Explain useful and dispersed energy qualitatively
Compare the share reaching the intended task while keeping unwanted transfers in the conservation account.
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028, section 1.7.3. Resource comparison and qualitative efficiency are Core. Solar origins and exceptions, fusion and its electricity research, and numerical efficiency are Supplement. Compare the same useful task, consistent system boundary and stated interval. Written work is self-checked. Human subject-teacher review, supervised practical experience and cumulative assessment 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
- Balance input and distinct destinations.
- Specify a device's intended useful task.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which quantity is the whole input in a 200 J input with 120 J useful output?
What you will learn
- Explain efficiency as the useful share of a supplied input.
- Distinguish efficient conversion from a large output or a renewable source.
The task defines the useful share
A transfer is more efficient when a greater share of the total input reaches the intended useful output. A lamp's desired output may be visible radiation, while a water heater's desired output is heating the water system. Unwanted warming is not always the same judgement across both tasks. Compare each device with a clearly stated useful output and system boundary.
Energy reaching other internal stores or waves is conserved even when called wasted for that task. It is often more dispersed and less useful for that purpose. For the same input, a device sending more energy to the desired output and less elsewhere is qualitatively more efficient, provided both accounts use the same useful-output definition.
Amount and proportion are different
Device A receives 200 J and supplies 120 J usefully; device B receives 1000 J and supplies 350 J usefully. B supplies a larger useful amount, but A directs a larger fraction of its input to the task. Core work can compare these as shares without requiring the percentage-efficiency formula, which is labelled Extended in the following lessons.
Renewable describes the source's replenishment, not the conversion's useful fraction. A renewable source can feed a conversion with substantial other transfers, and an efficient conversion can still use a finite resource. Neither efficiency alone nor useful amount alone determines whether local demand, cost, reliability and environmental requirements are met.
Pause and explain
Does a larger useful amount necessarily mean greater efficiency if inputs differ?
Worked example
For the same 200 J input and same task, device A gives 120 J usefully while B gives 160 J. Compare efficiency qualitatively.
Show the worked solution
- Both have the same input and useful-output definition.
- B sends a larger share, 160 rather than 120 J, to that task.
- B is more efficient for this comparison; the remainder remains conserved elsewhere.
Answer B is more efficient for the same input and task.
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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.
Compare 120 J and 160 J useful outputs from equal 200 J inputs for the same task.
Give me a hint
The identical inputs make the useful-share comparison direct.
Compare my reasoning
- A supplies 120 J to the task.
- B supplies 160 J from the same input.
- B's useful share is larger and the other destinations still conserve energy.
B is more efficient in the stated comparison.
Look for these in your work
- Use the common input and task.
- Keep other transfers in the account.
A renewable-source device produces considerable unwanted warming. Does the source's renewal prove the conversion is perfectly efficient?
Give me a hint
Keep source replacement separate from useful-output share.
Compare my reasoning
- Renewal describes the resource's replenishment.
- Unwanted destinations reduce the fraction reaching the stated useful task.
- A renewable label does not establish perfect conversion.
No; renewability and conversion efficiency are separate factors.
Look for these in your work
- Define the two factors separately.
- Account for the unwanted transfer.
Two devices have different input amounts and different desired outputs. What must be made consistent before a meaningful efficiency comparison?
Give me a hint
Specify a common task and a complete input account.
Compare my reasoning
- Define which output is useful for the intended task.
- Identify each full input with a consistent boundary and interval.
- Compare the useful share, not just the larger output number.
Use a consistent task, boundary and interval, and compare useful fractions of complete inputs.
Look for these in your work
- State the useful-output definition.
- Avoid a comparison from amounts alone.
Common mistakes
- Wasted energy is not destroyed.
- Larger useful output does not alone prove a larger useful fraction.
- Renewable and efficient are different descriptions.
What can you explain now?
Can a useful heating transfer count as desirable in a heater rather than wasted?
Compare with the explanation
Yes.
The stated useful task determines how that output is classified.
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Make it stick
Explain useful share, conserved dispersed energy and the difference between efficiency, output amount and resource renewal.
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