Power: watts and energy per second · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Power: how fast energy moves

Science · Energy · ages 12-13
Name ______________________   Date ____________

  1. A device runs for one hour. What time in seconds should you use with power in watts to calculate energy in joules?

    • 60 s
    • 3600 s
    • 360 s
    • 600 s

  2. What does the word 'power' mean in science?

    • How much energy an object stores
    • How fast energy is transferred
    • How long a device has been running

  3. A watt measures how much energy is stored in a battery.

    Circle one:   True   False

  4. What unit do we use to measure power?

    • Joule
    • Watt
    • Second

  5. Which uses more energy over an hour: a 100 W bulb or a 10 W LED, and why?

    LED10Bulb100
    • The bulb, because it transfers more energy every second
    • The LED, because it has a smaller number so it must be quicker
    • They use the same energy because they are on for the same amount of time

  6. A kettle operates at 2000 W and a charger at 5 W. Which comparison of their energy-transfer rates is correct?

    • The kettle's rate is 400 times higher
    • The charger's rate is 400 times the kettle's
    • The kettle transfers only 400 J more each second
    • Both use the same total energy during every task

  7. A device transfers 1800 J in 2 s, then 4200 J in 3 s. What is its average power over both intervals in W?

    Answer: ______________

  8. A toaster has a power of 800 W. How much energy does it transfer in 5 seconds?

    Answer: ______________

  9. A 500 W heater must transfer 4000 J. It has already transferred 1000 J. At constant power, how many more seconds are needed?

    Answer: ______________

  10. Two trials use the same hairdryer with equal efficiency and equal useful-energy requirements. Its high setting finishes sooner. What is true about that setting?

    • It transfers the exact same energy per second as low power, just louder
    • It transfers energy each second at a slower rate, so it saves energy
    • It transfers energy each second at a faster rate, doing the same job quicker
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Power: how fast energy moves W1-mt_ckpA3oZQ44-s1

  1. 3600 s · An hour contains 60 minutes of 60 seconds: 60 × 60 = 3600 s.
  2. How fast energy is transferred · Power is the rate of energy transfer, not the amount an object stores or how long it runs.
  3. False · A watt measures the rate energy is transferred each second, not the total energy stored somewhere.
  4. Watt · The watt (W) measures power. The joule measures energy itself, and the second measures time.
  5. The bulb, because it transfers more energy every second · The bulb's rating is 10 times higher, so it transfers 10 times more energy every second, using more energy over the same hour.
  6. The kettle's rate is 400 times higher · Compare rates by division: 2000 ÷ 5 = 400. This does not decide total energy over different durations.
  7. 1200 · Total energy is 6000 J and total time is 5 s. Average power is 6000 ÷ 5 = 1200 W.
  8. 4000 · Energy equals power times time: E = P × t = 800 × 5 = 4000 joules.
  9. 6 · It still needs to transfer 3000 J. Remaining time is 3000 ÷ 500 = 6 s.
  10. It transfers energy each second at a faster rate, doing the same job quicker · Equal useful energy in less time requires greater useful power. Equal efficiency then means greater input-energy transfer each second.
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