Efficiency

Overview

You should explain where the rest of the energy goes, not just calculate a percentage.

What You Need to Know

• Build from the energy-transfer work already completed and identify useful output versus total input.
• Use everyday devices so you can see why some energy becomes less useful, often as heating or sound.
• Work through both efficiency equations clearly and keep percentage form explicit.
• Compare systems with different efficiencies to make the interpretation meaningful, not just procedural.
• Reinforce that 100% efficiency is not realistic for everyday devices because some energy is always transferred in unwanted ways.

How to Work Through It

1. Start with a retrieval task on useful and wasted energy transfers from earlier lessons.
2. Introduce efficiency qualitatively, then work through the percentage equations.
3. Practise calculations using both energy and power forms of the equation.
4. Finish with short interpretation questions where you explain what an efficiency value means.

Check Your Understanding

• Can you explain whether a device with higher efficiency transfers more useful energy from the same input and give a reason.
• Use a hinge question where you choose the correct efficiency equation from a short scenario.
• Try one efficiency calculation and check whether you can explain where the rest of the input went.

Common Mistakes

• Confusing useful output with total input. Keep the fraction structure visible.
• Percentage conversion errors are common, especially between decimal and percent forms.
• Some think “wasted” means energy disappeared. Revisit conservation of energy and transfer to less useful stores.

Next Steps

• Set short mixed questions on energy efficiency and power efficiency.
• Carry efficiency into the comparison of energy resources.