4.14PH1 · Section 4: Energy resources and energy transfer

Energy stores and transfers

Key learnings

The 8 energy stores
The eight stores in 4PH1 are: kinetic, gravitational potential (GPE), elastic potential, chemical, thermal, nuclear, magnetic, electrostatic. A store is a noun — WHERE energy is held — not a verb. In transfer questions name the specific store that increases or decreases (e.g. "chemical energy store"), never just "energy". "Electrical" and "light" are NOT stores — they are pathways.
The 4 energy transfer pathways
Energy moves between stores by four pathways: mechanically (a force acting through a distance — work done), electrically (a current through a potential difference), by heating (conduction, convection or radiation across a temperature difference), and by radiation (light or sound waves). A pathway is HOW energy moves. The mark scheme accepts store-change language ("GPE store decreases, KE store increases") or pathway language — both at equivalent depth.
Conservation of energy and efficiency
Conservation of energy: energy cannot be created or destroyed, only transferred between stores; the total is unchanged. Efficiency $= \dfrac{\text{useful energy output}}{\text{total energy input}} \times 100\%$ — a ratio with no unit, shown as a percentage, and never over 100%. Example: 2400 J of chemical energy giving 720 J of useful KE is $\frac{720}{2400} \times 100 = 30\%$ efficient; the other 1680 J is dissipated to thermal stores, not destroyed.

Common mistakes

Drawn from real examiner reports.

Saying energy is "used up" or "lost"

Phrases like "the energy is used up", "the battery runs out", or "energy is lost" break conservation and score nothing. Correct language: the chemical store of the battery decreases; the thermal store of the surroundings increases; the total is unchanged. "Wasted" energy is fine only if you add it has gone to a less useful (thermal) store — it still exists.

June 2024 Paper 2P Q7(b): candidates who wrote that "energy is lost at the turbine" rather than naming the thermal store gained as waste did not score the transfer mark. The examiner noted that both the store-change route and the pathway route required specific language, not vague descriptions.

Confusing stores with pathways

A store is where energy resides (kinetic, GPE, chemical, thermal…); a pathway is how it moves (mechanically, electrically, by heating, by radiation). Do not mix them: "transferred by kinetic energy" (kinetic is a store) or "the electrical store" (electrical is a pathway) both lose marks. "Electrical energy" and "light energy" are pathways, not stores.

November 2024 Paper 2P Q7(a): candidates who named stores that do not change in the scenario (e.g. chemical or nuclear) when only kinetic and thermal were increasing during turbine spin-up lost marks because they confused what they had memorised about stores with the specific scenario given.

Efficiency over 100% (wrong denominator)

In $\text{efficiency} = \dfrac{\text{useful output}}{\text{total input}} \times 100\%$ the denominator is always total input. Inverting it gives a value over 100%, which is impossible. You may use power instead of energy, but be consistent. On a Sankey diagram the output arrow widths must sum to the input width — never exceed it.

November 2024 Paper 2P Q7(b)(ii): a significant number of candidates either did not identify KE as the useful output or used total energy as both numerator and denominator, giving 100% — the examiner noted the multi-step structure required identifying the useful store first.

Distance left in cm for work done

Convert lengths to metres before a work-done or energy calculation: $W = F \times d$ needs $d$ in metres, so 40 cm = 0.40 m. In June 2024 Paper 2P Q2(c) candidates left the distance in cm, giving N·cm instead of joules and losing the unit mark. Check the unit on the answer line is J.

June 2024 Paper 2P Q2(c)(i)–(ii): candidates forgot to convert distance from cm to m before calculating work done, losing the unit mark (N·cm instead of J).

Not squaring v or forgetting the root

Two calculation slips: in $\text{KE} = \tfrac{1}{2}mv^2$ , square $v$ first (forgetting it makes the answer far too small); and in GPE use $g = 9.8$ N/kg (10 accepted if used consistently). After solving KE for $v$ , take the square root — $v^2 = 78.4$ means $v = 8.9$ m/s, not 78.4.

(general exam technique)

Not identifying the useful store first

In a multi-step efficiency question, identify the useful store before calculating. Ask "what is the device for?" — that store is the useful output; everything else is wasted (usually thermal). Miss this and you cannot pick the right numerator, so the whole efficiency calculation collapses.

Exam tips

Trace the energy chain, stage by stage
For "describe the energy transfers" questions, go stage by stage: name (a) the store that decreases, (b) the store that increases, (c) the pathway linking them. Don't skip stages — each arrow scores marks.
Use the Store → pathway → Store template
Use the "[Store A decreases] → [pathway] → [Store B increases]" template for every stage. Naming the pathway (mechanically / electrically / by heating / by radiation) as well as both stores earns the depth the mark scheme wants, on either the store route or the pathway route.
Rearrange efficiency, then find wasted
Rearrange the efficiency formula for the missing quantity: useful $=$ efficiency $\times$ total $\div 100$ ; total $=$ useful $\div$ efficiency $\times 100$ . Then wasted $=$ total $-$ useful (never "destroyed"). Keep useful as the numerator so it stays $\le 100\%$ .
Sankey: proportional widths, label all
For a Sankey diagram: the input arrow width is the total energy; it splits into a forward useful arrow and a downward wasted (thermal) arrow. Make widths proportional to the values, label each with its value and store/pathway, and check the outputs sum to the input.

Revision notes