Predict warming and cooling from unequal energy rates
Use signed net power and elapsed time before making a temperature claim.
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
- Balance complete energy-transfer rates.
- Distinguish fixed-state warming from phase change.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Input is 20 W and total outgoing is 35 W. What is the signed net rate?
What you will learn
- Predict warming or cooling when received and outgoing rates differ in a fixed-state account.
- Calculate net energy change while retaining all transfer routes and assumptions.
Find the signed net rate into the object
For a fixed-state positive-heat-capacity object with no other changing stores, receiving more energy per second than it transfers away raises its internal energy and temperature. Receiving less gives a loss and cooling. Define net rate into the object as total input minus total output, so a negative sign describes a loss from that boundary.
An input of 50 W and outputs totalling 35 W give +15 W net. If rates remain constant for 120 s, the gain is 1800 J. An input of 20 W with outputs 35 W gives −15 W and a −1800 J change over the same duration. These are supplied energy accounts; a temperature rise in degrees needs the object's thermal properties as well.
Do not omit a mechanism or predict forever
The outgoing radiation rate often changes as surface temperature changes, and other thermal routes may also vary. A constant-rate calculation is for the supplied interval, not a prediction of unlimited warming at the same rate. A later steady temperature can result when the complete rates become equal.
During a state change, a net energy gain can change phase without the same temperature rise. Other stores or internal sources also require the stated boundary to be expanded. Use the fixed-state assumption explicitly; an infrared-emission reading alone does not establish net warming or cooling when input and other outputs are unknown.
Pause and explain
A fixed-state object gains 1800 J, but mass and c are unknown. What can be stated?
Worked example
A fixed-state object receives 50 W, loses 35 W in total and keeps these rates for 120 s. Find its net energy change and qualitative response.
Show the worked solution
- Net rate is 50 − 35 = +15 W.
- ΔE = 15 × 120 = +1800 J.
- With the stated unchanged state and no other stores, its temperature rises; an exact rise is not determined here.
Answer +1800 J; warming under the stated fixed-state account.
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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 net energy change for 50 W input, 35 W total output and 120 s at constant rates.
Give me a hint
Use a signed net rate into the object.
Compare my reasoning
- Net rate is +15 W.
- Multiply by 120 s.
- The gain is +1800 J.
+1800 J.
Look for these in your work
- Use the full output.
- Retain the stated interval.
Find the signed change for 20 W input, 35 W output and 120 s, then describe a fixed-state response.
Give me a hint
Subtract output from input before multiplying.
Compare my reasoning
- Net rate is −15 W.
- ΔE = −15 × 120 = −1800 J.
- The fixed-state object loses internal energy and cools under the stated assumptions.
−1800 J; cooling in the supplied fixed-state account.
Look for these in your work
- Preserve the loss sign.
- State the qualitative assumption.
A surface emits 25 W by radiation. Can you determine whether it warms from that reading alone?
Give me a hint
Received energy and other outgoing routes are missing.
Compare my reasoning
- 25 W is only one outgoing rate.
- Input and other outputs are unknown.
- The signed complete net rate, and therefore the temperature trend, is not determined.
No; one emission rate alone does not determine net warming or cooling.
Look for these in your work
- Identify the missing rates.
- Avoid using emission as a net-change measurement.
Common mistakes
- Use input minus every output.
- A supplied constant rate is not an unlimited-time prediction.
- An exact temperature change needs thermal properties and state conditions.
What can you explain now?
What does a negative net rate into the object mean?
Compare with the explanation
It loses energy overall.
Outputs exceed inputs for that specified boundary and interval.
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Make it stick
Find signed net rates and explain why temperature trends and numerical rises need a complete fixed-state account.
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