Understand, qualitatively, the concept of efficiency of energy transfer

1.7.3 Energy Resources and Efficiency

⚡️ Energy resources are the raw materials we use to produce energy. They can be renewable (like wind or solar) or non‑renewable (like coal or oil). Understanding how efficiently we can convert these resources into useful work is key to judging their value and environmental impact.

What is Efficiency?

Efficiency, denoted by the Greek letter η, is the ratio of useful work output to the energy input. It is always expressed as a percentage.

$$\eta = \frac{W_{\text{out}}}{Q_{\text{in}}}\times 100\%$$

• • Useful work output – the energy that actually does something useful (e.g., turning a wheel, lighting a bulb).
• • Energy input – the total energy supplied by the source.
• • If η = 100 %, all input energy is converted to useful work (ideal, but impossible in reality).

Typical Efficiencies of Energy Resources

Analogy: The Water Wheel

Imagine a water wheel turning in a river. The water’s kinetic energy is the input energy. The wheel’s rotation is the useful work output. If the wheel is made of a very slippery material, most of the water’s energy will just flow past – low efficiency. If it’s a well‑designed wheel with good paddles, most of the water’s energy turns the wheel – high efficiency. This is exactly how we think about energy conversion in power plants and engines.

💡 Tip for exams: When you see a question about efficiency, always write the formula, identify the work and heat terms, and remember that efficiencies are less than 100 % for real devices.

Exam Practice Questions

1. Calculate the efficiency of a device that produces 250 J of work when 500 J of energy is supplied.
2. Why is the efficiency of a hydroelectric dam higher than that of a coal power plant?
3. Explain, using the water wheel analogy, why no energy conversion is 100 % efficient.

📝 Exam Tip: Always express your answer as a percentage and round to the nearest whole number unless the question specifies otherwise.