Energy Stores and Transfers

This lesson explains Energy Stores and Transfers for Cambridge IGCSE Physics 0625. It separates the Core requirements from the additional Supplement work for Extended candidates. Focus on the cause-and-effect explanation and the exact quantities being compared. Work through the example before testing the same skill without notes.

What are the energy stores in IGCSE Physics?

The 0625 syllabus lists eight stores: kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic, internal (thermal) and magnetic. Energy moves between stores by four transfer pathways: mechanical work (forces), electrical work (currents), heating, and waves (electromagnetic or sound). The principle of conservation of energy states that energy cannot be created or destroyed, only transferred between stores. Total energy stays constant in a closed system.

Two equations matter here. Both are Extended (Supplement) only as calculations; Core candidates describe the transfers in words.

Quantity	Symbol	Unit
Kinetic energy	$E_k$	J
Gravitational potential energy change	$\Delta E_p$	J
Mass	$m$	kg
Speed	$v$	m/s
Gravitational field strength	$g$	9.8 N/kg
Height change	$\Delta h$	m

$\text{Kinetic energy} = \dfrac{1}{2} \times \text{mass} \times \text{speed}^2$. In symbols: $E_k = \dfrac{1}{2}mv^2$. $\text{Change in gravitational potential energy} = \text{mass} \times \text{gravitational field strength} \times \text{height change}$. In symbols: $\Delta E_p = mg\Delta h$.

Use $g = 9.8\ \text{N/kg}$, the 0625 standard. Some papers state 10 N/kg instead, so read the question.

How do I describe an energy transfer for the available marks?

Use the formula sentence: energy transfers from [store] to [store] by [pathway]. For a falling durian: from the gravitational potential store to the kinetic store by mechanical work (gravity acting). On impact: from kinetic to internal stores of the fruit and ground, by mechanical work, then dissipated by heating. Naming both stores and the pathway is what separates 2 marks from 1.

Worked example

A 60 kg student runs up a flight of stairs 4.5 m high.

(a) Calculate the gain in gravitational potential energy. Use $g = 9.8\ \text{N/kg}$. [2] (b) At the top, the student is moving at 2.0 m/s. Calculate their kinetic energy. [2] (c) State the store from which this energy originally came. [1]

Solution (a). Equation: $\Delta E_p = mg\Delta h$. Substitute: $\Delta E_p = 60 \times 9.8 \times 4.5$. Answer: $\Delta E_p = \textbf{2646 J} \approx \textbf{2600 J}$ (2 s.f.).

Solution (b). Equation: $E_k = \dfrac{1}{2}mv^2$. Substitute: $E_k = 0.5 \times 60 \times 2.0^2$. Answer: $E_k = \textbf{120 J}$.

Solution (c). The chemical store (in the student’s muscles/food).

Original marking points

• M1: $\Delta E_p = mg\Delta h$ with correct substitution.
• A1: 2600 J (accept 2646 J) with unit.
• M1: $\dfrac{1}{2} \times 60 \times 2.0^2$ (speed must be squared).
• A1: 120 J.
• B1: chemical (energy) store.

These marking points belong to this original example. They are not an official Cambridge mark scheme.

Common errors and how to correct them

• Forgetting to square the speed. $\dfrac{1}{2} \times 60 \times 2.0 = 60\ \text{J}$ does not show the required method. Fix: square $v$ before multiplying anything else.
• Saying energy is “lost” or “used up”. Energy is transferred or dissipated, never destroyed. Fix: write “dissipated to the internal store of the surroundings”.
• Using weight instead of mass in $E_k = \dfrac{1}{2}mv^2$. Fix: $m$ is in kg; if you are given weight in N, divide by 9.8 first.
• Naming pathways as stores. “Electrical energy” and “heat energy” are not stores in 0625 language. Fix: electrical work and heating are pathways.
• Wrong g value. Fix: use 9.8 N/kg unless the paper states 10.

How to apply this in an exam

When a question links height and speed (a falling or rolling object), equate the two equations: $mg\Delta h = \dfrac{1}{2}mv^2$. Mass cancels, so $v = \sqrt{2g\Delta h}$. Extended Paper 4 sets this almost every year, and showing the cancellation line earns the method mark even if the arithmetic goes wrong.

Where this skill matters

Stores, pathways and conservation appear on all four written papers. Papers 1 and 3 (Core) ask descriptive questions: name the store, complete the transfer sentence, interpret a simple flow diagram. Papers 2 and 4 (Extended) add the $E_k$ and $\Delta E_p$ calculations and the $mg\Delta h = \dfrac{1}{2}mv^2$ link. Paper 6 (and Paper 5) can use energy ideas in pendulum or ramp experiments. Core students should master the vocabulary; Extended students need the equations fluent with $g = 9.8\ \text{N/kg}$, since these calculations feed directly into the work, power and efficiency questions that follow in the same paper.