Conservation of energy and system efficiency — AC9S9U05

• Apply conservation of energy to a clearly defined system.
• Distinguish useful output from dissipated outputs such as heating and sound.
• Calculate efficiency = useful energy output ÷ total energy input × 100%.
• Evaluate realistic design changes without confusing efficiency with speed or power.

Recall energy transfer and transformation, joules, kinetic/potential energy and heating. You should be comfortable with percentages and ratios.

Conservation means total energy is accounted for when the system boundary and time interval are consistent. A device can be inefficient even though energy is conserved because some output is not useful for the intended purpose.

Efficiency = useful output / total input × 100%. If a motor receives 200 J and gives 140 J useful movement, the other 60 J must appear in other transfers such as heating and sound. For the same system boundary, efficiency cannot exceed 100%.

• Inefficient systems destroy energy. Energy is conserved; it becomes less useful outputs.
• Efficiency can exceed 100%. Not for consistent energy input/output accounting.
• Efficiency means speed. Efficiency is a ratio of useful output to total input.
• Only useful output counts in conservation. All outputs must be included in the energy account.

1. A lamp takes 100 J and produces 25 J useful light. Identify dissipated energy.
2. Calculate the lamp efficiency.
3. Explain why the remaining energy has not disappeared.
4. Suggest one change that could increase useful output for the same input.

1. Define system boundary and efficiency.
2. A device receives 500 J and has 325 J useful output. Calculate efficiency.
3. For the same device, calculate dissipated output.
4. Explain why a 120% efficiency claim signals an accounting/measurement problem.
5. Compare two devices with different inputs and useful outputs using percentages.
6. Evaluate whether reducing sound output necessarily improves efficiency for the device’s intended purpose.

Two motors produce the same useful mechanical output. Motor A draws less input energy but runs hotter at one measured point. Decide what can and cannot be concluded about efficiency and propose measurements needed for a fair comparison.

1. Is energy lost in an inefficient device? No; it is transferred into outputs that are less useful for the purpose.
2. What is efficiency? The fraction/percentage of input energy transferred to the intended useful output.
3. Why define the system? What counts as input and output depends on the chosen boundary and interval.
4. Can efficiency be over 100%? Not with correct, like-for-like energy accounting.

1. A pump receives 800 J and transfers 520 J usefully. Calculate efficiency and account for the remaining energy.
   Show the ratio, percentage and dissipated output.
2. Explain why conservation of energy and low efficiency can both be true.
   Separate total energy accounting from usefulness.
3. Evaluate a proposed design improvement that claims to double efficiency without changing measured input/output data.
   Use the definition of efficiency and identify what evidence must change.

• I can balance an energy account.
• I can calculate efficiency correctly.
• I can explain dissipated energy.
• I can evaluate a design claim.

Exit ticket: A 60%-efficient device receives 300 J. How much useful energy does it produce, and where might the rest go?

Australian Curriculum v9.0 — AC9S9U05: conservation of energy and system efficiency through inputs, outputs, transfers and transformations.

Victoria Levels 9–10 — VC2S10U15: Exact direct content relationship.

NSW Stage 5 — SC5-EGY-01: Partial related through evaluation of energy use and sustainability; it is not a one-to-one efficiency outcome.

• AC9S9U04 energy transfer models
• AC9S9I04 data representation

• Australian Curriculum v9.0
• Victorian Curriculum F–10 Version 2.0
• NSW Science 7–10 Syllabus (2023)

Curriculum equivalents for Apply the law of conservation of energy to analyse system...

Mapped skill: apply the law of conservation of energy to analyse system efficiency in terms of energy inputs, outputs, transfers and transformations

These references identify matching or closely related learning. Curriculum sequence, terminology and depth vary, so teachers should use the mapped skill and lesson difficulty to confirm suitability.

Australian Curriculum v9.0 is the canonical source for this SkillrHub lesson. Victoria and NSW entries name the closest published state codes or outcomes; international entries are planning references rather than claims of identical curricula.