Energy transfers, power and fuels
Energy is never used up: it is transferred from one store to another. Learning to describe transfers precisely is what separates a physics answer from a vague one.
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 pathways
- • Power
- • Comparing fuels and foods
- • Wasted energy and efficiency
This lesson at a glance
- 30 minutes
- 21 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 21
Visual introductionPicture this
Energy transfers, power and fuels
Energy is never used up: it is transferred from one store to another. Learning to describe transfers precisely is what separates a physics answer from a vague one.
In a nutshell
Understand energy changes and transfers, including comparing power ratings and energy values of fuels and foods
Learning cycle
Part 2 of 21
Learning cycle 1 of 2
Part 1 · Stores and pathways
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 3 of 21
Learn
Stores and pathways
Energy is held in stores: kinetic, gravitational, thermal, chemical, elastic and nuclear. It moves between stores by pathways: mechanical work, electrical work, heating and radiation. A falling ball transfers energy from a gravitational store to a kinetic store mechanically.
Reset break
Part 4 of 21
Pause
That's sitting 1 of 5 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 21
Learn
Power
Power is the rate of energy transfer, measured in watts: one watt is one joule per second. A 2,000 W kettle transfers 2,000 J every second, so in 90 seconds it transfers 180,000 J. Comparing power ratings tells you how fast appliances use energy, not how much they use in total.
Quick check
Part 6 of 21
Quick check
Part 7 of 21
Reset break
Part 8 of 21
Pause
That's sitting 2 of 5 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 21
Learning cycle 2 of 2
Part 2 · Comparing fuels and foods
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 10 of 21
Learn
Comparing fuels and foods
Energy values are quoted per gram or per 100 g, which lets very different substances be compared fairly. Petrol carries about 46 kJ per gram and fat about 37 kJ per gram, far more than carbohydrate at about 17 kJ per gram. Comparing per-gram values, rather than per-serving, is what makes the comparison valid.
Explore the idea
Part 11 of 21
Learn
Wasted energy and efficiency
Some energy in every transfer is dissipated to the surroundings, usually by heating, and becomes too spread out to be useful. Efficiency is the useful energy transferred divided by the total supplied. A filament bulb wastes most of its energy as heat, while an LED transfers far more of it usefully as light.
Reset break
Part 12 of 21
Pause
That's sitting 3 of 5 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 13 of 21
Quick check
Part 14 of 21
Explore the idea
Part 15 of 21
Worked example
Worked answer: how much energy does a 2,000 W kettle transfer in 90 seconds? (3 marks)
Power is the energy transferred per second, so energy = power × time (1). Energy = 2,000 × 90 (1) = 180,000 J, or 180 kJ (1). The time must be in seconds; using 1.5 minutes without converting gives an answer 60 times too small.
Reset break
Part 16 of 21
Pause
That's sitting 4 of 5 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 17 of 21
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 18 of 21
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
____ is held in stores: kinetic, gravitational, thermal, chemical, elastic and nuclear.
A falling ball ____ energy from a gravitational store to a kinetic store mechanically.
____ power ratings tells you how fast appliances use energy, not how much they use in total.
Challenge round
Part 19 of 21
Game · Recall cards
What does a 2000 W power rating mean?
Card 1 of 4
Mastery quiz
Part 20 of 21
Marked quiz
End of lesson quiz: Energy transfers, power and fuels
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 21 of 21
Lesson round-up
Ready when you are
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Understand energy changes and transfers, including comparing power ratings and energy values of fuels and foods