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?
Figure 1: A Sankey diagram — the width of arrows represents the amount of energy. Useful energy output is smaller than total input because energy is wasted.
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.
Efficiency = (Useful output ÷ Total input) × 100 = (350 ÷ 500) × 100 = 70%. The motor is 70% efficient. Wasted energy = 500 − 350 = 150 J (likely as heat from friction).