Energy Transformations, Conservation & Dissipation · Grade 6 Science
Chapter: Energy, Heat & Thermal DesignEnergy Transformations, Conservation & Dissipation
Learning goals
- I can calculate efficiency as useful output divided by total input.
- I can explain how energy can be conserved and wasted at the same time.
- I can explain why dissipated energy cannot easily be collected and reused.
Readings for this unit
Before this unit
Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.
Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.
An old filament light bulb drawing 60 J every second turns about 3 J of that into light and about 57 J into heat. It is a 5 per cent efficient lamp and a 95 per cent efficient heater, which is a fair description of what it actually is. An LED doing the same lighting job converts around 40 per cent into light.
In both cases every joule is accounted for. Nothing vanished; the filament bulb simply sent most of its energy into warming the room instead of lighting it. Conservation of energy says the total before equals the total after, and it says nothing at all about how much of the total ends up where you wanted it.
The energy that goes where you did not want it is said to be dissipated. It is still there, spread through the air and the walls as a very slight temperature rise, and the problem is precisely that it is spread: there is no practical way to gather it back up.
Build your understanding
Where this fits
The last lesson named the stores and the pathways. This one follows energy through a change and introduces the two ideas that make the accounting useful: the total is conserved, and the useful fraction is almost always small. Efficiency figures from this lesson come back in the chapter's design task, where improving one is the whole point.
Use three flexible 40-45-minute sessions: conservation and the useful fraction; calculating efficiency from real figures; dissipation and why waste heat cannot be recovered, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.
Part 1 · Conserved and wasted at the same time
These two statements sound contradictory and are not. Energy cannot be created or destroyed, so the total in a closed system is fixed; that is conservation. Meanwhile most of the energy a machine takes in ends up somewhere useless; that is waste. The resolution is that waste describes where energy went, not whether it still exists. When a filament bulb turns 57 J out of every 60 J into heat, those 57 J are entirely present in the room, raising its temperature by an amount far too small to notice but perfectly real.
Keeping this straight matters because the everyday phrase using up energy is misleading in a way that blocks understanding. Nothing uses energy up. A phone battery that has run flat contains the same total energy it had when charged; what changed is that the energy moved out of a concentrated chemical store and into the dispersed thermal store of the phone and the air around it. The thing that runs out is not energy but concentrated energy, and that distinction is what the rest of this lesson is about. Physicists sometimes make the point by saying that what we buy from an electricity company is not energy but concentration. The energy was already in the coal or the wind; what the system delivers is energy gathered into a form concentrated enough to turn a motor or light a room. Once it has done that job it is still present and no longer concentrated, which is exactly why you cannot run the bill backwards.
Pause and explain
How can energy be conserved and wasted at the same time?
Check your reasoning after answering
Because the two words answer different questions. Conservation says the total amount is unchanged, so the 57 J a filament bulb does not turn into light still exists in the room as a slight warming. Waste says that energy went somewhere it cannot be used. Nothing was destroyed; it was moved into a store too dispersed to be worth collecting.
Part 2 · Efficiency is a fraction
Efficiency is the useful energy out divided by the total energy in, usually written as a percentage. A filament bulb producing 3 J of light for every 60 J supplied is 3 divided by 60, which is 5 per cent. An LED producing 24 J of light for the same 60 J is 40 per cent. An electric kettle is around 90 per cent efficient, which is unusually high and happens for a simple reason: the job you want done is heating, so most of what would count as waste in another device is exactly the output you are paying for.
That last point is worth dwelling on, because efficiency is not a property of a device alone. It depends on what you decide counts as useful. The filament bulb is 5 per cent efficient as a lamp and roughly 95 per cent efficient as a room heater, and both figures describe the same object doing the same thing. When you read an efficiency number anywhere, the first question is always what the person quoting it has counted as the useful output, because changing that definition changes the number without changing the machine. The same question applies to figures you meet outside school. A petrol engine quoted at 30 per cent efficiency is being judged on energy delivered to the wheels, but a figure for the whole journey would also have to count the energy spent refining and transporting the fuel, and that number is lower. Neither is dishonest; they are answers to different questions, and the useful habit is to ask which question was asked.
Pause and explain
The same filament bulb can be described as 5 per cent efficient or 95 per cent efficient. Explain how both can be right.
Check your reasoning after answering
Because efficiency depends on what counts as useful output. Judged as a lamp, only the 3 J of light out of 60 J counts, giving 5 per cent. Judged as a heater, the 57 J of thermal energy is the wanted output, giving about 95 per cent. The device is unchanged; the definition of useful has changed.
Part 3 · Why waste heat stays wasted
Dissipated energy is energy that has spread into the thermal store of the surroundings, and the reason it cannot be recovered is about concentration rather than quantity. The 57 J the bulb releases each second ends up shared among an enormous number of air and wall particles, raising each by an immeasurably small amount. To get useful work back out you would need a temperature difference to drive it, and the whole point is that the energy has levelled out to match everything around it.
A rough comparison makes the scale clear. A room containing about 60 kg of air needs roughly 60,000 J to raise its temperature by 1 degree Celsius. A filament bulb left on for an hour delivers around 216,000 J, so in a sealed room it could warm the air by perhaps 3 degrees Celsius. Spread over the walls, furniture and the air leaking out of the room, the actual rise is far smaller, and no device could usefully collect a fraction of a degree spread through a whole room. This one-way tendency is a genuine law of nature rather than an engineering shortcoming. Concentrated energy spreads out by itself and never gathers itself back up, which is why a cup of tea cools to room temperature and a room-temperature cup never spontaneously heats itself while chilling the room. You can move energy back into a concentrated store, as a refrigerator does, but only by spending more concentrated energy elsewhere to do it.
Pause and explain
Why can the thermal energy released by a light bulb not be collected and used again?
Check your reasoning after answering
Because it has been spread out rather than destroyed. Getting useful work from thermal energy requires a temperature difference, and dissipation means the energy has levelled out to match the surroundings. A few hundred thousand joules shared across a room's air and walls raises everything by a fraction of a degree, which no device can usefully draw on.
Part 4 · Chains of changes
Most real systems put several transfers in a row, and the losses compound. Consider electricity reaching a filament bulb: a coal power station converts the chemical store of the fuel to electricity at about 35 per cent efficiency, transmission lines deliver perhaps 95 per cent of that to the house, and the bulb converts 5 per cent of what arrives into light. Multiplying the three fractions gives roughly 1.7 per cent of the original fuel energy arriving as light in the room.
That figure explains things a single efficiency cannot. It is why replacing the bulb with an LED saves far more fuel than the lamp's own 60 J per second suggests, since every joule saved at the end of the chain saves many times that at the power station. It is also the reasoning behind choosing where to improve a system: the biggest gain comes from the least efficient stage, which here is the power station and the bulb rather than the wires. The arithmetic also explains a claim that sounds wrong at first: an electric heater is about 100 per cent efficient in the room and still may be a poor way to heat a house. Every joule of electricity becomes heat, so nothing is wasted locally, but roughly three joules of fuel were burned at the power station to deliver one joule to the socket. Where you draw the boundary of the system decides which answer you get, which is the lesson from chapter 1 arriving again.
Pause and explain
A chain runs at 35 per cent, then 95 per cent, then 5 per cent. Where should an engineer look for improvement, and why?
Check your reasoning after answering
At the 5 per cent stage first and the 35 per cent stage second, because the overall efficiency is the product of the three and the smallest fraction limits it most. Improving the 95 per cent transmission step can gain at most a twentieth, while raising the final stage from 5 to 40 per cent multiplies the whole chain's output eightfold.
Apply the whole unit · Compare two engines
A petrol car engine converts about 20% of the chemical energy in its fuel into movement. A modern diesel engine manages about 30%. Each car is given 50,000,000 J of fuel.
Your task
Calculate the useful energy from each, compare them, and say where the rest of the energy goes.
What your response should show
- Calculates 10,000,000 J and 15,000,000 J from the stated percentages
- Expresses the difference as a comparison rather than only a subtraction
- Names the thermal store of exhaust, engine and air as the destination
- Supplies observable evidence for the waste rather than asserting it
Response guidance — after your attempt
Calculate the useful energy delivered by each engine from the 50,000,000 J supplied.
Express the diesel's advantage as a fraction of the petrol engine's output.
State where the wasted energy ends up and give the observable evidence.
At 20% efficiency the petrol engine delivers 20 per cent of 50,000,000 J, which is 10,000,000 J of movement. At 30% the diesel delivers 15,000,000 J. The diesel therefore produces 5,000,000 J more from the same fuel, which is half as much again as the petrol engine, so it would travel roughly 1.5 times as far on one tank. The remaining energy, 40,000,000 J in the petrol case and 35,000,000 J in the diesel, ends up almost entirely in the thermal store of the exhaust gases, the engine block, the coolant and the air, with a small amount as sound. The observable evidence is direct: the exhaust is hot, the bonnet is hot, the radiator exists specifically to carry heat away, and the engine is audible from some distance. Nothing is destroyed, but four fifths of the fuel in the petrol car ends up warming the road and the sky.
Scientific references for this unit
- US Department of Energy: where the energy goes in a petrol or diesel vehicle
- US Energy Information Administration: electricity generation and transmission losses
- US Department of Energy: lighting efficiency, filament bulbs against LEDs
- Institute of Physics: energy dissipation and why waste heat is unrecoverable
Parent / teacher teaching notes
Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.
The key idea
Energy is never used up and never created, yet machines still waste it. Both statements are true because waste does not mean destruction: it means energy has spread out into the thermal store of the surroundings, where it is too thinly scattered to do anything useful.
Worked examples
Example 1 · Compare two lighting models
- Divide visible-light output by input.
- Account for the remaining stated energy.
- Keep usefulness and conservation distinct.
Example 2 · Transfer to a different case
- Identify the distinction and conditions in this new case.
- Use the relevant mechanism or quantity; preserve units and the chosen boundary.
- Explain why the answer fits and what further evidence would be needed.
Example 3 · Choose a boundary for conservation · a focused example
- State the literal condition or observation in the example.
- Apply the distinction explained in this teaching part.
- Check whether the claim stays within the given evidence.
Avoid these mistakes
- Misleading claim: Energy that is less useful has been destroyed.
- Misleading claim: Conservation means every individual object must keep the same energy.
- Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
- Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.
Check your understanding
Does stopping mean all energy vanished?
Discuss the answer
No. The energy formerly associated with the motion is transferred or redistributed into other changes and surroundings.
Why is the cup alone not a closed energy account?
Discuss the answer
Energy crosses its boundary to the surroundings. The total account must include that transfer or a larger system.
Could thermal output be useful in a different device?
Discuss the answer
Yes. Usefulness depends on the intended function; conservation is not a judgment about whether the output is desirable.
Can the missing 10 J be declared destroyed?
Discuss the answer
No. Check omitted pathways, stored changes, and measurement uncertainty. The record is incomplete under its stated account.
Sustained scientific explanation and revision
These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.
Activity 1 · Energy Transformations, Conservation & Dissipation · Model and explain
Read Two Cups and One Question and the linked unit teaching; use its complete supplied case and diagrams.
Your task
For Energy Transformations, Conservation & Dissipation, study Parts 3–4 and attempt the worked case: Calculate each light fraction and explain a limited recommendation for reading. Explain the steps before opening the model.
Success criteria
- An accurate scientific claim answering the actual question.
- Relevant evidence or a labeled model with quantities and units when applicable.
- Reasoning that connects evidence with the mechanism rather than only repeats it.
- A specific method, boundary, scale, or alternative-explanation limit.
Hint
Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.
Response guidance — discuss after your attempt
A gives 24 ÷ 120 = 20% as visible light and 96 J in thermal pathways. B gives 30% as visible light and 84 J in thermal pathways. B has a larger modeled light fraction for this purpose, but brightness at the page, price, lifetime, and actual measurements would also matter. Both conserve the stated input energy.
Activity 2 · Energy Transformations, Conservation & Dissipation · Practice, revise and exit
Read Two Cups and One Question and the linked unit teaching; use its complete supplied case and diagrams.
Your task
For Energy Transformations, Conservation & Dissipation, complete practice questions 1–6 and both choice checks, then answer independently: A bicycle slows with friction. What larger system would make the account clearer? Save a correction explaining one changed claim or method.
Success criteria
- An accurate scientific claim answering the actual question.
- Relevant evidence or a labeled model with quantities and units when applicable.
- Reasoning that connects evidence with the mechanism rather than only repeats it.
- A specific method, boundary, scale, or alternative-explanation limit.
Hint
Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.
Response guidance — discuss after your attempt
Include the bicycle, contact surfaces, and surrounding air, or list their energy exchanges. Reduced kinetic energy can become thermal changes and sound without being destroyed.
Words to know
- conservation of energy
- Energy remains accounted for across a complete system and its transfers.
- dissipation
- Energy spreading into less readily useful forms or surroundings for the chosen purpose.
- evidence
- A relevant observation, measurement, or record used to evaluate a claim.
- system boundary
- The chosen limit specifying what is included and which exchanges cross it.
- uncertainty
- A stated limit in measurement or knowledge, distinct from a careless mistake.
In the real world
Hybrid and electric cars are designed to capture the kinetic energy of braking and transform it back into stored electrical energy in the battery, instead of wasting all of it as heat in the brakes.
Exit check
A bicycle slows with friction. What larger system would make the account clearer?
Check your explanation after answering
Include the bicycle, contact surfaces, and surrounding air, or list their energy exchanges. Reduced kinetic energy can become thermal changes and sound without being destroyed.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.