Calculate source work per charge as electromotive force
Use the e.m.f. equation with a named source work account and distinguish it from a component-only work value.
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
- Define source e.m.f.
- Convert joules and coulombs and divide quantities.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which unit is equivalent to J/C?
What you will learn
- Use e.m.f. = W/Q for source electrical work.
- Match consistent energy and charge units to the source boundary.
Apply the source equation
For the stated source account, e.m.f. = W/Q, where W is electrical work done by the source and Q the charge moved around the complete circuit. In J and C, the result is V. If 18 J of source work accompanies 3 C, the e.m.f. is 6 V. The historical symbol E for e.m.f. is distinguished here from an electrical-energy symbol by naming the source quantity.
Use a non-zero charge amount for the ratio. A zero-work, zero-charge interval gives 0/0 and does not determine a unique e.m.f. A source can retain non-zero e.m.f. even though no charge passed during a selected open-circuit interval.
Keep the work boundary consistent
A lamp-only work value is not automatically the total source work. The source may supply energy to other components or transfer some internally. Identify which energy account is actually given before dividing by charge and naming the result e.m.f.
Convert prefixed units before the calculation. 24 mJ of source work for 4 mC gives (24 × 10⁻³)/(4 × 10⁻³) = 6 V. Matching prefixes cancel here, but unlike prefixes need their separate conversion factors. The result is a work-per-charge value, not total work or current.
Pause and explain
24 mJ of source work accompanies 4 mC. What e.m.f. follows?
Worked example
A source does 18 J of electrical work while 3 C passes around the complete circuit. Calculate its e.m.f.
Show the worked solution
- Use e.m.f. = W/Q for the source account.
- W = 18 J and Q = 3 C.
- 18/3 = 6 V.
Answer 6 V.
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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.
Find source e.m.f. for 18 J of work and 3 C passed charge.
Give me a hint
Use the source's work and the same charge account.
Compare my reasoning
- E.m.f. = W/Q.
- 18/3 = 6.
- The unit J/C is V.
6 V.
Look for these in your work
- Name the source boundary.
- State volts.
Calculate e.m.f. for 24 mJ and 4 mC, and for 12 J and 2 mC in separate source accounts.
Give me a hint
Convert both quantities in each trial before dividing.
Compare my reasoning
- 0.024/0.004 = 6 V.
- 12/0.002 = 6000 V.
- Only the first trial has matching prefixes to cancel.
6 V; 6000 V.
Look for these in your work
- Treat both accounts separately.
- Do not cancel unlike scales.
An open circuit has no passed charge or source work during a chosen interval. Does W/Q establish zero e.m.f.?
Give me a hint
Check whether the ratio is defined for Q = 0.
Compare my reasoning
- The supplied interval gives W = 0 and Q = 0.
- The ratio 0/0 is indeterminate.
- The source e.m.f. requires other suitable information.
No; 0/0 does not determine e.m.f., which can remain non-zero.
Look for these in your work
- Identify the indeterminate ratio.
- Keep source ability separate from interval transfer.
Common mistakes
- Use source work, not an isolated component's work.
- Convert energy and charge prefixes separately.
- An open interval's 0/0 does not determine e.m.f.
What can you explain now?
What quantity belongs in W when finding source e.m.f.?
Compare with the explanation
Electrical work done by the source.
The formula requires the source account rather than an arbitrary component-only energy.
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Make it stick
Calculate source e.m.f. from a matching non-zero work–charge account with consistent units.
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