Edexcel IGCSE Physics · Spec 4.2
Energy Stores and Transfer Pathways
Covers recall all required energy stores, Identify transfer pathways and Describe complete energy changes.
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Energy Stores and Transfer Pathways
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Explained
Stores and pathways, and why the distinction is marked
Energy is not created or destroyed. It moves between stores, and describing where it started and where it ended up is what these questions are asking for.
Eight stores are named in the specification: kinetic, gravitational, elastic, chemical, thermal, magnetic, electrostatic and nuclear. That list is worth learning as a list, because a question can ask which store an object possesses and the answer must be one of those eight.
Stores and pathways are different things
A store is where energy is held. A pathway is how it moves between stores.
There are four pathways: mechanically, by a force doing work; electrically, by a current; by heating; and by radiation, meaning light or sound.
So light, sound and electricity are not stores. Nothing holds energy as light; light is how energy travels. This is the distinction that trips people up, because everyday speech treats light energy and sound energy as things.
Describing a transfer properly
Name the object, name the store it started in, name the object it ended up in, and name the store it ended in.
A falling ball: the gravitational store of the ball empties and its kinetic store fills. A kettle: the chemical store of the fuel at the power station empties, energy is transferred electrically, and the thermal store of the water fills. A stretched catapult: the elastic store of the rubber empties and the kinetic store of the stone fills.
Notice how each of those names both the store and the object it belongs to. Saying that energy changes from elastic to kinetic is only half of it.
What the examiners and mark scheme say
A principal examiner report for Edexcel International GCSE Physics records that about two thirds of learners scored zero on one such question, often because they were not specific enough about which objects had changed which energy stores. Those who did score usually did so by referring to a mechanical transfer of energy.
Another report notes that all of the stores an object possesses are listed in the specification, and that in the question concerned only two of them, the kinetic and thermal stores, were relevant. Listing every store you can remember is not the same as identifying the ones that changed.
A mark scheme on a related question credits the idea that energy cannot be destroyed, allowing the statement that energy is conserved, and then credits the energy being transferred to a thermal or heat store, to the surroundings, or by friction. It allows wasted, lost or dissipated in place of transferred, and instructs the examiner to ignore transfers to gravitational or kinetic stores.
That last instruction is the useful one. When a question asks where the missing energy went, it went to a thermal store and then to the surroundings. Gravitational and kinetic are where it came from, not where it was lost to.
Dissipation
No transfer is fully useful. Some energy always ends up in the thermal store of the surroundings, usually through friction or air resistance, and once it has spread out into the surroundings it cannot be gathered back and used.
The energy still exists, so it has not been lost in the everyday sense. It has been dissipated, meaning spread out too thinly to be useful. That word is the one mark schemes look for, and it is more accurate than lost.
Efficiency
Efficiency is the useful energy transferred divided by the total energy supplied, usually written as a percentage.
It is always less than one hundred per cent, because some energy is always dissipated. An answer above one hundred per cent means the two figures have been divided the wrong way round, and it would also mean energy had been created, which is why the check is worth doing.
Spec 4.2
What you need to know
- Recall all required energy stores
- Identify transfer pathways
- Describe complete energy changes
Active recall
Quick check
Answer each question before opening the answer.
Name the two often-missed stores
Magnetic and electrostatic
Is sound an energy store?
No, it is a transfer pathway
How is energy transferred when a force moves an object?
Mechanically
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