Apply energy conservation with a stated system
Account for stores and transfers across a boundary instead of saying energy has disappeared.
Section P1.7.1 · Energy
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Identify decreasing and increasing stores.
- Add quantities measured in the same unit.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How much remains from 120 J after a 70 J accounted transfer?
What you will learn
- State that total energy is conserved.
- Distinguish a closed energy account from transfers into or out of a selected system.
A complete account balances
Energy cannot be created or destroyed: it can be transferred between stores and systems. In a closed energy account with no transfer across the selected boundary, the total remains the same. If 120 J leaves a source store and 70 J appears in a desired store, the other 50 J must have another accounted destination, such as internal stores of the surroundings.
Energy described as 'wasted' is still transferred or stored elsewhere; it has not ceased to exist. It is often more dispersed and less useful for the intended task. A 120 J input with alleged outputs 70 J and 60 J fails that stated budget unless another input, decrease of a stored amount or a measurement issue is identified.
The boundary determines what is inside
For a selected system, change in total stored energy equals energy transferred in minus energy transferred out over the same interval. If 100 J enters and 30 J leaves, the system's total stored amount increases by 70 J. If 20 J enters and 50 J leaves, its total decreases by 30 J. These are changes, not necessarily the complete initial or final stored totals.
A cooling cup alone is not a closed energy account if it transfers energy to the room. A larger cup-and-surroundings account can include that receiving store. State the objects, interval and relevant transfers before claiming that one store's change must equal a second store's change. An incomplete boundary or missing observation should prompt investigation rather than inventing destroyed energy.
Sources for this lesson
Pause and explain
A cup loses internal energy while warming its room. Has energy been destroyed?
Worked example
A source releases 120 J. A desired store gains 70 J, and the only other destination is the surroundings. Find that gain.
Show the worked solution
- Use a complete budget: 120 = 70 + other gain.
- Subtract the known desired gain.
- The surroundings gain 50 J; total energy is conserved.
Answer 50 J in the surroundings.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Account for 120 J released when a desired store gains 70 J and the surroundings are the only other destination.
Give me a hint
Subtract the supplied store gain from the total release.
Compare my reasoning
- 120 J is the total released.
- 70 J is already accounted for.
- The remaining gain is 120 − 70 = 50 J.
50 J to the surroundings.
Look for these in your work
- Balance all destinations.
- Avoid calling the remainder destroyed.
Find stored changes when a system receives 100 J and transfers out 30 J, and when it receives 20 J and transfers out 50 J.
Give me a hint
Use input minus output for both changes.
Compare my reasoning
- First change = 100 − 30 = +70 J.
- Second change = 20 − 50 = −30 J.
- A negative change is a decrease, not a negative absolute store.
+70 J; −30 J.
Look for these in your work
- Use one boundary and interval.
- Distinguish change from total stored energy.
A claimed closed 120 J release gives 70 J to one store and 60 J to another. Explain why the account needs checking.
Give me a hint
Compare the total claimed gains with the stated release.
Compare my reasoning
- The reported gains sum to 130 J.
- This exceeds 120 J by 10 J.
- Investigate omitted inputs, other store decreases or measurements; do not invent energy creation.
The gains exceed the release by 10 J; the specified closed account is inconsistent.
Look for these in your work
- Calculate the discrepancy.
- Name evidence that could repair the account without violating conservation.
Common mistakes
- Wasted energy is not destroyed.
- State the selected system and interval.
- A stored change is not the final stored total.
What can you explain now?
What stored change follows from 20 J entering and 50 J leaving?
Compare with the explanation
−30 J; a decrease of 30 J.
Net input is 20 − 50 = −30 J over the stated interval.
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Make it stick
State conservation, explain dispersed wasted energy and write input minus output for a selected system's stored change.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.