The Law of Conservation of Energy
In 1842, Julius von Mayer measured heat and work and proved they are the same thing, discovering a law that governs every device ever built.
Printable Worksheets
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Q1 · A bouncing ball gets lower and lower with each bounce. Have you ever wondered, if energy can't be destroyed, where is the energy going each time it bounces?
Q2 · If you could somehow recover all the "lost" energy from a bouncing ball, could you get it to bounce forever? Why or why not?
Key Relationships, This Lesson
● Know
- The law of conservation of energy
- The difference between a closed system and an open system
- That waste energy is usually thermal energy lost to surroundings
● Understand
- Why energy appears to "disappear" even though it is conserved
- How the law applies to real-world devices and living things
- Why no real process is 100% efficient
● Can do
- Explain where energy goes in a described process
- Identify useful and waste energy in a system
- Apply conservation of energy to predict energy outputs
Wrong: "When a ball stops rolling, its energy is gone."
Right: The ball's kinetic energy transforms into thermal energy through friction with the surface and air resistance. The energy hasn't gone, it's spread out as heat in the ground and air around it.
Wrong: The kinetic energy transforms into thermal energy through friction with the ground and air resistance. The energy is conserved but becomes spread out and less useful.
Right: Energy is conserved, the total amount stays the same. The kinetic energy transforms into thermal energy that dissipates into the surroundings, making it very hard to recover and reuse.
Wrong: "A machine can be 100% efficient if it is well made."
Right: No real machine can be 100% efficient. Friction, air resistance, and heat loss are unavoidable in any physical process, some energy always ends up as waste thermal energy that dissipates into the surroundings.
Wrong: Some waste energy is unavoidable in every real process due to friction, air resistance, and heat loss. Even the best machines convert some input energy to waste thermal energy.
Right: Even a perfectly designed machine loses some energy as heat, because friction and resistance cannot be completely eliminated. The goal of good engineering is to minimise waste, not eliminate it entirely.
Drop a basketball from 1 metre, it bounces back to about 60 cm, then 36 cm, then lower again. Each bounce is visibly smaller, yet the ball never gains extra energy from nowhere. Everything you observe follows a strict rule: the total energy before an event equals the total energy after. When you flick on a light switch, chemical energy in the power station's fuel becomes thermal, then kinetic (spinning turbine), then electrical, then light and heat in your bulb. Not a single joule disappears; it merely changes address.
The reason we care about wasting energy is that once it spreads out as low-grade thermal energy, it becomes practically impossible to gather back into a useful form. The energy still exists, but it is no longer doing the job we wanted.
A car engine converts about 20% of petrol's chemical energy into kinetic energy that moves the wheels. The other 80% becomes thermal energy in the engine, exhaust and brakes. The total energy is conserved, but most of it is not doing useful work.
Many students believe energy disappears when a ball stops bouncing or a car comes to rest. In reality, the kinetic energy transforms into thermal energy in the tyres and road, and sound energy in the air. The ball does not lose energy, it loses useful, organised energy.
Click each stage to follow how energy transforms through a typical process.
Store
Energy is stored in a source: chemical bonds in fuel, height in a raised object, or compression in a spring.
Transform
Energy changes from one form to another: chemical to thermal, thermal to kinetic, or potential to kinetic.
Transfer
Energy moves from one object to another: heat flows from hot to cold, or sound waves travel through air.
Dissipate
Energy spreads out as low-grade thermal energy and sound, becoming practically impossible to recover.
To track energy through a transformation, you need to name the forms involved. Kinetic energy belongs to anything moving, from a thrown ball to wind. Gravitational potential energy depends on height: lift an object and you store energy in it. Elastic potential energy lives in stretched or compressed materials.
Thermal energy, light, sound and electrical energy are also common forms. Most real situations involve several at once. A flying bird has kinetic energy (motion), gravitational potential energy (height) and chemical energy (food in its muscles) all simultaneously.
A hydroelectric dam stores gravitational potential energy in the elevated water. When the water falls, that becomes kinetic energy that spins turbines, which generate electrical energy. Each stage is a named transformation in a long chain.
Wattle Point Wind Farm in South Australia converts the kinetic energy of moving air into electrical energy. On a windy day, the spinning blades demonstrate energy transformation you can see and hear.
The conservation law applies everywhere, from atoms to galaxies. Physicists use it as a bookkeeping tool: they add up all forms of energy at the start, add them up at the end, and check the totals match. If they do not, it means some energy form has been missed, not that energy vanished.
This principle is especially powerful because it works even when we do not understand the details. Nineteenth-century physicists discovered neutrinos precisely because energy seemed to disappear in radioactive decay. Conservation demanded a new particle, and experiments later found it.
When an asteroid enters Earth's atmosphere, its enormous kinetic energy transforms into thermal energy (the glowing fireball), sound energy (sonic boom) and light energy. The total energy of the asteroid-atmosphere system is unchanged, just redistributed.
Click each point on the roller coaster track to see the energy forms
Copy Into Your Books
▼Conservation of Energy
- Energy cannot be created or destroyed
- It can only be transferred or transformed
- Total energy in a closed system stays constant
Closed vs Open Systems
- Closed: no energy or matter enters or leaves
- Open: energy and/or matter can enter and leave
- Most real systems are open
Useful vs Waste Energy
- Useful energy does the intended job
- Waste energy is usually thermal energy
- Waste energy is dissipated into surroundings
- Dissipated energy is hard to recapture
this level Language
- Say "transforms into" not "turns into"
- Say "dissipated as thermal energy" not "lost"
- Account for ALL energy in any explanation
Account for the Energy
1 A student rides an electric scooter across a flat playground in Brisbane. The battery provides 1,000 J of electrical energy.
2 A boulder rolls down a steep slope in the Blue Mountains and comes to rest at the bottom.
3 A coal-fired power station in the Latrobe Valley burns coal to generate electricity for 100,000 homes. For every 100 J of chemical energy in the coal, only 35 J becomes electrical energy.
4 A swimmer dives off a 10-metre platform at the Sydney Olympic Park Aquatic Centre. Describe the energy transformations from the moment they stand on the platform to when they enter the water.
The "Missing" Energy Mystery
At the start of this lesson you were asked about a rubber ball that bounces lower and lower, if energy cannot be destroyed, where are all those missing joules going with each bounce?
Now that you've worked through conservation of energy, can you explain exactly what is happening to that energy? How has your understanding shifted?
Q1. 6. State the law of conservation of energy. Explain why a ball rolling on grass eventually stops, and describe where the energy went.
1 mark for stating the law. 1 mark for explaining friction and air resistance. 1 mark for identifying thermal energy dissipated to surroundings.Q2. 7. A torch converts 10 J of chemical energy from its batteries into 1 J of light energy. Identify the useful energy output, the waste energy output, and the total energy output. Explain why the torch is not violating the law of conservation of energy.
1 mark for useful energy. 1 mark for waste energy. 1 mark for total energy. 1 mark for explanation using conservation law.Q3. 8. A student claims: "If energy is conserved, then we never need to worry about saving energy, we can just transform it back into useful forms whenever we want." Analyse this claim using the concepts of waste energy, dissipation, and the difference between closed and open systems.
1 mark for identifying the flaw in the claim. 1 mark for explaining dissipation. 1 mark for explaining why dissipated energy is hard to reuse. 1 mark for linking to open systems. 1 mark for a balanced conclusion.Model answers (click to reveal)
Comprehensive Answers
▼Activity 1, Account for the Energy
1. Electric scooter: Input = 1,000 J electrical energy. Useful output = kinetic energy of the scooter and rider moving across the playground. Waste output = thermal energy from friction in bearings, air resistance, and heat from the motor. Total output = 1,000 J (conserved). The waste energy is dissipated to the air and ground.
2. Boulder in Blue Mountains: Initial energy = gravitational potential energy (due to height). Final energy forms = some kinetic energy just before stopping, but most transformed into thermal energy through friction with the ground and air resistance, plus sound energy from collisions. The boulder stopped because all its kinetic energy was transformed into waste thermal and sound energy that dissipated to the surroundings.
3. Coal power station: Input = 100 J chemical energy. Useful output = 35 J electrical energy. Waste output = 65 J thermal energy (through exhaust gases, cooling towers, friction). The waste thermal energy transfers to the air and water in the Latrobe Valley, slightly warming the local environment. This is why power stations need cooling systems.
4. Diver at Sydney Olympic Park: On platform: gravitational potential energy maximum, kinetic energy zero. During dive: potential energy transforms into kinetic energy as height decreases. At water entry: kinetic energy maximum, potential energy minimum. After entering water: kinetic energy transforms into thermal energy (water and diver warm slightly) and sound energy (splash). Some kinetic energy also transfers to moving water molecules.
Activity 2, The "Missing" Energy Mystery
The student's conclusion is incorrect [1 mark]. The ball and Earth system they considered is open, not closed, because air resistance acts on the ball as it falls [1 mark]. The "missing" energy was transformed into thermal energy through air resistance (friction between the ball and air molecules) and also into sound energy [1 mark]. Some energy may also have been transferred to the air as kinetic energy of air movement. The total energy of the ball + Earth + air system is conserved, but the student's measurement only tracked the ball's kinetic energy, not the thermal energy dissipated to the surroundings [1 mark]. A better conclusion would be: "The prediction assumed no air resistance. In reality, some gravitational potential energy transforms into thermal energy through air resistance, so the measured speed is slightly lower than predicted. This is consistent with conservation of energy once all forms are accounted for." [1 mark]
Multiple Choice
1. BThis is the exact statement of the law. Option A is wrong because energy cannot be created. Option C contradicts the law. Option D is false, conservation applies to all systems.
2. CFriction and air resistance transform kinetic energy into thermal energy, which dissipates. The energy is conserved but becomes spread out. Option A violates conservation. Option B is not what happens. Option D confuses mass-energy equivalence (E=mc²), which is irrelevant at these energies.
3. AReal systems exchange energy with surroundings. The law applies to the total energy of the universe, but if we only look at a subsystem, energy can appear to change. Option B understates the law's validity. Option C is false, closed systems can be approximated. Option D violates conservation.
4. D100 J = 25 J useful + 75 J waste. All energy is accounted for; none is lost. The thermal energy dissipates to surroundings. Options A, B, and C all incorrectly claim energy is lost or violated.
5. BFriction in pipes and turbines transforms some energy into thermal energy that dissipates. This is unavoidable in real systems. Option A violates conservation. Option C is wrong, water does not store chemical energy. Option D is irrelevant.
Short Answer Model Answers
Q6 (3 marks): The law of conservation of energy states that energy cannot be created or destroyed, only transferred or transformed [1 mark]. The ball stops because friction between the ball and grass, and air resistance, oppose its motion [1 mark]. The kinetic energy transforms into thermal energy that spreads out (dissipates) into the ground and surrounding air [1 mark].
Q7 (4 marks): Useful energy output = 1 J light energy [1 mark]. Waste energy output = 9 J thermal energy [1 mark]. Total energy output = 10 J [1 mark]. The torch does not violate conservation because 10 J input = 1 J useful + 9 J waste = 10 J total output. All energy is accounted for; it has just been transformed into a less useful form [1 mark].
Q8 (5 marks): The claim is flawed because it ignores the concept of dissipation [1 mark]. When energy is transformed into waste thermal energy, it spreads out among billions of particles in the surroundings [1 mark]. This dissipated energy is extremely difficult to collect and convert back into a useful form because it is so spread out and low-grade [1 mark]. Real systems are open, meaning energy constantly transfers to the surroundings and becomes unavailable for useful work [1 mark]. Therefore, even though energy is conserved in the universe, we must still conserve useful energy in practice because once it is dissipated, it is effectively lost to us for doing work [1 mark].
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