Ch 15 · 1.7.4 Efficiency · O Level Physics 5054

Topic 15 of 17 in Motion, Forces and Energy — O Level Physics 5054 — 19 lessons.

Introduction

No machine turns all of the energy you put in into the energy you want. A filament lamp gets hot, a car engine warms the air, a motor hums. Efficiency tells you what fraction of the input is usefully transferred. (Syllabus section 1.7.4.)

Key ideas

efficiency = useful energy output/total energy input (× 100\%)

🧑‍🏫 Teacher's explanation

Imagine you are buying a bag of 100 candies. You pay for 100 candies, but when you open the bag, only 80 are actually edible sweets, and 20 are just pieces of plastic packaging. The 'efficiency' of the bag is how much actual candy you got compared to what you paid for. In physics, no machine is perfect; some energy always 'escapes' into the surroundings, usually as heat or sound.

📖 Definitions

🔍 Worked example — Easy

An electric motor uses 500 J of electrical energy to lift a weight. The useful work done by the motor is 350 J. Calculate the efficiency of the motor as a percentage.

🔍 Worked example — Standard

A lightbulb has a power input of 60 W. It produces 12 W of useful light power. Calculate the efficiency of the bulb.

🔍 Worked example — Exam-style

A car engine consumes 2.5× 10^6 J of fuel during a journey. The kinetic energy gained by the car is 6.0× 10^5 J. Calculate the energy wasted as heat and sound, and then calculate the efficiency of the engine.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

Example 1: a motor. An electric motor is supplied with 2000 J and lifts a load, giving it 1500 J of gravitational potential energy.

🔍 Worked example — Easy

An electric motor is supplied with 2000 J of electrical energy. It uses this to lift a load, resulting in 1500 J of gravitational potential energy. Calculate the efficiency of the motor.

🔍 Worked example — Standard

A light bulb has a total power input of 60 W. It produces 9.0 W of light energy. Calculate the efficiency of the bulb.

🔍 Worked example — Exam-style

A power station is 38\% efficient. It takes in 500 MW of fuel energy. Calculate the useful power output of the station in MW.

⚠️ Common errors

Common mistakes

Practice

Answers: (1) 170/200 = 0.85 = 85\%. (2) 0.60 × 400 = 240\ W. (3) The useful output cannot be more than the input, because energy cannot be created (conservation of energy).

Summary

All chapters in Motion, Forces and Energy

  1. 1.1 Physical Quantities and Measurement Techniques
  2. 1.2 Motion
  3. 1.3 Mass and Weight
  4. 1.4 Density
  5. 1.5.1 Balanced and Unbalanced Forces
  6. 1.5.2 Friction
  7. 1.5.3 Elastic Deformation
  8. 1.5.4 Circular Motion
  9. 1.5.5 Turning Effect of Forces
  10. 1.5.6 Centre of Gravity
  11. 1.6 Momentum
  12. 1.7.1 Energy
  13. 1.7.2 Work
  14. 1.7.3 Energy Resources
  15. 1.7.4 Efficiency
  16. 1.7.5 Power
  17. 1.8 Pressure