Compare source and component work accounts for the same charge
Check a supplied electrical work budget with consistent charge amounts and distinguish ideal-source assumptions from omitted energy shares.
Section 4.2.3 · Electromotive force and potential difference
Exam scope and assessment notes
Extended only (Supplement) · Physics 0972, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is 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
- Use source e.m.f. = W/Q and component V = W/Q.
- Add work amounts for a stated same-charge account.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What work corresponds to constant 3 V across a component for 2 C?
What you will learn
- Compare source e.m.f. and component work-per-charge values.
- Check energy budgets without mixing different charge amounts or incomplete boundaries.
Match charge before comparing work
Suppose the same 2 C passes in a supplied ideal-source account and through two sequential component accounts. A 9 V source supplies 18 J. Component p.d.s of 3 V and 6 V correspond to 6 J and 12 J respectively. The component work totals 18 J, matching the supplied source work under the stated ideal conditions.
The sum is valid because the question supplies the same charge amount and complete sequential accounts. Do not add work-per-charge values from unrelated trials with different boundaries and call the result a new source e.m.f. General series and parallel circuit laws are taught in the next circuit section.
Account for missing shares
If a source supplies 20 J for 2 C while the supplied external component accounts total 18 J, 2 J is not yet assigned. The source e.m.f. is 10 V, whereas the named external work per charge is 9 J/C. Internal source transfer is one possible stated explanation, but the arithmetic alone does not identify a unique hidden mechanism.
Keep source, component and useful-output accounts separate. A useful light value is not necessarily the entire component electrical work. A budget mismatch prompts checking the boundaries, shared charge and omitted transfers rather than claiming energy conservation fails. These exercises are supplied accounts, not evidence of a completed circuit experiment.
Pause and explain
A source gives 20 J for 2 C and named external accounts total 18 J. What is proved?
Worked example
The same 2 C receives work from an ideal 9 V source and passes through supplied 3 V and 6 V component accounts. Check the work budget.
Show the worked solution
- Source work = 9 × 2 = 18 J.
- Component works = 3 × 2 = 6 J and 6 × 2 = 12 J.
- 6 + 12 = 18 J matches the source account.
Answer Source 18 J; components 6 J and 12 J; the supplied budget balances.
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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.
Check work for a 9 V ideal source and supplied 3 V and 6 V sequential accounts, all for the same 2 C.
Give me a hint
Multiply each per-charge value by the common charge before comparing totals.
Compare my reasoning
- Source work = 18 J.
- Components transfer 6 J and 12 J.
- Their total equals 18 J.
18 J supplied; 6 J + 12 J transferred; balanced.
Look for these in your work
- Use the same 2 C.
- State the ideal complete-account assumptions.
A source does 20 J for 2 C, while supplied external component work totals 18 J. Find e.m.f., external work per charge and the unassigned work.
Give me a hint
Separate the source ratio, external ratio and missing work difference.
Compare my reasoning
- E.m.f. = 20/2 = 10 V.
- External work per charge = 18/2 = 9 J/C.
- Unassigned share = 20 − 18 = 2 J.
E.m.f. 10 V; external work per charge 9 J/C; unassigned 2 J.
Look for these in your work
- Keep both boundaries.
- Do not diagnose a unique hidden transfer.
One 3 V component trial uses 2 C and a separate 6 V trial uses 5 C. Can their work values simply be called the budget of a 9 V source moving 2 C?
Give me a hint
Check the incompatible charge amounts and separate trial boundaries.
Compare my reasoning
- The work values are 6 J and 30 J.
- They arise from different supplied charge amounts.
- They do not establish the claimed same-2-C complete source budget.
No; the trials use different charges and do not establish that common source account.
Look for these in your work
- Identify the charge mismatch.
- Require the proposed complete-circuit conditions.
Common mistakes
- Use the same charge and explicit boundaries.
- A missing share is not a proven unique mechanism.
- Do not substitute useful output for complete electrical work.
What can you explain now?
What must match when comparing sequential work-per-charge accounts using a common charge?
Compare with the explanation
The passed-charge amount and account boundaries.
A valid budget depends on the supplied same-charge conditions rather than unrelated trial labels.
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Make it stick
Compare source and component work with matching charge amounts and explicit complete energy boundaries.
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.