 = 380 J

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Leander

Author: Leander

Expertise: Physics

Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.