Energy you can count: calculating and conserving it
Energy is not a substance you can see, but it is a quantity you can count in joules - and the total never changes. Getting that idea straight makes the rest of physics much easier.
Part of your national curriculum
- Physics: energy: Understand energy as a quantity that can be quantified and calculated, and the total energy has the same value before and after a change
Lesson overview
What you'll learn in this lesson
Understand energy as a quantity that can be quantified and calculated, and the total energy has the same value before and after a change
Key learning points
- • Stores and transfers
- • Putting numbers on it
- • The total does not change
- • Useful versus wasted
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 you can count: calculating and conserving it
An English teaching video on this topic from Massimo Boscherini. 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 as a quantity that can be quantified and calculated, and the total energy has the same value before and after a change
What you already know
Part 2 of 19
Before we start
What you already know
You should know that energy is measured in joules and can be transferred between stores.
Key words
Part 3 of 19
Key words
Words you'll need today
- Joule
- The unit of energy, symbol J.
- Energy store
- Where energy is held, such as kinetic, thermal or chemical.
- Conservation of energy
- Total energy is the same before and after a change.
- Dissipated
- Spread out to the surroundings, usually as heating, and no longer useful.
- Power
- Energy transferred each second, measured in watts (1 W = 1 J/s).
Learning cycle
Part 4 of 19
Learning cycle 1 of 2
Counting energy in joules
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 5 of 19
Learn
Stores and transfers
Describe a change by naming the store energy comes from and the store it ends in. A kettle: chemical or electrical supply to the thermal store of the water. A dropped ball: gravitational potential store to kinetic store. Words like 'heat energy travelling' hide what is happening; naming stores does not.
Explore the idea
Part 6 of 19
Learn
Putting numbers on it
Energy is a quantity, so it can be calculated. Power tells you how fast it moves: energy = power x time. A 2000 W kettle running for 90 s transfers 2000 x 90 = 180 000 J. Comparing a 9 W lamp with a 60 W lamp for the same hour tells you exactly why the swap saves money - the same brightness for a sixth of the joules.
Quick check
Part 7 of 19
Quick check
A 1500 W heater runs for 60 s. Energy transferred?
Quick check
Part 8 of 19
Quick check
A dropped ball transfers energy from which store to which?
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
Nothing lost, only spread out
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 11 of 19
Learn
The total does not change
In any change, the energy you end with equals the energy you started with. If a 100 J push on a trolley leaves only 70 J in the kinetic store, the missing 30 J has not vanished - friction warmed the wheels, axle and air. Add it back and the books balance. That is conservation of energy, and it holds everywhere without exception.
Explore the idea
Part 12 of 19
Learn
Useful versus wasted
'Wasted' energy is not destroyed energy; it is energy dissipated to the surroundings so thinly that you cannot use it. A filament lamp puts about 5 J of every 100 J into light and the rest into heating the room. Insulation, lubrication and better designs raise the useful share, but nothing can produce more energy than went in - which is why perpetual motion machines never work.
Quick check
Part 13 of 19
Quick check
100 J in, 70 J in the kinetic store. The other 30 J...
Quick check
Part 14 of 19
Quick check
Conservation of energy states that...
Common mix-ups
Part 15 of 19
Common mix-ups
Things people often get wrong
- Energy is never used up or created - it is transferred and often dissipated.
- Wasted energy is spread out, not destroyed.
- Power (watts) is a rate; energy (joules) is a total.
- 'Heat' is a transfer; the store is the thermal store.
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.
Describe a change by naming the store ____ comes from and the store it ends in.
Energy is a ____, so it can be calculated.
Mastery quiz
Part 18 of 19
Marked quiz
End of lesson quiz: Energy you can count: calculating and conserving it
6 questions, marked with the reasoning shown. No timer.
1. Energy is measured in...
2. 1 watt is equal to...
3. Which lamp transfers fewer joules in an hour?
4. 'Wasted' energy in a machine has usually been...
5. A perpetual motion machine is impossible because it would...
6. A 2000 W kettle for 30 s transfers...
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 as a quantity that can be quantified and calculated, and the total energy has the same value before and after a change