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
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
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.
Sources for this lesson
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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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.
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.
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.