Year 9 Science · AC9S9U05

Conservation of energy and system efficiency

Define a system, account for all energy outputs, calculate efficiency and explain why “wasted” energy is transferred to less useful stores rather than destroyed

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Key conceptTeach from the board

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%.

Worked examplesWe do

Worked examples

AC9S9U05 - Conservation of energy and system efficiency
Example 1

A lamp takes 100 J and produces 25 J useful light. Identify dissipated energy.

Example 2

Calculate the lamp efficiency.

Example 3

Explain why the remaining energy has not disappeared.

Example 4

Suggest one change that could increase useful output for the same input.

Curriculum examplesCopied content

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.

ComponentAC v9VictoriaNSW
Energy account/efficiencyDirectDirectSupporting
EvaluationDirect applicationBand-level applicationStrong partial SC5-EGY-01
Questions and answersWith answers
  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.
Practice and reviewReady for practice
  • 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.
Curriculum alignmentStart here
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