Apply energy conservation with a stated system
Account for stores and transfers across a boundary instead of saying energy has disappeared.
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028, section 1.7.1. Energy stores, transfers and simple conservation are Core; kinetic and gravitational potential energy equations, multistage conservation and Sankey diagrams are Supplement. State the system, interval and model assumptions. Written practice is self-checked. Supervised practical experience, cumulative assessment and human subject-teacher review remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Identify decreasing and increasing stores.
- Add quantities measured in the same unit.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How much remains from 120 J after a 70 J accounted transfer?
What you will learn
- State that total energy is conserved.
- Distinguish a closed energy account from transfers into or out of a selected system.
A complete account balances
Energy cannot be created or destroyed: it can be transferred between stores and systems. In a closed energy account with no transfer across the selected boundary, the total remains the same. If 120 J leaves a source store and 70 J appears in a desired store, the other 50 J must have another accounted destination, such as internal stores of the surroundings.
Energy described as 'wasted' is still transferred or stored elsewhere; it has not ceased to exist. It is often more dispersed and less useful for the intended task. A 120 J input with alleged outputs 70 J and 60 J fails that stated budget unless another input, decrease of a stored amount or a measurement issue is identified.
The boundary determines what is inside
For a selected system, change in total stored energy equals energy transferred in minus energy transferred out over the same interval. If 100 J enters and 30 J leaves, the system's total stored amount increases by 70 J. If 20 J enters and 50 J leaves, its total decreases by 30 J. These are changes, not necessarily the complete initial or final stored totals.
A cooling cup alone is not a closed energy account if it transfers energy to the room. A larger cup-and-surroundings account can include that receiving store. State the objects, interval and relevant transfers before claiming that one store's change must equal a second store's change. An incomplete boundary or missing observation should prompt investigation rather than inventing destroyed energy.
Pause and explain
A cup loses internal energy while warming its room. Has energy been destroyed?
Worked example
A source releases 120 J. A desired store gains 70 J, and the only other destination is the surroundings. Find that gain.
Show the worked solution
- Use a complete budget: 120 = 70 + other gain.
- Subtract the known desired gain.
- The surroundings gain 50 J; total energy is conserved.
Answer 50 J in the surroundings.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Account for 120 J released when a desired store gains 70 J and the surroundings are the only other destination.
Give me a hint
Subtract the supplied store gain from the total release.
Compare my reasoning
- 120 J is the total released.
- 70 J is already accounted for.
- The remaining gain is 120 − 70 = 50 J.
50 J to the surroundings.
Look for these in your work
- Balance all destinations.
- Avoid calling the remainder destroyed.
Find stored changes when a system receives 100 J and transfers out 30 J, and when it receives 20 J and transfers out 50 J.
Give me a hint
Use input minus output for both changes.
Compare my reasoning
- First change = 100 − 30 = +70 J.
- Second change = 20 − 50 = −30 J.
- A negative change is a decrease, not a negative absolute store.
+70 J; −30 J.
Look for these in your work
- Use one boundary and interval.
- Distinguish change from total stored energy.
A claimed closed 120 J release gives 70 J to one store and 60 J to another. Explain why the account needs checking.
Give me a hint
Compare the total claimed gains with the stated release.
Compare my reasoning
- The reported gains sum to 130 J.
- This exceeds 120 J by 10 J.
- Investigate omitted inputs, other store decreases or measurements; do not invent energy creation.
The gains exceed the release by 10 J; the specified closed account is inconsistent.
Look for these in your work
- Calculate the discrepancy.
- Name evidence that could repair the account without violating conservation.
Common mistakes
- Wasted energy is not destroyed.
- State the selected system and interval.
- A stored change is not the final stored total.
What can you explain now?
What stored change follows from 20 J entering and 50 J leaving?
Compare with the explanation
−30 J; a decrease of 30 J.
Net input is 20 − 50 = −30 J over the stated interval.
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Make it stick
State conservation, explain dispersed wasted energy and write input minus output for a selected system's stored change.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.