A Tesla battery in a Sydney garage stores 75 kilowatt-hours of electrical energy. When the car drives to Newcastle and back, where did that energy go? By quantifying energy — measuring it in joules — we can account for every single joule and prove that conservation of energy is not just a theory. It is a calculable fact.
A Tesla Model 3 driving from Sydney to Newcastle uses about 150 megajoules of energy from its battery. When it arrives, the battery has less energy stored in it. But the car has not "lost" energy — it has transformed and transferred it.
Before reading on, estimate: of those 150 MJ, roughly how much do you think became kinetic energy of the car, how much became thermal energy (heating tyres, air, brakes), and how much became sound energy? Write your estimates — you will compare them to real data at the end of the lesson.
📚 Core Content
Wrong: "If a device is 50% efficient, half the energy disappears."
Right: 50% efficiency means half the input energy becomes useful output, and the other half becomes waste energy (usually thermal). All energy is still accounted for — none disappears.
Wrong: "A bigger number for efficiency always means a better device."
Right: Efficiency must be considered alongside the total energy used. An LED bulb at 20% efficiency might use far less total energy than an incandescent bulb at 5% efficiency because the LED needs much less input energy to produce the same light.
The law of conservation of energy is not just a philosophical idea. It is testable and calculable. When scientists and engineers design power stations, electric vehicles, or heating systems, they use energy accounting to predict performance and identify where improvements can be made.
The fundamental equation for energy accounting is:
Energy input = Useful energy output + Waste energy output
This equation must balance. If you know any two values, you can calculate the third. This is how engineers diagnose problems: if the useful output is lower than expected, they know more energy is being wasted than designed — perhaps through friction, heat loss, or electrical resistance.
A solar panel on a roof in Brisbane receives 800 J of light energy from the Sun in one second. It produces 160 J of electrical energy. Calculate the waste energy and the efficiency.
Identify what you know: Energy input = 800 J, Useful output = 160 J.
Calculate waste energy: Waste = Input − Useful = 800 − 160 = 640 J.
Calculate efficiency: Efficiency = (160 ÷ 800) × 100 = 20%.
Answer: Waste energy = 640 J. Efficiency = 20%. The remaining 640 J becomes thermal energy that heats the solar panel and is dissipated to the surrounding air.
An electric kettle has an efficiency of 90%. If 1,800 J of electrical energy is supplied, how much thermal energy is actually transferred to the water, and how much is wasted?
Identify what you know: Input = 1,800 J, Efficiency = 90% = 0.90.
Calculate useful output: Useful = Input × Efficiency = 1,800 × 0.90 = 1,620 J (thermal energy in the water).
Calculate waste: Waste = 1,800 − 1,620 = 180 J (thermal energy lost to the kettle body and air).
Answer: Useful thermal energy in water = 1,620 J. Waste energy = 180 J. Even a highly efficient kettle loses some energy to its surroundings.
Numbers make abstract ideas concrete. Here are real efficiency figures for electricity generation methods used in Australia. These are approximate values — exact figures vary by specific technology and age of equipment.
| Source | Typical Efficiency | Input Energy → Output Energy |
|---|---|---|
| Coal-fired power station | ~35% | Chemical → Electrical |
| Natural gas combined cycle | ~50% | Chemical → Electrical |
| Solar photovoltaic (PV) | ~20% | Light → Electrical |
| Wind turbine | ~45% | Kinetic → Electrical |
| Hydroelectric | ~90% | Gravitational potential → Electrical |
| Incandescent light bulb | ~5% | Electrical → Light |
| LED light bulb | ~20% | Electrical → Light |
These numbers tell a story. Hydroelectric power is extremely efficient because water flowing downhill has minimal waste — most of the gravitational potential energy converts directly into kinetic energy of spinning turbines. In contrast, incandescent light bulbs are extraordinarily inefficient because 95% of the electrical energy becomes waste thermal energy rather than light.
The black-box flight recorder was invented in Australia by Dr David Warren in 1953. While not directly about energy, it relies on precise energy accounting: the device must operate on minimal battery power for weeks after a crash, storing data in memory that requires carefully managed electrical energy. Australian engineers are world leaders in low-power device design.
A professional cyclist in the Tour Down Under produces about 400 watts of useful mechanical power. But the human body is only about 25% efficient at converting chemical energy in food into mechanical work. This means the cyclist must actually metabolise food at a rate of about 1,600 watts — four times the useful output. The other 1,200 watts becomes thermal energy, which is why cyclists overheat and need to drink litres of water per hour to cool down through sweating.
1 A solar panel in Alice Springs receives 2,000 J of light energy and produces 340 J of electrical energy. Calculate the waste energy and the efficiency.
2 A coal power station has an efficiency of 35%. If it burns coal that releases 10,000 MJ of chemical energy, how much electrical energy does it produce, and how much is wasted?
3 An LED bulb is supplied with 60 J of electrical energy. If it is 20% efficient, how much light energy does it produce? How does this compare to an incandescent bulb that produces 3 J of light from the same 60 J input?
4 The Snowy Hydro scheme has turbines that are approximately 90% efficient. If water flowing through delivers 5,000 MJ of gravitational potential energy, calculate the electrical energy output and the waste energy.
1. A device uses 500 J of energy input and produces 150 J of useful output. How much waste energy is produced?
2. A wind turbine receives 2,000 J of kinetic energy from the wind and generates 900 J of electrical energy. What is its efficiency?
3. An LED bulb has an efficiency of 20% and produces 12 J of light energy. How much electrical energy was supplied?
4. A coal power station has an efficiency of 35%. For every 1,000 MJ of chemical energy in the coal, which statement is correct?
5. Which change would most likely improve the overall energy efficiency of a typical Australian home?
6. A natural gas power station has an efficiency of 50%. It burns gas that releases 8,000 MJ of chemical energy. Calculate the electrical energy output and the waste energy output. Show all working. 1 mark for correct formula. 1 mark for correct electrical output. 1 mark for correct waste energy.
7. The following data compares two light bulbs used in Australian homes:
Incandescent: 60 W input, 5% efficient, produces 3 W of light.
LED: 10 W input, 20% efficient, produces 2 W of light.
A student concludes: "The incandescent bulb is better because it produces more light." Evaluate this conclusion using energy accounting concepts. 1 mark for calculating waste energy for each bulb. 1 mark for identifying the flaw in the student's reasoning. 1 mark for explaining efficiency in context. 1 mark for a balanced, evidence-based conclusion.
8. The Snowy 2.0 pumped hydro scheme stores energy by pumping water uphill, then releases it to generate electricity later. For every 100 MJ of electrical energy used to pump water, about 80 MJ of electrical energy can be generated when the water flows back down. Some politicians have claimed this means "20% of energy is lost, so pumped hydro is a waste." Analyse this claim using the law of conservation of energy, the concept of efficiency, and the value of storing energy for later use. 1 mark for explaining where the 20 MJ goes. 1 mark for showing that energy is conserved, not lost. 1 mark for explaining why pumped hydro is valuable despite inefficiency. 1 mark for linking to intermittency of solar/wind. 1 mark for a balanced conclusion.
1. Alice Springs solar panel: Waste = 2,000 − 340 = 1,660 J. Efficiency = (340 ÷ 2,000) × 100 = 17%.
2. Coal power station: Useful = 10,000 × 0.35 = 3,500 MJ. Waste = 10,000 − 3,500 = 6,500 MJ.
3. LED vs incandescent: LED light = 60 × 0.20 = 12 J. LED waste = 60 − 12 = 48 J. Incandescent waste = 60 − 3 = 57 J. The LED produces 4 times as much light from the same input, or could use far less power to produce the same light.
4. Snowy Hydro: Electrical output = 5,000 × 0.90 = 4,500 MJ. Waste = 5,000 − 4,500 = 500 MJ. The waste energy becomes thermal energy through friction in turbines, generators, and water turbulence.
Current: 100 bulbs × 60 W = 6,000 W total. Each bulb produces 3 W light (5% of 60 W). Total light = 300 W. Waste = 5,700 W as thermal energy [1 mark]. With LEDs: to produce 300 W of light at 20% efficiency, need 300 ÷ 0.20 = 1,500 W input. Or using 10 W LEDs: each produces 2 W light, so need 150 bulbs × 10 W = 1,500 W for same light [1 mark]. Energy saved = 6,000 − 1,500 = 4,500 W. Over 8 hours = 36 kWh saved per day [1 mark]. Annual saving ≈ 36 × 200 school days = 7,200 kWh. At $0.30/kWh = ~$2,160/year [1 mark]. Recommendation: LED replacement is the most cost-effective first step because it has immediate impact, low installation cost, and rapid payback. Solar panels and AC improvements are also valuable but have higher upfront costs [1 mark].
1. C — 500 − 150 = 350 J waste. Option A gives useful output. Option B incorrectly says energy disappears. Option D adds instead of subtracts.
2. B — (900 ÷ 2,000) × 100 = 45%. Option A reverses the division. Option C is the waste percentage. Option D is not supported.
3. A — Input = Useful ÷ Efficiency = 12 ÷ 0.20 = 60 J. Option B doubles incorrectly. Option C divides by 5 instead of 0.20. Option D multiplies by 20 instead of dividing.
4. D — 35% of 1,000 = 350 MJ electrical. 1,000 − 350 = 650 MJ waste thermal. Options A, B, and C all contain errors in accounting or description.
5. C — Both improve efficiency, but B saves more total energy because less input is needed. Option A is true but incomplete. Option B is true but incomplete. Option D is false — any improvement below 100% still helps.
Q6 (3 marks): Useful output = Input × Efficiency = 8,000 × 0.50 = 4,000 MJ [1 mark]. Waste = 8,000 − 4,000 = 4,000 MJ [1 mark]. The 4,000 MJ of waste energy becomes thermal energy through exhaust gases, friction, and heat loss to the surroundings [1 mark].
Q7 (4 marks): Incandescent waste = 60 − 3 = 57 W [0.5 mark]. LED waste = 10 − 2 = 8 W [0.5 mark]. The student's conclusion is flawed because they only compared light output without considering the input energy required [1 mark]. The LED uses only 10 W to produce 2 W of light, while the incandescent uses 60 W to produce 3 W. The LED is four times more efficient and uses one-sixth the power [1 mark]. A better conclusion: "The LED is superior because it produces almost the same light using far less electrical energy, dramatically reducing waste thermal energy and electricity costs." [1 mark]
Q8 (5 marks): The 20 MJ becomes thermal energy through friction in pipes, turbines, and generators, plus turbulence in the water [1 mark]. Energy is conserved because 100 MJ input = 80 MJ useful electrical output + 20 MJ waste thermal energy. Nothing has disappeared [1 mark]. Pumped hydro is valuable because it stores energy when supply exceeds demand (e.g., midday solar peak) and releases it when demand exceeds supply (evening peak) [1 mark]. Solar and wind are intermittent — they do not generate on demand. Without storage, excess renewable energy would be curtailed (wasted). Pumped hydro captures energy that would otherwise be lost [1 mark]. Conclusion: While 20% energy transformation to waste is a real inefficiency, the ability to time-shift renewable energy makes pumped hydro economically and environmentally valuable. The claim ignores the context of grid stability and renewable intermittency [1 mark].
Q6 (3 marks): (1) Correct formula used. (2) Correct electrical output calculated. (3) Correct waste energy calculated.
Q7 (4 marks): (1) Calculates waste for both bulbs. (2) Identifies flaw in reasoning. (3) Explains efficiency difference. (4) Balanced conclusion.
Q8 (5 marks): (1) Accounts for 20 MJ waste. (2) Explains conservation. (3) Values storage. (4) Links to intermittency. (5) Balanced evaluation.
Want to review any section before moving on?
Tick when you can calculate waste energy, efficiency, and use energy accounting to justify conclusions.