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
- 14 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 14
Watch first
Comparing power and energy values
An English teaching video on this topic from Lammas Science. Watch it, then scroll down for the lesson.
In a nutshell
Understand energy changes and transfers, including comparing power ratings and energy values of fuels and foods
Learning cycle
Part 2 of 14
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 14
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.
Explore the idea
Part 4 of 14
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 5 of 14
Quick check
What does a 2000 W power rating mean?
Reset break
Part 6 of 14
Pause
That's sitting 1 of 2 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.
Quick check
Part 7 of 14
Quick check
Which type of food generally has a higher energy value per 100 g?
Learning cycle
Part 8 of 14
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 9 of 14
Learn
Worked comparison
A 100 g chocolate bar with about 2200 kJ has roughly ten times the energy value of a 100 g apple with about 200 kJ — comparing per-100g values lets you fairly judge foods of different sizes.
Quick check
Part 10 of 14
Quick check
A 3000 W kettle compared to a 1500 W kettle, boiling the same water, will:
Challenge round
Part 11 of 14
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 12 of 14
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 about 2200 kJ has roughly ten times the energy value of a 100 g apple with about 200 kJ — ____ per-100g values lets you fairly judge foods of different sizes.
Mastery quiz
Part 13 of 14
Marked quiz
End of lesson quiz: Comparing power and energy values
3 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:
Lesson round-up
Part 14 of 14
Lesson round-up
Ready when you are
Quiz score
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Points this lesson
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Best run
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Luna: 0 out of 3 on the practice checks. Only if you feel up to it — one more?
Ask LunaDo the work
Your written work for this lesson
This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.
Why the school greenhouse failed
A science club grew tomato plants in the school greenhouse. In June the plants grew well: leaves were dark green and the fruit set quickly. In July the club went away for two weeks and left the greenhouse door shut. When they came back, the leaves were curled and pale and most of the fruit had dropped.
The club had a data logger inside. It showed the temperature reached 46 °C on several afternoons and the soil moisture fell close to zero. The club knew photosynthesis needs light, water and carbon dioxide, and that enzymes in a plant work best in a narrow range of temperature. Above that range the enzymes stop working properly.
The club repeated the summer with the door propped open and a water bottle drip on each pot. Temperature peaked at 31 °C and the plants survived.
Written for Good Learning Co.
Reading questions
- Name the variable the club changed the second time, and one variable they kept the same.
- Explain, using the words photosynthesis and enzymes, why the plants were damaged in July.
- Write a conclusion for the club's report, saying how far the evidence supports it.
Method and results
WritingWrite the aim, what you changed, what you measured and what you kept the same, then record your results in a table.
1 page + results table
Conclusion
WritingExplain what your results show and why, using the scientific words from the lesson. Say how you know you can trust them.
6–8 sentences
Labelled diagram
DiagramDraw the apparatus or the process in pencil, label it fully, and photograph it.
1 labelled diagram
Reading and notes
ReadingRead the lesson text again and write the key ideas in your own words.
Half a page of notes
Saved as you type.
Part 1 of 14 · Watch & discover
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Understand energy changes and transfers, including comparing power ratings and energy values of fuels and foods