Interpret simple energy flow diagrams
Read the source, destinations and missing branch of a clearly labelled energy account.
Section P1.7.1 · Energy
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Apply conservation to a complete account.
- Distinguish stores from transfer pathways.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What unit labels an energy amount?
What you will learn
- Interpret a simple flow diagram.
- Complete a budget without inferring an unstated arrow scale or time rate.
Read what the arrows and labels specify
A simple energy flow diagram shows where energy transfers from and where it goes. In the original lamp example, 120 J of electrical transfer supplies 40 J of visible radiation and 80 J transferred through heating and non-visible radiation over the same interval. The outputs sum to the input. The drawing's ordinary arrows show connections; their widths are not a quantitative scale.
The source description can be traced back to a battery's decreasing chemical store. A receiving surface can absorb radiation and increase its internal store. 'Useful' depends on the task: visible radiation is useful for this lamp's lighting task, but a heater's intended output is different. Do not declare all internal-energy transfers undesirable regardless of purpose.
A missing branch is a missing destination
For a device with 200 J input and 150 J in a stated desired output, an exhaustive two-branch account assigns 50 J to the other branch. If the diagram is not exhaustive, a subtraction finds only the unaccounted total, not its exact destination. A branch label such as 'internal stores and other waves' may group more than one physical recipient.
An amount in joules does not state a rate in watts unless the interval is supplied. Comparing arrow thickness is valid only when the diagram provides a consistent energy scale, as the Extended Sankey lesson does. First read the labels, unit, boundary and interval; then balance distinct branches without counting an intermediate arrow again as an additional final output.
Sources for this lesson
Pause and explain
Does 120 J on a diagram imply a power of 120 W?
Worked example
A lamp flow shows 120 J electrical input and 40 J visible radiation, with the only other branch marked X. Find X.
Show the worked solution
- Treat the two outputs as an exhaustive account for the same interval.
- 120 = 40 + X.
- X = 80 J in the other transfer branch.
Answer 80 J in the other branch.
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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.
Find a lamp's other branch when 120 J enters and 40 J leaves as visible radiation in an exhaustive two-output account.
Give me a hint
Balance the two outputs with the supplied input.
Compare my reasoning
- Input = visible output + other output.
- 120 = 40 + X.
- X = 80 J.
80 J.
Look for these in your work
- Use the exhaustive-account condition.
- Keep energy units.
An exhaustive diagram has 200 J input and 150 J desired output. Find the remaining branch and explain whether 200 W follows.
Give me a hint
Separate the amount budget from the missing time information.
Compare my reasoning
- The remaining output is 200 − 150 = 50 J.
- The input amount is 200 J.
- No duration is supplied, so its power is not established.
50 J remains; input power is not known without time.
Look for these in your work
- Balance distinct outputs.
- Do not replace joules with watts.
A lamp and heater both transfer energy into surroundings' internal stores. Must that transfer be called wasted in both tasks? Explain.
Give me a hint
Define the intended useful outcome of each device.
Compare my reasoning
- A lamp's task is usually illumination, so unwanted warming can be a less useful output.
- A heater's intended task is to increase relevant internal energy.
- Usefulness depends on the task and receiver, not a universal ban on internal-energy gains.
No; heating can be a desired output for the heater, so usefulness depends on the intended task.
Look for these in your work
- Name each purpose.
- Retain conservation for both devices.
Common mistakes
- Read labels before inferring geometry.
- Joules are an amount, not a power without time.
- Useful depends on the intended task.
What can you explain now?
Does a narrow ordinary flow arrow prove a smaller energy amount if no scale is stated?
Compare with the explanation
No.
Connections may be schematic rather than drawn with quantitative widths.
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Make it stick
Read a simple two-branch energy diagram, complete the missing amount and explain why arrow geometry and joule labels do not supply power.
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.