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What Is Efficiency?

Part of Efficiency · GCSE GCSE Physics revision

This deep dive covers What Is Efficiency? within Efficiency for GCSE Physics. Revise Efficiency in Energy for GCSE Physics with 19 exam-style questions and 4 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 2 of 13 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.

Topic position

Section 2 of 13

Practice

19 questions

Recall

4 flashcards

📊 What Is Efficiency?

Studio Render Sankey diagram showing energy flow through a motor. A thick amber input arrow (100 J) enters from the left. The motor splits it into a cyan useful output arrow (40 J, kinetic/light/sound) going right and a thicker amber wasted energy arrow (60 J, heat) angled downward. Arrow widths are proportional to energy. The efficiency equation at the bottom: efficiency = useful output / total input = 40/100 = 0.40 (40%).

Figure 1: A Sankey diagram — arrow width is proportional to energy. 100 J in → 40 J useful (cyan) + 60 J wasted as heat (amber). Efficiency = 40%.

Efficiency is a measure of how well a device transfers input energy into useful output energy. It is expressed as a decimal (0 to 1) or as a percentage (0% to 100%).

Efficiency = Useful output energy ÷ Total input energy

To express as a percentage, multiply by 100:

Efficiency (%) = (Useful output energy ÷ Total input energy) × 100

You can also use power values instead of energy values:

Efficiency = Useful output power ÷ Total input power

Key facts about efficiency:

  • Efficiency has no unit — it is a ratio (or percentage)
  • Efficiency is always less than 100% (or less than 1 as a decimal)
  • Wasted energy = Total input energy − Useful output energy
  • Most wasted energy becomes thermal energy (heat) in the surroundings

Quick Check: A motor receives 500 J and produces 350 J of useful kinetic energy. Calculate its efficiency as a percentage.

Keep building this topic

Read this section alongside the surrounding pages in Efficiency. That gives you the full topic sequence instead of a single isolated revision point.

The Leaky Bucket ProblemSankey DiagramsWhy Is Efficiency Never 100%?Efficiency of Common Devices

Practice Questions for Efficiency

Which equation correctly defines efficiency?

  • A. efficiency = total input energy / useful output energy
  • B. efficiency = useful output energy transfer / total input energy transfer
  • C. efficiency = wasted energy / total input energy
  • D. efficiency = total input energy / wasted energy
1 markfoundation

Explain one method that can be used to reduce unwanted energy transfers in a machine and state how it reduces waste.

2 marksstandard

Quick Recall Flashcards

As a percentage
85% (multiply decimal by 100)
As a decimal
0.85 (between 0 and 1)

19 questions on Efficiency — practise free

Instant marking, adaptive difficulty, and 4 spaced repetition flashcards. Free until your GCSEs.

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The Leaky Bucket Problem

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Sankey Diagrams