Energy transfers in everyday appliances - Teacher Slides.pptx

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Energy transfers in everyday appliances

Electricity

Lesson 7 of 10

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Warm-up

Electricity

Lesson 7 of 10

Answer each one, then check.

1

What is the unit of energy?

2

What is the unit of charge?

3

Convert 3 minutes to seconds.

4

What is P = VI?

5

What does a motor transfer energy to?

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Warm-up - Answers

Electricity

Lesson 7 of 10

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

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Learning Objectives

Electricity

Lesson 7 of 10

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.

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Electricity

Lesson 7 of 10

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.

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Appliances and Their Energy Transfers

Electricity

Lesson 7 of 10

Different appliances give different useful transfers.

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Energy from Power and Time

Electricity

Lesson 7 of 10

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)

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Energy from Charge and pd

Electricity

Lesson 7 of 10

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

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Comparing Appliances

Electricity

Lesson 7 of 10

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.

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Power Ratings

Electricity

Lesson 7 of 10

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.

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Key Terms

Electricity

Lesson 7 of 10

Electrical work

Energy transferred when charge flows in a circuit.

Power rating

The power of an appliance in normal use.

Appliance

A device that transfers electrical energy usefully.

Kinetic store

The energy of a moving object.

Thermal store

The energy of hot objects.

Charge

Measured in coulombs.

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Electricity

Lesson 7 of 10

YOUR TASK

Energy Bill

12 minutes

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.

WHAT 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.

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Can I...?

Electricity

Lesson 7 of 10

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.

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Summary & Exam Focus

E = Pt and E = QV.

Kettle: electrical to thermal store.

Motor: electrical to kinetic store.

Power rating: energy per second.

EXAM FOCUS

A 2000 W kettle is on for 3.0 minutes. Calculate the energy transferred. (3 marks)

Convert minutes to seconds first.

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Exam Practice: Energy transfers in everyday appliances

Electricity

Lesson 7 of 10

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.

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Exam Practice: Energy transfers in everyday appliances (cont.)

Electricity

Lesson 7 of 10

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).

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Question 1 · 3 marks · Calculate

Electricity

Lesson 7 of 10

“

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.

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Question 1 · mark scheme

Electricity

Lesson 7 of 10

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

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Question 2 · 2 marks · Calculate

Electricity

Lesson 7 of 10

“

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.

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Question 2 · mark scheme

Electricity

Lesson 7 of 10

2 marks available. Award a mark for each point made.

Correct substitution

1 mark

6000 J

1 mark

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Question 3 · 2 marks · Describe

Electricity

Lesson 7 of 10

“

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.

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Question 3 · mark scheme

Electricity

Lesson 7 of 10

2 marks available. Award a mark for each point made.

Electrical energy transferred

1 mark

To the kinetic store of the drill

1 mark

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Question 4 · 3 marks · Explain

Electricity

Lesson 7 of 10

“

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.

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Question 4 · mark scheme

Electricity

Lesson 7 of 10

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

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Question 5 · 4 marks · Calculate

Electricity

Lesson 7 of 10

“

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.

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Question 5 · mark scheme

Electricity

Lesson 7 of 10

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

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