AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER
Energy transfers in everyday appliances
Electricity · Lesson 7 of 10
Teacher copy - includes the notes for whoever is teaching from it.
Warm-up
Answer each one, then check.
1. What is the unit of energy?
Joule (J)
2. What is the unit of charge?
Coulomb (C)
3. Convert 3 minutes to seconds.
180 s
4. What is P = VI?
Power = pd × current
5. What does a motor transfer energy to?
The kinetic store
Learning Objectives
1. Describe how domestic appliances transfer energy from batteries or the mains to kinetic energy stores or heating.
2. Recall and apply E = Pt.
3. Recall and apply E = QV.
4. Relate the power rating of appliances to changes in stored energy.
Electrical Work
energy transferred = power × time E = Pt and energy transferred = charge flow × potential difference E = QV
Work is done when charge flows in a circuit. The energy an appliance transfers depends on how long it is on for and its power.
Appliances and Their Energy Transfers
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Different appliances give different useful transfers. |
A kettle, a drill and a hair dryer with the energy transfers and power rating of each.
Energy from Power and Time
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A 2000 W kettle is switched on for 3.0 minutes. Calculate the energy it transfers. |
1. Convert the time
3.0 min = 180 s
2. Substitute
E = Pt = 2000 × 180
3. Answer
E = 360 000 J
Answer: 360 000 J (360 kJ)
Energy from Charge and pd
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500 C of charge flows through a 12 V battery-powered motor. Calculate the energy transferred. |
1. Write the equation
E = QV
2. Substitute
E = 500 × 12
3. Answer
E = 6000 J
Answer: 6000 J
Comparing Appliances
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A 700 W drill is used for 60 s. A 1500 W hair dryer is used for 60 s. Which transfers more energy? |
1. Drill
700 × 60 = 42 000 J
2. Hair dryer
1500 × 60 = 90 000 J
Answer: The hair dryer transfers more energy because its power is greater for the same time.
Power Ratings
What the rating tells you.
▸ Meaning. The power rating is the energy the appliance transfers each second.
▸ Energy used. Energy depends on the power rating and how long the appliance is on.
▸ Motors. Transfer electrical energy to the kinetic energy store of the motor.
▸ Heaters. Transfer electrical energy to the thermal energy store.
Key Terms
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Electrical work Energy transferred when charge flows in a circuit. |
Power rating The power of an appliance in normal use. |
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Appliance A device that transfers electrical energy usefully. |
Kinetic store The energy of a moving object. |
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Thermal store The energy of hot objects. |
Charge Measured in coulombs. |
Your Task: Energy Bill
12 minutes
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A 3.0 kW electric shower is used for 8.0 minutes. Calculate the energy transferred, in joules. Explain what happens to the energy. 1. Convert kW and minutes. 2. Multiply. |
A good answer shows: 3000 × 480 = 1 440 000 J (1.44 MJ). Electrical energy is transferred to the thermal store of the water; some is dissipated.
Note: Ask learners to compare this with a kettle.
Can I...?
☐ Describe the energy transfer in a kettle.
☐ Describe the energy transfer in a motor.
☐ Recall E = Pt.
☐ Recall E = QV.
☐ Convert time to seconds.
☐ Compare appliances.
☐ Explain power rating.
☐ Give units correctly.
Summary
✓ E = Pt and E = QV.
✓ Kettle: electrical to thermal store.
✓ Motor: electrical to kinetic store.
✓ Power rating: energy per second.
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EXAM FOCUS A 2000 W kettle is on for 3.0 minutes. Calculate the energy transferred. (3 marks) Convert minutes to seconds first. |
Exam Practice: Energy transfers in everyday appliances
Answer all questions. Use the mark allocation as a guide to how much to write. · 15 minutes
▸ Question 1 · 3 marks · Calculate. A kettle has a power of 2000 W. It is switched on for 3.0 minutes. Calculate the energy transferred by the kettle. Use the equation: energy…
▸ Question 2 · 2 marks · Calculate. A charge of 500 C flows through a motor connected to a 12 V battery. Calculate the energy transferred to the motor. Use the equation…
▸ Question 3 · 2 marks · Describe. Describe the main energy transfer when an electric drill is used.
▸ Question 4 · 3 marks · Explain. A hair dryer has a power of 1500 W. A phone charger has a power of 10 W. Explain why the hair dryer transfers more energy in one minute.
▸ Question 5 · 4 marks · Calculate. A 3.0 kW electric shower is used for 8.0 minutes. Calculate the energy transferred in megajoules (MJ).
Question 1 · 3 marks · Calculate
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“A kettle has a power of 2000 W. It is switched on for 3.0 minutes. Calculate the energy transferred by the kettle. Use the equation: energy transferred = power × time” |
HOW TO ANSWER IT Command word: Calculate. Worth 3 marks, so plan before writing.
Question 1 · mark scheme
3 marks available. Award a mark for each point made.
▸ Converts to 180 s. 1 mark
▸ Correct substitution. 1 mark
▸ 360 000 J. 1 mark
▸ Model answer. t = 180 s; E = 2000 × 180 = 360 000 J
Question 2 · 2 marks · Calculate
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“A charge of 500 C flows through a motor connected to a 12 V battery. Calculate the energy transferred to the motor. Use the equation: energy transferred = charge flow × potential difference” |
HOW TO ANSWER IT Command word: Calculate. Worth 2 marks, so plan before writing.
Question 2 · mark scheme
2 marks available. Award a mark for each point made.
▸ Correct substitution. 1 mark
▸ 6000 J. 1 mark
▸ Model answer. E = 500 × 12 = 6000 J
Question 3 · 2 marks · Describe
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“Describe the main energy transfer when an electric drill is used.” |
HOW TO ANSWER IT Command word: Describe. Worth 2 marks, so plan before writing.
Question 3 · mark scheme
2 marks available. Award a mark for each point made.
▸ Electrical energy transferred. 1 mark
▸ To the kinetic store of the drill. 1 mark
▸ Model answer. Electrical energy from the mains is transferred to the kinetic energy store of the drill bit (and some to the thermal store and sound).
Question 4 · 3 marks · Explain
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“A hair dryer has a power of 1500 W. A phone charger has a power of 10 W. Explain why the hair dryer transfers more energy in one minute.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 4 · mark scheme
3 marks available. Award a mark for each point made.
▸ Power is energy transferred per second. 1 mark
▸ 1500 J each second and 10 J each second. 1 mark
▸ Same time so hair dryer transfers more. 1 mark
▸ Model answer. Power is the energy transferred each second, so 1500 W transfers 1500 J each second and 10 W transfers 10 J each second. In the same time the hair dryer transfers much more energy.
Question 5 · 4 marks · Calculate
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“A 3.0 kW electric shower is used for 8.0 minutes. Calculate the energy transferred in megajoules (MJ).” |
HOW TO ANSWER IT Command word: Calculate. Worth 4 marks, so plan before writing.
Question 5 · mark scheme
4 marks available. Award a mark for each point made.
▸ Converts to 3000 W. 1 mark
▸ Converts to 480 s. 1 mark
▸ 1 440 000 J. 1 mark
▸ 1.44 MJ. 1 mark
▸ Model answer. P = 3000 W; t = 480 s; E = 1 440 000 J = 1.44 MJ