AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER
Energy transfers in everyday appliances
Electricity · Lesson 7 of 10
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.
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. |