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Electrical power and energy Why can a 10 W bulb transfer more energy than a 3 kW kettle?

Foundation and Higher · AQA · 45 minutes

Students vote on whether a kettle boiling for three minutes or a light left on all day transfers more energy, then find out with E = P t. Power from current and p.d., energy from charge, mains electricity as the setting, and a virtual heater whose power they measure. Slides to present, a vote that shows who's stuck, practice and homework.

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The idea in one minute

Power is how fast energy is transferred: the energy transferred each second, P = E ÷ t, in watts (1 W is 1 joule each second). So the energy an appliance transfers depends on its power and on how long it is on: E = P t, with t in seconds. In a circuit the power of a component is the p.d. across it times the current through it, P = V I, and since V = I R it is also P = I² R. The p.d. is the energy each coulomb of charge transfers, so the energy is also E = Q V. At home all this happens on the mains: an alternating supply at about 230 V and 50 Hz, through live, neutral and earth wires. An appliance's power rating is its power in normal use.

The lesson starts with a vote before anyone explains: a 3 kW kettle for three minutes, or a 10 W bulb left on all day? Then the equations, each with its units, and a running circuit where students read the current and each p.d., work out each resistor's power and predict what doubling the battery does. A virtual heater lets them measure power at different p.d.s (a model, not a practical in a lab). Kilowatt-hours, the unit on a bill, come as an aside: the exam works in joules. It is for Foundation and Higher; the stretch, two steps from a power rating to a resistance, leans towards Higher.

By the end, students can

  • say that power is the energy transferred each second, and use P = E ÷ t and E = P t with time in seconds
  • work out the power of a component with P = V I and with P = I² R, and choose between them
  • work out the energy transferred with E = Q V, and see that it agrees with E = P t
  • describe the UK mains supply (a.c., about 230 V, 50 Hz) and the jobs of the live, neutral and earth wires

Key words

  • power (P, watts, W)
  • energy transferred (E, joules, J)
  • power rating
  • kilowatt (kW)
  • alternating current (a.c.)
  • direct current (d.c.)
  • frequency (hertz, Hz)
  • live, neutral and earth wires
  • kilowatt-hour (kWh)

Before this lesson

Students should use V = I R and Q = I t, change minutes to seconds and kilowatts to watts, and square a number. The lesson on current, p.d. and resistance comes first.

Taught in Current, p.d. and resistance.

The lesson, step by step

30 minutes in total. Each step says what changes for this length.

  1. 1 · Think3 min

    Kettle or light?

    A 3 kW kettle boils for 3 minutes.

    A 10 W bulb is left on all day: 24 hours.

    Which transfers more energy?

    A: The kettle

    B: The bulb

    C: About the same

    Vote first. Then say why.

    Write here

    3 min 1 · Hook · students think · the slides say “Think”

    Kettle or light?

    A powerful kettle for a few minutes, a weak bulb all day: students vote on which transfers more energy before anyone explains.

    For this length: Vote once, no discussion

    Say

    “A 3 kW kettle boils for 3 minutes. A 10 W light bulb is left on all day, 24 hours. Which one transfers more energy? The kettle, the bulb, or about the same?”

    Ask
    • What does the 3 kW on the kettle tell you?
    • If the bulb were on for only an hour, would your answer change?
    Watch for

    Saying the kettle, because 3 kW is 'more electricity': keep the answer, the next step needs it.

    For the teacher

    Give a minute of silent thinking, then vote. Most say the kettle: its power is 300 times the bulb's.

    Don't explain yet. Power is how fast energy is transferred, and time counts too: that is the next step. (The bulb transfers more: 864,000 J against 540,000 J.)

    Take a vote with Ask the class: the kettle, the bulb, or about the same. Keep the votes on the board for the explain step.

    Shorter: Vote once, no discussion (−2 min).

  2. 2 · Watch8 min

    Power: energy each second

    Power: the energy transferred each second

    P = E ÷ t: watts (W), joules (J), seconds (s)

    1 W = 1 J each second; 3 kW = 3000 J each second

    So the energy transferred: E = P t

    Kettle: 3000 × 180 = 540,000 J

    Bulb: 10 × 86,400 = 864,000 J: the bulb wins

    Power in watts, time in seconds

    2 · Watch

    Mains electricity

    UK plugs: about 230 V a.c., 50 Hz

    a.c.: the current goes back and forth, 50 times a second

    A battery gives d.c.: one way only

    Live (brown): about 230 V. Neutral (blue): about 0 V

    Earth (green and yellow): safety, no current normally

    A power rating: its power in normal use

    Live, neutral, earth: say each one's job

    2 · Watch

    Power in a circuit: P = V I

    Power = p.d. × current

    P = V I: watts = volts × amps

    A 230 V kettle takes 10 A: P = 230 × 10 = 2300 W

    V = I R, so P = I × I R: P = I² R

    2 A through 6 Ω: P = 2² × 6 = 4 × 6 = 24 W

    Square the current, then times R

    Know V and I? P = V I. Know I and R? P = I² R.

    2 · Watch

    Try it: power in each resistor

    6 V, 10 Ω and 20 Ω in series

    Read the current and each p.d.

    Each one's power: P = V I

    Check it with P = I² R

    Predict: the battery at 12 V?

    Then move it and work it out again

    Predict first: twice the volts, ? the power

    CircuitCurrent 0.2 A · across resistor 1: 2 V · across resistor 2: 4 V

    2 · Watch

    Energy from charge: E = Q V

    P.d. is the energy each coulomb transfers: 1 V = 1 J/C

    E = Q V: joules = coulombs × volts

    60 C through a 12 V lamp: E = 60 × 12 = 720 J

    It agrees with E = P t, as Q = I t and P = V I

    Which equation? Write down what you know first

    What you know, its units, then the equation

    2 · Watch

    An aside: kilowatt-hours

    Bills count energy in kilowatt-hours (kWh)

    1 kWh: 1 kW for 1 hour

    = 1000 W × 3600 s = 3,600,000 J

    A 2 kW heater for 3 hours: 2 × 3 = 6 kWh

    In exam answers: joules, unless told otherwise

    Exams: watts, seconds and joules

    8 min 2 · Explain · you explain · the slides say “Watch”

    Power, the mains, and the four equations

    Power as energy each second answers the hook; the mains as the setting; P = V I and P = I² R, tried in a circuit; E = Q V; kilowatt-hours as an aside.

    For this length: Skip the mains and kilowatt-hour slides; say each in a line

    Say

    “Power is how fast energy is transferred: joules each second. Energy is power times time, in seconds. In a circuit, power is p.d. times current.”

    Ask
    • A 2 kW heater: how many joules does it transfer each second?
    • Which wire in a plug is at about 230 V?
    • You know the current and the resistance. Which equation gives the power?
    Watch for

    Leaving kilowatts or minutes in E = P t: ask what 1 W means, in joules and seconds.

    The hook, worked

    Kettle: 3 kW = 3000 W; 3 minutes = 180 s; E = 3000 × 180 = 540,000 J

    Bulb: 24 hours = 86,400 s; E = 10 × 86,400 = 864,000 J

    864,000 > 540,000: the bulb transfers more

    Power two ways

    A 230 V kettle takes 10 A: P = V I = 230 × 10 = 2300 W

    2 A through 6 Ω: P = I² R = 2² × 6 = 4 × 6 = 24 W

    For the teacher

    Power is the energy transferred each second: P = E ÷ t, power in watts (W), energy in joules (J), time in seconds (s). 1 W is 1 J each second; 3 kW is 3000 J each second. So E = P t. Work the hook: the bulb transfers more, because it is on 480 times as long.

    The mains: UK plugs give an alternating supply (a.c.) at about 230 V and 50 Hz: the current turns back and forth 50 times a second. A battery gives direct current (d.c.), one way only. The live wire (brown) is at about 230 V; the neutral (blue) completes the circuit, at or near 0 V; the earth (green and yellow) is a safety wire that carries a current only if something goes wrong. A power rating is the appliance's power in normal use.

    In a circuit: P = V I. With V = I R, P = I² R: square the current, then times the resistance. Use whichever fits what you know.

    Try it: read the current and the p.d. across each resistor, work out each resistor's power with P = V I, and check it with P = I² R. Then double the battery: the current and the p.d. both double, so the power is four times.

    P.d. is the energy each coulomb transfers (1 V = 1 J/C), so E = Q V. It agrees with E = P t, because Q = I t and P = V I.

    Aside: bills count energy in kilowatt-hours. 1 kWh is 1 kW for an hour, 3,600,000 J. In exam answers, joules.

    On the Try it slide, ask the class to predict before anyone moves the battery from 6 V to 12 V: double the power, or more?

    Shorter: Skip the mains and kilowatt-hour slides; say each in a line (−4 min).

  3. 3 · Your turn8 min

    Measure a heater's power

    Open the quiz: the virtual practical.

    R1 is a 4 Ω heater. Change the cell's p.d.

    Record 4 readings, including 6 V and 12 V.

    Power at each: P = V I. Check one with P = I² R.

    From 6 V to 12 V: what happens to the power?

    A virtual experiment, not a practical in a lab.

    Predict the current before each reading.

    3 · Your turn

    Worked, then yours

    I do: 12 V, 2 A: P = 12 × 2 = 24 W

    You do: 230 V and 0.5 A. P?

    I do: 800 W for 2 minutes: 800 × 120 = 96,000 J

    You do: 1.5 kW for 4 minutes. E?

    I do: 0.5 A through 40 Ω: 0.5² × 40 = 10 W

    You do: 3 A through 8 Ω. P?

    Watts and seconds first, then the equation.

    Write here

    Its answers' slide comes with your copy.

    8 min 3 · Your turn · students write

    Measure a heater's power, then yours

    Students measure the power of a virtual heater at different p.d.s and see doubling the p.d. give four times the power, then solve twins of worked examples.

    For this length: After the practical, the first two pairs of examples only

    Say

    “Open the virtual practical. R1 is a 4 Ω heater. Change the cell's p.d., record four readings, and work out the power at each.”

    Ask
    • Before each reading: what power do you expect?
    • From 6 V to 12 V: what happened to the current? And to the power?
    Watch for

    Doubling the power when the p.d. doubles: ask what the current did.

    Power from p.d. and current

    I do: a 12 V lamp takes 2 A

    P = V I = 12 × 2 = 24 W

    Then theirs

    You do: 230 V and 0.5 A. P?

    Energy: seconds first

    I do: 800 W for 2 minutes

    t = 120 s; E = P t = 800 × 120 = 96,000 J

    Then theirs

    You do: 1.5 kW for 4 minutes. E?

    Power from current and resistance

    I do: 0.5 A through 40 Ω

    P = I² R = 0.5² × 40 = 0.25 × 40 = 10 W

    Then theirs

    You do: 3 A through 8 Ω. P?

    A virtual experiment: the circuit follows a model, and it is not a practical in a lab. R1 is a 4 Ω heater. Record your readings first, then answer the questions about them.

    1. Virtual practical. R1 is a 4 Ω heater. Change the cell's potential difference (B1) and record at least 4 readings, each at a different p.d. Include 6 V and 12 V. Before each one, predict the current.They set up a virtual circuit and record its meters' readings, each checked.
    2. Use your reading at 6 V and P = V I. What is the heater's power, in watts?
    3. Now use your current at 12 V and P = I² R, with R = 4 Ω. What is the heater's power, in watts?
    4. From 6 V to 12 V the p.d. across the heater doubled. What happened to its power, and why?
      1. It doubled: power goes up as the p.d. goes up
      2. It stayed the same: the heater's resistance didn't change
      3. It became four times as big: the current doubled too
      4. It halved: more p.d. means less current
    5. The heater is left at 12 V for 2 minutes. How much energy does it transfer, in joules?
    6. How much charge flows through the heater in those 2 minutes, in coulombs? (Then work out E = Q V: does it agree with your last answer?)
    For the teacher

    The virtual practical: a cell B1 whose p.d. students set from 0 to 12 V in steps of 2 V, ammeter A1, a 4 Ω resistor R1 (the heater) and voltmeter V1 across it. They record at least four readings, each checked against the circuit.

    Then the questions on their readings: the power at 6 V with P = V I, at 12 V with P = I² R, what doubling the p.d. did, and the energy and charge in two minutes at 12 V, where E = P t and E = Q V give the same answer.

    Say plainly that this is a virtual experiment: a real heater warms up and its resistance changes a little.

    Then each worked example aloud, its twin straight after.

    Give the virtual practical first; once most have their readings, ask a student to read out the power at 6 V and at 12 V. Then the worked examples and their twins; open the answers slide when most have finished.

    Shorter: After the practical, the first two pairs of examples only (−4 min).

    Longer: Each student writes a question for a partner using E = Q V, and works it out first (+3 min).

  4. 4 · Vote and practise7 min

    Vote first

    A current of 2 A flows through a 6 Ω resistor.

    What is the power transferred in it?

    A: 12 W

    B: 24 W

    C: 3 W

    D: 144 W

    Choose on your own, then say why.

    4 · Vote and practise

    Practise: equation first

    1. A 230 V hairdryer takes 5 A. Power?

    2. A 60 W lamp for 5 minutes. Energy, in J?

    3. 3 A through 5 Ω. Power?

    4. A 2.3 kW kettle on 230 V. Current?

    5. 40 C through a 12 V lamp. Energy?

    Stuck on 2? 5 minutes is 300 s.

    Write here

    4 · Vote and practise

    Reason it out

    6. Which wire in a plug is at about 230 V?

    7. 2 kW for 10 minutes or 500 W for 30: more energy?

    8. a.c. at 50 Hz: what does it mean?

    9. A 12 V motor takes 2 A for 5 minutes. Energy?

    10. A 23 Ω heater on 230 V. Power?

    Stretch: a 9.2 kW shower on 230 V. Resistance?

    10 takes two steps: the current first.

    Write here

    7 min 4 · Who's stuck? · students vote · the slides say “Vote and practise”

    Vote, then practise

    One question shows who forgets to square the current; then practice in levels, with a two-step stretch.

    For this length: After the vote, questions 1–5 only

    Say

    “Choose an answer on your own. Nobody talks until everyone has chosen.”

    Ask
    • What do you know, and in which units? Which equation joins them?
    Watch for

    2 × 6 for P = I² R: ask what the little 2 means.

    Show your working on paper: what you know with its units, the equation, then the answer with its unit. Change minutes to seconds and kilowatts to watts first. The stretch is optional.

    The question that shows who's stuck

    1. A current of 2 A flows through a 6 Ω resistor. What is the power transferred in the resistor?
      1. 12 W
      2. 24 W
      3. 3 W
      4. 144 W
    In your Classroom, as they answer (an example)
    • A10
    • B9
    • C3
    • D4

    Each wrong answer is a different mistake. While they answer, your Classroom shows who chose which, so you know in seconds whom to help with what.

    1. A hairdryer on the 230 V mains takes a current of 5 A. What is its power, in watts?
    2. A 60 W lamp is on for 5 minutes. How much energy does it transfer, in joules?
    3. A current of 3 A flows through a 5 Ω resistor. What is its power, in watts?
    4. A 2.3 kW kettle runs on the 230 V mains. What current does it take, in amps?
    5. 40 C of charge flows through a lamp with a p.d. of 12 V across it. How much energy is transferred, in joules?
    6. In a plug, which wire is at a p.d. of about 230 V?
      1. The live wire (brown)
      2. The neutral wire (blue)
      3. The earth wire (green and yellow)
      4. All three: they share the 230 V
    7. Which transfers more energy: a 2 kW heater on for 10 minutes, or a 500 W heater on for 30 minutes?
      1. The 2 kW heater
      2. The 500 W heater: it is on three times as long
      3. They transfer the same
      4. You can't tell without the p.d.
    8. UK mains electricity is a.c. at 50 Hz. What does that mean?
      1. The current flows one way only, as from a battery, at 50 A
      2. The current goes one way, then back, 50 complete times each second
      3. The p.d. of the supply is 50 V
      4. The current stops and starts 50 times each minute
    9. A 12 V motor takes a current of 2 A for 5 minutes. How much energy does it transfer, in joules?
    10. A heater with a resistance of 23 Ω runs on the 230 V mains. What is its power, in watts? (Find the current first.)
    11. Stretch: an electric shower is rated 9.2 kW on the 230 V mains. What is its resistance, in ohms?

    Support · Write down what you know with its unit, change minutes to seconds and kilowatts to watts, then choose the equation that has those letters and the one you want.

    For the teacher

    Everyone chooses before anyone explains. Then ask for the working behind each option: 12 W is I × R (the p.d., not the power), 3 W is R ÷ I, 144 W squares I × R.

    Questions 1–3 build fluency; 4–5 change the look (kilowatts, E = Q V); 6–8 are about the mains and comparing energy; 9–10 are exam-style, in two steps; the stretch goes from a power rating to a resistance.

    Students show their working on paper or the board; the quiz keeps their final answers.

    If many chose 12 W or 144 W, write I² R as I × I × R and work 2 × 2 × 6 together before they practise.

    Shorter: After the vote, questions 1–5 only (−5 min).

    Longer: Question 10 and the stretch, gone through together (+8 min).

  5. 5 · Show what you know4 min

    Show what you know

    1. A 1150 W microwave runs for 2 minutes 30 seconds.

    How much energy does it transfer, in joules?

    2. 2 A flows through a 15 Ω resistor. Its power?

    Seconds first; square the current.

    Write here

    4 min 5 · Exit · students show · the slides say “Show what you know”

    Show what you know

    An energy with time to change into seconds, and a power with a current to square, show who has it.

    Say

    “Write the equation first, then the numbers in watts and seconds.”

    Watch for

    2.5 for the time, or 30 W for the power.

    Write the equation first, then the numbers in watts and seconds.

    1. A 1150 W microwave oven runs for 2 minutes 30 seconds. How much energy does it transfer, in joules?
    2. A current of 2 A flows through a 15 Ω resistor. What is its power, in watts?
    For the teacher

    A 1150 W microwave runs for 2 minutes 30 seconds: the energy it transfers.

    2 A through a 15 Ω resistor: its power.

    Those who kept minutes or forgot the square need it again next lesson; set the homework.

    Longer: Which transfers more: a 2 kW heater for an hour or a 100 W TV for 5 hours? Work it out in joules. (+4 min).

One to one: before any calculation, ask the student to say what they know and in which units, then which equation joins them. Let them drive the circuit while you only ask what they predict before each move.

Homework: Electrical power and energy

For each calculation: write what you know with its units, change minutes to seconds and kilowatts to watts, write the equation, then the answer with its unit.

  1. A toaster on the 230 V mains takes a current of 4 A. What is its power?
  2. A 40 W lamp is on for 2 hours. How much energy does it transfer, in joules? (Extra: and in kilowatt-hours?)
  3. A current of 0.5 A flows through a 120 Ω resistor. What is its power?
  4. A 1.5 kW heater transfers 270,000 J. How long was it on, in seconds and in minutes?
  5. While a phone charges, 7200 C of charge flows at a p.d. of 5 V. How much energy is transferred?
  6. A 2.3 kW kettle and a 23 W lamp both run on the 230 V mains. Work out the current in each. Then use P = I² R to explain why the kettle's cable is thicker (a thicker wire has less resistance).

Print the worksheet with working space and homework. Teacher answers come with your Library copy.

Teaching electrical power and energy: common questions

Is this lesson for Foundation or Higher, and is it in combined science?

Both tiers: mains electricity, power and energy transfers in AQA GCSE Physics (8463) are for Foundation and Higher, and AQA's combined science has them too. Foundation students can stop at the standard questions; the exam-style questions and the stretch suit Higher.

Do students have to learn these equations?

Yes. AQA expects students to recall and use P = V I, P = I² R, E = P t and E = Q V, as well as V = I R and Q = I t from the lesson before; they are not on the equation sheet. The lesson's habit (what you know, its units, then the equation that joins them) is what makes choosing between them quick.

Why are kilowatt-hours only an aside?

Exam questions on this topic work in watts, seconds and joules, and mixing kilowatt-hours into an E = P t calculation is a common way to lose marks. Kilowatt-hours are worth a slide because students meet them on bills; the lesson shows how one becomes 3,600,000 J.

Is the virtual heater a required practical?

No: AQA has no required practical on electrical power. The virtual heater follows an exact model (meters with no errors, a resistance that never changes as it warms), so it shows the pattern cleanly, above all that twice the p.d. gives four times the power. A real resistor or lamp in a lab is a good way to compare.

Where are the National Grid and transformers?

They are a topic of their own, after this one. This lesson gives what it needs: power as energy each second, P = V I, and the idea that the power heating a cable is I² R, which is why the grid sends power at a high p.d. and a small current.

Can I use the lesson without creating an account?

Yes. Read the lesson, print the worksheet and open its slides without signing in; the circuit on the slide runs there too, and nothing you draw is kept. A free teacher account keeps the whole lesson in your Library to change and teach, with the virtual practical, the quizzes, the answers and the answer slides.

Curriculum reference: AQA GCSE Physics 8463: 4.2.3 (domestic uses and safety: direct and alternating potential difference, mains electricity, the live, neutral and earth wires) and 4.2.4.1–4.2.4.2 (power, P = V I and P = I² R; energy transfers in everyday appliances, E = P t and E = Q V), with 4.1.1.4 (power as the rate of energy transfer).

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