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Grade 11 · lesson 3 of 4

Distinguish Useful Efficiency from Conservation

Compute an output fraction and complete an energy ledger.

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Supplied materials

  • Use the supplied fictional observations or model. Paper and pencil are optional; spoken answers are welcome. No equipment or real experiment is needed.

Learn the idea

Efficiency describes the fraction of input energy serving a stated purpose; energy conservation accounts for all destinations. For a lighting task, useful efficiency here is emitted-light energy / electrical input energy × 100%. Thermal transfers count in conservation even when they are not the desired light output. Specify the device boundary and interval before adding amounts.

During a fictional interval, a lamp receives 300 J electrically and emits 75 J as light and 210 J as thermal transfers to its surroundings. Its stored internal energy increases by 15 J. These are the only listed transfers. The ledger balances: 75 + 210 + 15 = 300 J. Lighting efficiency is 75/300 = 25%, not 100%. This simple energy fraction does not measure human-perceived brightness, which also depends on wavelength.

  1. Write electrical input and the three accounted destinations.
  2. Add destinations to check 300 J.
  3. Calculate the lighting energy fraction as a percentage.
  4. Explain why 25% useful efficiency can coexist with a complete 100% energy ledger.
Worked example

See the method in action

Lighting efficiency is 25%. Thermal transfer plus storage account for the other 225 J, so the low useful fraction does not imply energy destruction.

Your turn

Check your reasoning

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A no-storage-change lighting model receives 500 J and emits 150 J light; all remaining energy is thermal output. Which pair is correct?

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See the answer and explanation

30% useful and 350 J thermal

150/500 × 100 = 30%, and 500 − 150 = 350 J thermal under the supplied complete-output model.

Take it further

Change the useful job to warming an enclosure and give 400 J input, 320 J thermal transfer into that enclosure, and 80 J to the outside. Useful efficiency for that stated job is 320/400 = 80%; usefulness depends on the goal.

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