Energy stores, transfers and conservation
Follow where energy starts, how it moves and where it ends. Build an energy account, explain a bouncing ball and read a Sankey diagram without confusing width with length.
Core: stores, pathways and simple conservation. Supplement / Extended: multi-stage calculations and Sankey diagrams. This is Cambridge IGCSE, not Egyptian Baccalaureate / Thanaweya Amma or Year 3.
A store tells us where energy is kept
Energy is measured in joules (J). It is not a material that gets used up. An energy store is a way of accounting for energy in an object or system. Kinetic means motion; gravitational potential means the position of objects attracting each other; chemical involves substances that can react; elastic (strain) involves stretching or squashing; nuclear involves atomic nuclei; electrostatic involves the arrangement of charges; internal (thermal) involves particles inside an object.
Worked example
A moving bicycle has kinetic energy. A raised book and Earth have gravitational potential energy. A battery has a chemical store. A stretched spring has an elastic store. The Sun releases energy from a nuclear store. Separated charges have an electrostatic store. A warm mug has internal energy. Internal energy includes microscopic particle motion and interactions, not just the mug moving as a whole.
Think first: Is a stationary, warm mug an object with no energy?
No. Its kinetic energy as a whole may be zero, but it has an internal store. Similarly, a stretched spring can hold elastic energy while stationary.
Make an energy account · Core
- Choose the system and the interval. Name the starting store.
- Identify the transfers: mechanical work, electrical work, heating or waves.
- List all known inputs and outputs in the same energy unit.
- Ask whether energy is accumulating. If not, the outputs add to the input.
- Subtract to find a missing amount, then add back to check conservation.
Worked example: a battery lifts a load
The battery’s chemical store decreases by 500 J while a load starts and ends at rest. The gravitational potential store of the load–Earth system rises by 350 J. Assume no other useful stored-energy increase. The remaining 500 − 350 = 150 J increases internal energy of the apparatus/surroundings or leaves as sound. The motor is a device, current and mechanical work are transfers, and gravitational potential is the useful final store. Do not call “electrical work” a store.
Simple flow, not drawn to scale
Chemical store in battery → electrical work by current → motor → mechanical work → increased gravitational potential store of load–Earth system. A separate branch accounts for energy transferred to surroundings.
Try explaining aloud: “The ___ store decreases; energy is transferred by ___; the ___ store increases.” Name the surroundings when they matter.
Where does the missing energy go?
Account for a lamp’s energy · Core
This idealised lamp has reached steady temperature. Over the chosen interval, no energy accumulates in it. Its only outputs are useful light and energy that warms the surroundings. Change the input and light share, then predict the other output.
Input: 400 J → lamp → useful light: 120 J; other output: ? J.
Investigate: keep the input fixed and increase the light share. Predict how the other output changes. Then keep the share fixed and double the input: both outputs double. Changing a control hides the previous answer.
Practise, then explain
Four multiple-choice questions, four numerical responses and four written tasks. Each question is labelled Core or Supplement. Numerical answers are checked automatically; written answers use a self-assessment guide. The suggested marks are for this original practice, not an official Cambridge mark scheme.
Practice stays only while this page is open. Refreshing or leaving clears these responses. Copy useful written work into your notes before leaving.
A rubber band is stretched and held still. Which store has increased?
Optional hint
The band is deformed, but is not moving while held.
Source corrections and careful wording
The original PDF is preserved. On printed p. 98, Q8(c) asks for a width at 100 MJ per centimetre: 380 ÷ 100 = 3.8 cm, not the printed 38%. Q6’s unconverted solar energy need not all immediately become thermal energy: reflected or transmitted light may also carry energy away. In Figure 14.1, sound and electrical work are transfer pathways, not stores.
Conservation does not mean every object keeps the same energy. An isolated system has no energy exchange; a “closed” system can still exchange energy. The shortcut input = useful output + wasted output assumes no net energy accumulation. We state that assumption in calculations.
Source alignment: supplied Physics-0625 Volume A, PDF pp. 101–107, including Figures 14.1–14.2, and the official Cambridge 2026–2028 syllabus, section 1.7.1. Original explanations, interactive diagram and practice. Kinetic and gravitational potential energy formula calculations follow in Lesson 15. Source-download access is unchanged.