Efficiency
This topic covers the calculation of energy efficiency for any energy transfer.
Full topic guide: the detailed syllabus page with worked examples and common mistakes lives at studyvector.co.uk/gcse/physics/energy/efficiency.
Topic preview: Efficiency
Sample stems from the StudyVector question bank (AQA · Edexcel · OCR) — not generic filler text.
More questions are being linked to this topic. You can still start adaptive practice after you create a free account.
Coverage and provenance
What this page is based on
StudyVector does not present unsupported question coverage as complete. Read how questions are selected and reviewed.
Topic explanation
Efficiency is a measure of how much useful energy you get out of a device compared to the total energy you put in. It is usually expressed as a percentage. No device is 100% efficient; some energy is always wasted, usually as heat.
Efficiency is easiest to revise when it is treated as a precise exam behaviour, not a loose note-taking category. In GCSE Physics, the goal is to recognise how the topic appears in a question, identify the command word, and decide what evidence, method, or vocabulary earns marks. StudyVector keeps this page tied to AQA · Edexcel · OCR language where coverage is available, then routes practice towards the same topic so revision moves from explanation into retrieval.
A strong revision session starts with a short recall check. Write down the rule, definition, process, or method linked to Efficiency before looking at any notes. Then answer one exam-style prompt and compare your answer with the mark-scheme logic: did you make a clear point, support it with the right step, and avoid drifting into a nearby topic? This matters because many lost marks come from almost-correct answers that do not match the expected structure.
Use this guide as the first layer: understand the topic, look at the worked examples, complete the mini quiz, then move into full practice. The full StudyVector practice loop is designed to capture whether mistakes are caused by knowledge, method, language, or timing. That distinction is important. If the error is factual, you need reteaching. If the error is method-based, you need a worked retry. If the error is wording, you need command-word calibration. That is how Efficiency becomes a controlled revision target rather than another page in a folder.
Lost marks → repair task
Why marks are usually lost here
These are the error patterns StudyVector looks for after an attempt. The goal is not a generic explanation; it is one repair move and one follow-up question.
Unit, formula, or method slip
Examiner move: Select the correct method and keep units, substitutions, signs, and rounding visible.
Repair drill: Redo the calculation or method line slowly, naming the formula before substituting values.
Missing chain of reasoning
Examiner move: Show the link between point, method, evidence, and conclusion instead of jumping to the final line.
Repair drill: Write the missing because/therefore step, then retry one isomorphic question.
Weak evidence or data reference
Examiner move: Use a precise value, quote, example, diagram feature, or syllabus term to support the claim.
Repair drill: Add one concrete reference to the answer and remove any generic sentence that does not earn a mark.
Mini quiz
Use these checks before full practice. They test topic recognition, exam technique, and whether you can connect the explanation to a marked response.
1. What should you check first when a Efficiency question appears in GCSE Physics?
- A.The command word and the exact topic focus
- B.The longest paragraph in your notes
- C.A memorised answer from a different topic
2. Which revision action gives the strongest evidence that Efficiency is improving?
- A.Rereading the explanation twice
- B.Answering a timed exam-style question and reviewing lost marks
- C.Highlighting every key phrase in the topic notes
Sample questions
Topic-specific public question previews are still being reviewed. We keep them off public pages until the topic match is safe.
Exam tips
- Read the command word carefully — "explain" needs reasons; "state" expects a short fact.
- For Efficiency, show structured working even when you are practising multiple choice — it builds accuracy under time pressure.
- Mark yourself against the mark scheme style: one clear point per mark, in logical order.
- Come back to this topic after a day or two; short spaced reviews beat one long cram.
Worked examples
Example 1
Modelled exam response
A light bulb is supplied with 100J of electrical energy and it produces 20J of light energy. Calculate its efficiency. Solution: Efficiency = (Useful Energy Out / Total Energy In) x 100%. Efficiency = (20J / 100J) x 100% = 20%.
Example 2
Identify the task before answering
Question type: a Efficiency prompt asks for a clear response in GCSE Physics. Step 1: underline the command word. Step 2: name the exact part of Efficiency being tested. Step 3: decide whether the mark scheme wants a definition, method, explanation, comparison, or calculation. Why it works: most weak answers fail before the content starts because they answer the topic generally rather than the exact exam task.
Example 3
Turn feedback into a repair task
Suppose your answer shows partial understanding but loses marks for precision. First, rewrite the missing mark as a short target: "I need to state the mechanism, unit, reason, or evidence explicitly." Then answer one similar question without notes. Finally, compare the second attempt with the first and check whether the same mark was recovered. Why it works: Efficiency improves faster when feedback creates a specific retry, not another passive reading session.
Next revision routes from this subject
Good topic pages should lead naturally into the next useful page. Use these links to stay inside the same strand or jump into the next topic area without starting your search again.
Stay in the same topic area
Explore the wider subject map
Common mistakes
- Stating that efficiency can be greater than 100%. This is impossible as it would violate the principle of conservation of energy.
- Confusing useful energy with wasted energy. Efficiency is about the useful output, not the total input or the wasted part.
- Leaving efficiency as a decimal instead of a percentage. The question will usually specify the required format.
Exam board notes
Covered by all major boards (AQA, Edexcel, OCR) at both Foundation and Higher tiers.
FAQs
How can you improve the efficiency of a device?
Efficiency can be improved by reducing the amount of wasted energy. For example, by lubricating moving parts to reduce friction, or by using insulation to reduce heat loss.
Why can't a machine be 100% efficient?
In any real-world machine, some energy is always converted into non-useful forms, such as heat due to friction or air resistance. This wasted energy means that the useful output energy is always less than the input energy.
More on StudyVector
Full practice set
The complete adaptive question bank for this topic — personalised to your weak areas — is available after you sign in. Your session can start on this topic immediately.