A typical coal power station in Australia converts only about 33% of its fuel's chemical energy into electricity — the remaining 67% becomes waste thermal energy. A cricket ball bowled at 140 km/h carries enormous kinetic energy, and when the batter hits it and the fielder catches it, that energy does not vanish. It transforms and transfers in ways that are predictable, measurable, and governed by one of the most important laws in physics: the law of conservation of energy.
A bowler runs in and delivers a cricket ball at high speed. The batter strikes it, sending the ball soaring toward the boundary. A fielder runs, dives, and catches it. The ball is now still in the fielder's hands.
Think carefully: the ball had kinetic energy when bowled. Estimate what percentage of that kinetic energy is still in the ball when the fielder catches it, and what percentage has transformed into other forms. Write your prediction and reasoning before reading on — you will verify it at the end of the lesson.
📚 Core Content
Wrong: "When a ball stops rolling, its energy is gone."
Right: 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.
Wrong: "A machine can be 100% efficient if it is well made."
Right: 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.
This is not a suggestion or a rough guide. It is a law of nature, tested billions of times and never found violated. Every joule of energy that existed at the beginning of the universe still exists today — just in different forms and places.
The law of conservation of energy states that in a closed system, the total amount of energy remains constant. Energy can change form. It can move from one object to another. But it cannot appear from nowhere, and it cannot vanish into nothing.
This law is why physicists and engineers can predict what will happen in a system. If you know the energy going in, you can account for where it must go — even if some of it becomes difficult to measure because it spreads out as waste thermal energy.
The law of conservation only applies exactly to closed systems. In the real world, most systems are open — which is why energy seems to disappear even though it is actually leaving the system we are observing.
A closed system does not exchange energy or matter with its surroundings. A perfectly insulated thermos flask comes close: thermal energy stays inside, so the total energy of the coffee + flask system is constant.
An open system exchanges energy and/or matter with its surroundings. A car engine is open: chemical energy enters as petrol, exhaust gases leave, and thermal energy transfers to the air through the radiator and exhaust. If you only look at the engine block, energy appears to decrease — but that is because it left the system, not because it was destroyed.
Even when energy is perfectly conserved, not all of it is useful. Understanding the difference between useful and waste energy is the foundation of efficiency — a concept you will explore in detail in the next lesson.
When you turn on an incandescent light bulb, electrical energy enters the filament. Some transforms into light — that is useful energy, because light is what you wanted. But about 95% transforms into thermal energy that heats the bulb and the surrounding air. That thermal energy is waste energy — it is conserved, but it does not help you see.
The waste energy is not "lost" in the sense of violating conservation. It is dissipated — spread out among billions of air molecules, making it practically impossible to collect and reuse. This is why dissipated thermal energy is sometimes called "low-grade" energy: it still exists, but it is not concentrated enough to do useful work.
In summer, road surfaces in the Australian outback can reach 70°C — hot enough to fry an egg. The asphalt absorbs enormous amounts of thermal energy from sunlight throughout the day. At night, that thermal energy transfers to the air and radiates into space, which is why desert temperatures can drop below 10°C even after a 45°C day. This massive energy transfer and dissipation happens in every outback town from Alice Springs to Birdsville.
When an elite long jumper leaves the take-off board, their kinetic energy transforms into gravitational potential energy as they rise through the air. At the peak of the jump, potential energy is maximum and kinetic energy is minimum (but not zero — they are still moving forward). On descent, potential energy transforms back into kinetic energy. The world's best long jumpers, like Australia's Brooke Buschkuehl, maximise jump distance by optimising the angle and speed at take-off to convert as much horizontal kinetic energy as possible into upward potential energy without losing too much to air resistance.
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.
1. Which statement best describes the law of conservation of energy?
2. A ball rolls along a flat surface and eventually stops. A student says "the energy has disappeared." What is the scientifically correct explanation?
3. Why does the law of conservation of energy apply exactly to a closed system but only approximately to most real-world situations?
4. A car engine transforms chemical energy in petrol into kinetic energy of the wheels. For every 100 J of chemical energy, 75 J becomes thermal energy (exhaust, radiator, friction) and only 25 J becomes kinetic energy. Which statement is correct?
5. Snowy 2.0 pumps water uphill using excess solar and wind energy, then releases it later to generate electricity. The system returns about 80% of the electrical energy used for pumping. What happened to the other 20%?
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.
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.
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.
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.
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]
1. B — This 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. C — Friction 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. A — Real 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. D — 100 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. B — Friction 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.
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].
Want to review any section before moving on?
Tick when you can explain conservation of energy, identify closed and open systems, and account for useful and waste energy.