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Physics · Electricity

Energy transfers in everyday appliances

Describe how appliances transfer energy, and use \(E = Pt\) and \(E = QV\) to calculate energy transferred by electrical work.

  • 6 key terms
  • All boards
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Warm-up

Answer each one, then check.

  1. 1

    What is the unit of energy?

    Show answerHide answer

    Joule (J)

  2. 2

    What is the unit of charge?

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    Coulomb (C)

  3. 3

    Convert 3 minutes to seconds.

    Show answerHide answer

    180 s

  4. 4

    What is \(P = VI\)?

    Show answerHide answer

    Power = pd × current

  5. 5

    What does a motor transfer energy to?

    Show answerHide answer

    The kinetic store

Learning Objectives

  1. 1Describe how domestic appliances transfer energy from batteries or the mains to kinetic energy stores or heating.
  2. 2Recall and apply \(E = Pt\).
  3. 3Recall and apply \(E = QV\).
  4. 4Relate the power rating of appliances to changes in stored energy.

ELECTRICAL WORK

energy transferred \(=\) power \(\times\) time \(E = Pt\) and energy transferred \(=\) charge flow \(\times\) 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.

Energy from Power and Time

A 2000 W kettle is switched on for 3.0 minutes. Calculate the energy it transfers.

Show the solutionHide the solution
  1. 1 Convert the time \(3.0\) min \(= 180\) s
  2. 2 Substitute \(E = Pt = 2000 \times 180\)
  3. 3 Answer \(E = 360\,000\) J

Answer360 000 J (360 kJ)

Energy from Charge and pd

500 C of charge flows through a 12 V battery-powered motor. Calculate the energy transferred.

Show the solutionHide the solution
  1. 1 Write the equation \(E = QV\)
  2. 2 Substitute \(E = 500 \times 12\)
  3. 3 Answer \(E = 6000\) J

Answer6000 J

Comparing Appliances

A 700 W drill is used for 60 s. A 1500 W hair dryer is used for 60 s. Which transfers more energy?

Show the solutionHide the solution
  1. 1 Drill \(700 \times 60 = 42\,000\) J
  2. 2 Hair dryer \(1500 \times 60 = 90\,000\) J

AnswerThe 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.

Energy Bill

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: \(3000 \times 480 = 1\,440\,000\) J (1.44 MJ). Electrical energy is transferred to the thermal store of the water; some is dissipated.

Can I...?

  1. 1Describe the energy transfer in a kettle.
  2. 2Describe the energy transfer in a motor.
  3. 3Recall E = Pt.
  4. 4Recall E = QV.
  5. 5Convert time to seconds.
  6. 6Compare appliances.
  7. 7Explain power rating.
  8. 8Give units correctly.

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) (3 marks)

Convert minutes to seconds first.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

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.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 Calculate 3 marks

    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

    Show answerHide answer

    Model answer

    \(t = 180\) s; \(E = 2000 \times 180 = 360\,000\) J

    Mark scheme

    • Converts to 180 s — 1 mark
    • Correct substitution — 1 mark
    • 360 000 J — 1 mark
  2. Question 2 Calculate 2 marks

    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

    Show answerHide answer

    Model answer

    \(E = 500 \times 12 = 6000\) J

    Mark scheme

    • Correct substitution — 1 mark
    • 6000 J — 1 mark
  3. Question 3 Describe 2 marks

    Describe the main energy transfer when an electric drill is used.

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    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).

    Mark scheme

    • Electrical energy transferred — 1 mark
    • To the kinetic store of the drill — 1 mark
  4. Question 4 Explain 3 marks

    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.

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    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.

    Mark scheme

    • 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
  5. Question 5 Calculate 4 marks

    A 3.0 kW electric shower is used for 8.0 minutes. Calculate the energy transferred in megajoules (MJ).

    Show answerHide answer

    Model answer

    \(P = 3000\) W; \(t = 480\) s; \(E = 1\,440\,000\) J = 1.44 MJ

    Mark scheme

    • Converts to 3000 W — 1 mark
    • Converts to 480 s — 1 mark
    • 1 440 000 J — 1 mark
    • 1.44 MJ — 1 mark

Quick check

  1. Energy transferred by an appliance is power multiplied by...

    1. Acurrent
    2. Bresistance
    3. Cmass
    4. Dtime
    Show answerHide answer

    D: time

    \(E = Pt\).

  2. A motor transfers electrical energy mainly to the...

    1. Akinetic store
    2. Bchemical store
    3. Cnuclear store
    4. Dmagnetic store
    Show answerHide answer

    A: kinetic store

    The motor turns.

  3. A 100 W lamp on for 10 s transfers...

    1. A10 J
    2. B1000 J
    3. C100 J
    4. D10 000 J
    Show answerHide answer

    B: 1000 J

    100 × 10.

  4. 10 C flows through a 12 V pd. Energy transferred is...

    1. A1.2 J
    2. B22 J
    3. C120 J
    4. D12 J
    Show answerHide answer

    C: 120 J

    \(E = QV = 10 \times 12\).

  5. Two appliances are on for the same time. The one with the higher power...

    1. Atransfers less energy
    2. Btransfers the same
    3. Chas less current
    4. Dtransfers more energy
    Show answerHide answer

    D: transfers more energy

    Energy = power × time.

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