Energy stores and transfersIB MYP Physics: Revision notes
Section 1
Energy and the joule
Energy is what allows things to change or work to be done. It is measured in joules (J); 1 kilojoule (kJ) is 1000 J.
Energy is not a substance. Instead we describe where it is kept (a store) and how it moves from one store to another (a transfer).
Section 2
Energy stores
Energy can be held in these stores:
- Kinetic: moving objects
- Gravitational potential: objects raised above the ground
- Elastic: stretched or squashed springs and elastic bands
- Chemical: food, fuels and batteries
- Thermal: hot objects (particles moving faster)
- Nuclear: the nuclei of atoms
- Electrostatic: separated electric charges
- Magnetic: magnets attracting or repelling
Section 3
Energy transfers
Energy moves between stores in four ways:
- Mechanical work: a force moves an object (pushing, stretching, friction)
- Electrical work: a current carries energy around a circuit
- Heating: energy moves from a hotter object to a cooler one
- Radiation (waves): light, sound or infrared waves carry the energy
For example, a kettle uses electrical work to transfer energy to the thermal store of the water.
Energy is not 'used up' or 'made'. It is transferred from one store to another.
Section 4
Energy chains
An energy chain shows how energy moves through a device, naming the stores and the transfers.
Example (a torch): chemical store of the battery → (electrical work) → lamp → (light) radiation + (heating) thermal store of the surroundings.
Example (freewheeling bicycle): gravitational potential store → (mechanical work) → kinetic store → (friction) → thermal store of the surroundings.
In every energy chain, remember the waste: most devices also transfer some energy to the thermal store of the surroundings.
Section 5
Sankey diagrams
A Sankey diagram shows the energy flowing through a device using arrows whose widths are drawn to scale.
- The input arrow on the left is the total energy supplied.
- The useful output continues straight on.
- The wasted energy leaves as a branch, often to the thermal store of the surroundings.
The widths of the output arrows add up to the width of the input arrow, showing that energy is conserved.
Must Know
- Energy is measured in joules
- Stores: kinetic, gravitational potential, elastic, chemical, thermal, nuclear, electrostatic, magnetic
- Transfers: mechanical work, electrical work, heating, radiation
- Energy chains name the stores and the transfers
- Sankey diagrams show energy flow, with arrow widths drawn to scale
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Energy stores and transfers
- A student in Mumbai rides a bicycle to the top of a hill, then freewheels down it without pedalling before braking to a stop at the bottom.Describe the energy changes as the student freewheels down the hill.2 marks
- A hand torch contains a battery that lights a small lamp. After a few minutes the casing of the lamp feels warm.Write an energy chain for the torch, naming the stores and the transfers.2 marks
- A student drops a tennis ball from heights of 0.50, 1.00, 1.50 and 2.00 m onto a hard floor and measures the height of the first bounce each time. The bounce heights are 0.32, 0.64, 0.95 and 1.30 m.Describe the trend in the results and calculate the bounce height as a percentage of the drop height for the 1.00 m drop.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).