Explain steady temperature through balanced energy rates
Balance all input and output rates for a fixed-state object without claiming every transfer has stopped.
Exam scope and assessment notes
Extended only (Supplement) · Physics 0972, 2026–2028, section 2.3.3. These lessons cover steady and unequal rates, Earth radiation balance, emitter/absorber experiments and temperature/area effects. Written calculations and described methods are self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Distinguish energy amounts from transfer rates.
- Identify the object boundary and all relevant transfers.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Input is 40 W and outputs are 25 W and 15 W. What is the net rate?
What you will learn
- Explain steady temperature using equal received and outgoing energy rates.
- Distinguish thermal steady state from zero individual transfers or a phase-change plateau.
Equal totals give zero net accumulation
For a fixed-mass object in an unchanged state with no other changing energy stores, a constant temperature requires its total received energy rate to equal its total rate of transfer away. Individual transfers can continue. The balance is between sums: radiation, convection, conduction and other specified inputs or outputs must be assigned to the same object boundary.
In a supplied account, an object receives 40 W and transfers away 25 W by radiation plus 15 W through other thermal routes. Total outgoing is 40 W, so the net rate is zero. Over 60 s, 2400 J enters and 2400 J leaves. Equal totals do not require each outgoing mechanism individually to equal the full input.
State the fixed-state assumption
A constant-temperature phase change is different: energy can change internal energy through melting or boiling while the temperature remains constant. Therefore a thermometer plateau alone is not proof of equal rates. This steady-object rule uses an unchanged state and no additional energy-store changes; retain those conditions rather than contradict the preceding phase-change lessons.
The diagram's arrows show named supplied rates, not measured universal values or proportional arrow widths. A complete account can establish steady state under its assumptions, while an incomplete account cannot prove that missing transfers vanish. The surface can still emit and absorb radiation at steady temperature.
Pause and explain
Does constant temperature during melting prove balanced input and output?
Worked example
A fixed-state object receives 40 W and loses 25 W by radiation plus 15 W otherwise. Describe its temperature and the transfers over 60 s.
Show the worked solution
- Total outgoing rate is 25 + 15 = 40 W.
- Net rate is 40 − 40 = 0 W.
- Over 60 s, 2400 J enters and 2400 J leaves; steady temperature is supported under the stated fixed-state account.
Answer Steady temperature with continuing balanced transfers, not zero individual transfers.
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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.
Balance a fixed-state object with input 40 W and outputs 25 W radiation and 15 W otherwise.
Give me a hint
Add all outputs before finding the net rate.
Compare my reasoning
- Outgoing total is 40 W.
- Input and outgoing rates are equal.
- The stated object can stay at steady temperature while each transfer continues.
Balanced rates support steady temperature under the supplied unchanged-state account.
Look for these in your work
- Include every output.
- Keep individual transfers continuing.
For the same account over 60 s, find received and outgoing energy amounts.
Give me a hint
Multiply each complete rate by duration.
Compare my reasoning
- Received energy is 40 × 60 = 2400 J.
- Outgoing total is (25 + 15) × 60 = 2400 J.
- Net change is zero under the stated complete boundary.
2400 J received and 2400 J transferred away.
Look for these in your work
- Distinguish joules from watts.
- Balance the same duration and boundary.
Ice melts at a constant reading while energy enters. Explain why this does not contradict the fixed-state balance rule.
Give me a hint
The phase and internal-energy conditions have changed.
Compare my reasoning
- The material changes state.
- Energy can increase internal energy without raising temperature during that change.
- The unchanged-state rule does not apply to the plateau as though melting were absent.
A phase change can gain energy at constant temperature; it is not the fixed-state case.
Look for these in your work
- Name the excluded condition.
- Preserve conservation.
Common mistakes
- Balance total rates rather than one route alone.
- Steady temperature can coexist with continuing transfers.
- A phase-change plateau is outside the unchanged-state inference.
What can you explain now?
Does a steady object have to stop emitting infrared?
Compare with the explanation
No.
It can continue emitting while received and total outgoing rates balance.
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Make it stick
Balance all rates for an unchanged-state object and distinguish this from a phase-change plateau.
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