The Life Starter
Sign up
Grade 4 · lesson 4 of 4

Find the Missing Energy Entry

Use a system boundary and storage change to complete a ledger.

Download the grade pack

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

Before checking an energy ledger, decide what is inside the system. Our fictional device system includes its housing but not the room. For one interval, 25 units enter, 8 leave as light, and 12 leave as thermal energy. The record also says the energy stored inside the device increases by 5 units as it warms.

Input can be accounted for by transfers out plus an increase in stored energy. Here 8 + 12 + 5 equals 25. If someone records only the two outputs, the 5 missing units are not proof that energy vanished. They are the supplied storage increase. This lesson revises the earlier no-storage-change model by adding a stated condition.

  1. Draw a boundary around the device and label the room outside.
  2. Write 25 in and the two outward amounts.
  3. Add the stated 5-unit storage increase inside the boundary.
  4. Check input = outputs + storage increase and explain the earlier incomplete ledger.
Worked example

See the method in action

8 light + 12 thermal out + 5 stored = 25 input units. The device has more stored energy at the interval’s end; the outside outputs alone total 20.

Your turn

Check your reasoning

Your answer is temporary and is not saved to an account. Download your current answer before leaving or reloading. Written work on paper or in another app stays in your own file.

A device receives 18 units, sends 4 light and 11 thermal units out, and has no other transfers. How much does its stored energy increase?

Feedback appears here after you check. This is practice, not a graded school assessment.

See the answer and explanation

3 units

Outputs total 15, so 18 − 15 = 3 units remain as the storage increase under these conditions.

Take it further

Change thermal transfer out from 11 to 14 while keeping input 18 and light 4. Outputs now total 18, so the storage change is zero.

Nothing here is saved to an account or sent to a teacher. The answer download includes your current choice and any checked feedback.

Check the subject sources

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.