Welcome! A kettle company is losing customers because its kettles waste too much energy. In this lesson you’ll learn about energy stores, transfers and efficiency, then test three kettles and decide which one could save the company. It takes about one hour. Work through the steps in order.
Before you start
- You need: your exercise book or paper, a pen and a calculator.
- Write the title and date: Energy and efficiency – Save the Kettle Company.
- The golden rule: write your answer first, then tap Show answer. Correct mistakes in a different colour.
- Look out for the blue 📝 boxes. Copy each one into your book neatly – these are your revision notes.
📝 Copy into your book
By the end of this lesson I will be able to:
1. name the energy stores and describe energy transfers
2. use E = Pt to calculate energy transferred
3. calculate efficiency and suggest ways to improve it.
Step 1: Starter (5 minutes)
When you boil a kettle, the outside of the kettle gets warm and you hear it rumble. Where does all the energy from the plug end up? List as many places as you can.
Show answer
Most goes into the thermal store of the water – that’s the useful part. Some heats the kettle itself, some heats the surrounding air, and a tiny amount is carried away as sound. Energy that isn’t useful is called wasted energy.
📝 Copy into your book
Energy cannot be created or destroyed – only transferred. Energy that isn’t transferred usefully is WASTED. It is usually dissipated (spread out) to the surroundings, heating them up.
Step 2: Read and write (8 minutes)
First, copy these three questions into your book. Then read the text below carefully and answer them in full sentences.
- Name four energy stores.
- How is energy transferred to a kettle from the mains?
- What does it mean if a device has an efficiency of 0.8?
📖 Read: Stores, transfers and efficiency
Energy is stored in different ways, called energy stores. They include the kinetic store (moving objects), the gravitational potential store (raised objects), the elastic potential store (stretched or squashed objects), the thermal store (hot objects), the chemical store (fuels, food and batteries), and the nuclear, magnetic and electrostatic stores.
Energy is transferred between stores in four ways: by a force doing work, electrically (when a current flows), by heating, and by radiation such as light or sound. In a kettle, energy is transferred electrically from the mains to the heating element, and then by heating to the thermal store of the water.
No device is perfect. Efficiency tells us what fraction of the energy supplied is transferred usefully. An efficiency of 0.8 (or 80%) means that 80% of the energy is transferred usefully, and 20% is wasted. Efficiency can never be more than 1 (100%). We can improve efficiency by reducing wasted energy – for example, with insulation or lubrication.
Show answers
(a) Any four: kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, magnetic, electrostatic.
(b) Electrically (by a current), from the mains to the heating element.
(c) 80% of the energy supplied is transferred usefully; 20% is wasted.
Step 3: Key facts (6 minutes)
Copy the two equations you’ll need:
📝 Copy into your book
energy transferred = power × time E = P t
E in J, P in W, t in s
efficiency = useful output energy transfer ÷ total input energy transfer
(multiply by 100 for a percentage)
Quick check: a motor is supplied with 500 J and does 400 J of useful work. What is its efficiency?
Show answer
400 ÷ 500 = 0.8 (or 80%). The other 100 J is wasted.
Step 4: Main task – Save the Kettle Company (16 minutes)
You’re the company’s new energy scientist. Three new kettle designs were each used to boil 1 litre of water. Heating that water to boiling needs 336 000 J of useful energy – the same for every kettle.
| Kettle | Power | Time to boil | Design | Price |
|---|---|---|---|---|
| Classic | 2000 W | 200 s | Thin metal body | £20 |
| Thermo | 2500 W | 150 s | Double-walled body with an air gap | £35 |
| Turbo | 3000 W | 140 s | Thin metal body, very powerful element | £30 |
Your task: write a report for the boss.
- For each kettle, use E = Pt to calculate the total energy supplied.
- Calculate each kettle’s efficiency as a percentage.
- Explain why the most efficient kettle wastes less energy.
- Recommend which kettle the company should sell, and why. Is the fastest kettle the best?
Stuck? Useful energy = 336 000 J for every kettle. Total energy = power × time.
Show a model report
1. Energy supplied:
Classic: 2000 × 200 = 400 000 J
Thermo: 2500 × 150 = 375 000 J
Turbo: 3000 × 140 = 420 000 J
2. Efficiency:
Classic: 336 000 ÷ 400 000 = 0.84 → 84%
Thermo: 336 000 ÷ 375 000 = 0.896 → about 90%
Turbo: 336 000 ÷ 420 000 = 0.80 → 80%
3. Why Thermo wastes less: its double wall traps a layer of air, which has a very low thermal conductivity. Less energy is transferred through the body to the surroundings, so more stays in the water.
4. Recommendation: sell the Thermo. It is the most efficient, so it uses the least energy and costs customers the least to run. The Turbo is fastest, but it wastes the most energy, so being fast doesn’t make it the best.
✅ Check: did you show all six calculations, and give a reason for your recommendation?
📝 Copy into your book
Ways to improve efficiency: THERMAL INSULATION (reduces energy lost by heating) and LUBRICATION (reduces friction between moving parts).
Step 5: Exam practice (14 minutes)
Look at the marks – they tell you how many points to make. Answer in full sentences, then mark yourself.
Q1 (3 marks) A 1500 W hairdryer is used for 120 s. Calculate the energy transferred.
Show model answer
E = P t (1)
E = 1500 × 120 (1)
E = 180 000 J (1)
Q2 (3 marks) An electric motor is supplied with 2400 J of energy. It transfers 1800 J to the kinetic store of a lift. Calculate the efficiency of the motor as a percentage.
Show model answer
efficiency = useful output ÷ total input (1)
= 1800 ÷ 2400 = 0.75 (1)
= 75% (1)
Q3 (4 marks) Describe the energy transfers when an electric kettle boils water, and suggest two ways a manufacturer could make it more efficient.
Show model answer
Energy is transferred electrically from the mains to the heating element (1). It is then transferred by heating to the thermal store of the water, which is useful (1). Some energy is wasted heating the kettle and the surroundings (1). Improvements: insulate the kettle body (e.g. a double wall with an air gap); use a lid that fits well; make the element touch as much water as possible (1 for any two).
✅ Add up your marks out of 10 and write the score in your book.
Step 6: Exit ticket (8 minutes)
- A device can never be 100% efficient because…
- The fastest kettle isn’t always the best because…
- One thing I’m still not sure about is…
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes for this topic. Want more? Try the Energy questions and the Cut the Bills lesson.
