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Grade 4 · lesson 2 of 4

Account for the Flashlight’s Outputs

Check a simple energy budget without losing or creating energy.

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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

An energy pathway can also include amounts. In this fictional steady-operation model, a flashlight receives 20 energy units from its battery during one interval. It transfers 6 units outward as light and 14 as thermal energy. We assume these are all outputs and no energy is accumulating inside the flashlight during the interval.

Add the outputs and compare the total with the input. A complete ledger has 6 + 14 = 20 units. Energy described as “wasted” has not vanished; it is transferred in a form less useful for the stated job. Here the job is lighting a sign, so light is the useful output. No battery or lamp is needed to check this paper ledger.

  1. Draw an input arrow labeled 20 units.
  2. Draw output arrows labeled light 6 and thermal 14.
  3. Add the two outputs and compare with 20.
  4. Explain why the thermal output belongs in the ledger.
Worked example

See the method in action

The outputs total 6 + 14 = 20 units, equal to the input. Six units serve the lighting goal, while 14 transfer as thermal energy; none is treated as disappearing.

Your turn

Check your reasoning

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A new complete ledger has input 30 units and light output 9 units. With no storage change, what is the thermal output?

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See the answer and explanation

21 units

All 30 input units must be accounted for. 30 − 9 = 21 thermal units under the stated two-output, no-storage-change model.

Take it further

Change the light output to 12 units with the input still 30. Thermal output becomes 18, and 12 + 18 = 30.

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These LifeStarter passages, scenarios and exercises are authored for this Academy with AI assistance and review. External resources keep their publishers’ terms. A grade label suggests a starting level; it does not establish alignment with your school’s complete requirements.