Comparing power and energy values
A kettle's power rating and a chocolate bar's energy value are measured in related units — and comparing them tells you how fast energy is used or how much is stored.
Part of your national curriculum
- Physics: energy: Understand energy changes and transfers, including comparing power ratings and energy values of fuels and foods
Lesson overview
What you'll learn in this lesson
Understand energy changes and transfers, including comparing power ratings and energy values of fuels and foods
Key learning points
- • Power ratings
- • Energy values of foods and fuels
- • Worked comparison
This lesson at a glance
- 26 minutes
- 17 parts to scroll through
- 3 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 17
Visual introductionPicture this
Comparing power and energy values
A kettle's power rating and a chocolate bar's energy value are measured in related units — and comparing them tells you how fast energy is used or how much is stored.
In a nutshell
Understand energy changes and transfers, including comparing power ratings and energy values of fuels and foods
Learning cycle
Part 2 of 17
Learning cycle 1 of 2
Part 1 · Power ratings
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 3 of 17
Learn
Power ratings
Power is the rate of energy transfer, measured in watts (W), where 1 watt = 1 joule per second. A 2000 W kettle transfers 2000 joules of energy every second — twice as fast as a 1000 W kettle, so it boils water in roughly half the time.
Reset break
Part 4 of 17
Pause
That's sitting 1 of 4 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Explore the idea
Part 5 of 17
Learn
Energy values of foods and fuels
The energy value of food is usually given in kilojoules (kJ) or kilocalories (kcal) per 100 g. Fatty and sugary foods tend to have higher energy values than fruit and vegetables, meaning the same mass releases more energy when 'burned' by the body.
Quick check
Part 6 of 17
Quick check
Part 7 of 17
Reset break
Part 8 of 17
Pause
That's sitting 2 of 4 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Learning cycle
Part 9 of 17
Learning cycle 2 of 2
Part 2 · Worked comparison
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 10 of 17
Learn
Worked comparison
A 100 g chocolate bar with 2200 kJ has roughly double the energy value of a 100 g apple with about 200 kJ times ten (roughly 2000 kJ vs 200 kJ) — comparing per-100g values lets you fairly judge foods of different sizes.
Quick check
Part 11 of 17
Reset break
Part 12 of 17
Pause
That's sitting 3 of 4 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Challenge round
Part 13 of 17
Game · Sort it
Which of these are true?
Drag each card into the right column. Tap a card first if dragging is fiddly.
True
Not true
Challenge round
Part 14 of 17
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
Power is the rate of ____ transfer, measured in watts (W), where 1 watt = 1 joule per second.
A 2000 W kettle ____ 2000 joules of energy every second — twice as fast as a 1000 W kettle, so it boils water in roughly half the time.
A 100 g chocolate bar with 2200 kJ has roughly double the energy value of a 100 g apple with about 200 kJ times ten (roughly 2000 kJ vs 200 kJ) — ____ per-100g values lets you fairly judge foods of different sizes.
Challenge round
Part 15 of 17
Game · Recall cards
What does a 2000 W power rating mean?
Card 1 of 4
Mastery quiz
Part 16 of 17
Marked quiz
End of lesson quiz: Comparing power and energy values
4 questions, marked with the reasoning shown. No timer.
1. What does a 2000 W power rating mean?
2. Which type of food generally has a higher energy value per 100 g?
3. A 3000 W kettle compared to a 1500 W kettle, boiling the same water, will:
4. One watt equals
Lesson round-up
Part 17 of 17
Lesson round-up
Ready when you are
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Ask LunaPart 1 of 17 · Watch & discover
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Understand energy changes and transfers, including comparing power ratings and energy values of fuels and foods