Energy transfers, power ratings and fuel values
Energy is not made or used up - it moves between stores, and some of it always ends up spread out as heat. This lesson covers stores, transfers and the numbers on the back of appliances and food packets.
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
- • Stores and the sankey idea
- • Conservation and waste
- • Power ratings
- • Energy values of fuels and food
This lesson at a glance
- 30 minutes
- 19 parts to scroll through
- 4 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 19
Watch first
Energy transfers, power ratings and fuel values
An English teaching video on this topic from Cognito. Watch it, then scroll down for the lesson.
Do it for real
The practicals below use what the film shows, with a full method you can follow at home.
In a nutshell
Understand energy changes and transfers, including comparing power ratings and energy values of fuels and foods
What you already know
Part 2 of 19
Before we start
What you already know
You should know energy is measured in joules.
Key words
Part 3 of 19
Key words
Words you'll need today
- Energy store
- Where energy is held: kinetic, thermal, chemical, gravitational, elastic, nuclear.
- Transfer
- Energy moving between stores, by heating, waves, electricity or forces.
- Dissipated
- Spread out to the surroundings, usually as heat, and no longer useful.
- Power
- The rate of energy transfer, in watts (joules per second).
- Efficiency
- The fraction of energy transferred usefully.
Learning cycle
Part 4 of 19
Learning cycle 1 of 2
Stores and transfers
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 5 of 19
Learn
Stores and the sankey idea
A ball thrown upwards: kinetic store empties as the gravitational store fills. A phone charging: electrical work fills a chemical store. Say where energy started and where it ended, and always name the wasted pathway too - usually heating the surroundings, which is dissipation.
Explore the idea
Part 6 of 19
Learn
Conservation and waste
The total energy before equals the total energy after; none is lost. 'Wasted' energy is real energy that has been dissipated into the surroundings and become too spread out to use. A filament bulb transferring 60 J of electrical energy might deliver 3 J as light and 57 J as heat, so it is only 5 per cent efficient.
Quick check
Part 7 of 19
Quick check
A ball thrown upwards transfers energy from...
Quick check
Part 8 of 19
Quick check
A 2000 W kettle for 90 s transfers...
Reset break
Part 9 of 19
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.
Learning cycle
Part 10 of 19
Learning cycle 2 of 2
Power, fuels and food
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 11 of 19
Learn
Power ratings
Power in watts is joules per second, so a 2000 W kettle transfers 2000 J every second. Energy transferred = power x time: that kettle running for 90 s uses 180 000 J. Comparing appliances by power rating tells you which uses energy fastest, but the total cost also depends on how long it runs - a 2000 W kettle for two minutes costs far less than a 100 W television all evening.
Explore the idea
Part 12 of 19
Learn
Energy values of fuels and food
Different fuels release different amounts of energy per gram, which is why petrol suits vehicles and coal does not. Food is labelled in kilojoules and kilocalories per 100 g: fats hold roughly twice the energy per gram of carbohydrates or proteins. Comparisons must be per gram or per 100 g, or they are meaningless.
Quick check
Part 13 of 19
Quick check
Wasted energy from a bulb is mostly...
Quick check
Part 14 of 19
Quick check
A bulb transferring 60 J to give 3 J of light is...
Common mix-ups
Part 15 of 19
Common mix-ups
Things people often get wrong
- Energy is not used up - it is transferred and often dissipated.
- Power is the rate of transfer, not the amount of energy.
- A higher power rating does not mean higher total energy use; time matters too.
- Compare fuels and foods per gram, not per portion.
Challenge round
Part 16 of 19
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 17 of 19
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
Say where ____ started and where it ended, and always name the wasted pathway too - usually heating the surroundings, which is dissipation.
Power in watts is joules per second, so a 2000 W kettle ____ 2000 J every second.
____ appliances by power rating tells you which uses energy fastest, but the total cost also depends on how long it runs - a 2000 W kettle for two minutes costs far less than a 100 W television all evening.
Mastery quiz
Part 18 of 19
Marked quiz
End of lesson quiz: Energy transfers, power ratings and fuel values
6 questions, marked with the reasoning shown. No timer.
1. Power is measured in...
2. Energy transferred equals...
3. Which store does a stretched band have?
4. Per gram, fats provide about...
5. Dissipated energy is...
6. Total energy after a transfer compared with before is...
Lesson round-up
Part 19 of 19
Lesson round-up
Ready when you are
Quiz score
Not sat
Games
Not played
Points this lesson
0
Best run
0 in a row
Luna: 0 out of 4 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 19 · Watch & discover
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Understand energy changes and transfers, including comparing power ratings and energy values of fuels and foods