Define electromotive force as source work per unit charge
Explain e.m.f. as the source's electrical work per unit charge around a complete circuit, rather than a mechanical force.
Section 4.2.3 · Electromotive force and potential difference
Core and Extended; Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Distinguish electric charge and energy.
- Recognise a complete circuit path.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which account concerns energy supplied by a source?
What you will learn
- Define electromotive force through source work per unit charge.
- Distinguish the source's energy-supplying role from mechanical force.
Name the source account
Electromotive force, e.m.f., is the electrical work done by a source in moving a unit charge around a complete circuit. It describes energy supplied per unit charge, not how much charge is present or how quickly charge passes. A cell can transfer energy from a chemical store into the circuit; a generator uses a different source pathway.
Despite the word 'force' in the historical name, e.m.f. is not a mechanical force in newtons. Identify the source and its complete-circuit energy-supplying account. The same source can supply different total amounts of energy when different amounts of charge pass, even if its e.m.f. is unchanged.
Separate a source property from an operating outcome
A source may have e.m.f. while an external switch is open and no sustained current flows. The open path does not erase the source's ability to supply energy per charge when connected appropriately. It also does not establish that any fixed total amount of work has already been done without passed-charge information.
For an ideal source, e.m.f. equals its terminal p.d. In a real source under load, internal energy transfers can make the terminal p.d. smaller. An open-circuit voltage measurement can approximate e.m.f. when the meter draws negligible current. Do not assume every loaded terminal reading is automatically the complete source e.m.f.
Pause and explain
What does e.m.f. describe?
Worked example
A source can supply 6 J of electrical work for each 1 C circulated. What does this describe, and is it a 6 N mechanical force?
Show the worked solution
- The account is source work per unit charge around the circuit.
- That quantity is e.m.f.
- Its unit is energy per charge, not the newton.
Answer It describes the source e.m.f., not a 6 N mechanical force.
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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.
State the definition of electromotive force.
Give me a hint
Include the source, electrical work and unit charge around the complete circuit.
Compare my reasoning
- Name electrical work done by the source.
- Express it per unit charge.
- Refer to moving that charge around a complete circuit.
Electrical work done by a source in moving a unit charge around a complete circuit.
Look for these in your work
- Include source work.
- Keep the per-unit-charge meaning.
Two sources supply different work for each unit charge. Which named quantity distinguishes that energy-supplying property?
Give me a hint
Compare work per unit charge rather than current or mechanical force.
Compare my reasoning
- The comparison concerns the source account.
- It is expressed per unit charge.
- The named quantity is e.m.f.
Their e.m.f. differs.
Look for these in your work
- Identify the quantity.
- Do not substitute current or newtons.
A loaded real cell has terminal p.d. below its e.m.f. Does that contradict the definition?
Give me a hint
Allow source-internal energy transfers rather than assuming an ideal source.
Compare my reasoning
- E.m.f. describes total source work per charge.
- Terminal p.d. concerns the available terminal account.
- Internal energy transfers can make the loaded terminal p.d. smaller.
No; real source-internal transfers can reduce the loaded terminal p.d.
Look for these in your work
- State the real-source condition.
- Separate source and terminal accounts.
Common mistakes
- E.m.f. is not a force in newtons.
- Identify source work per charge.
- Loaded terminal p.d. need not equal real source e.m.f.
What can you explain now?
Does an open external switch necessarily make a cell's e.m.f. zero?
Compare with the explanation
No.
The source can retain its energy-per-charge ability although sustained current through the open external path stops.
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Make it stick
Define source e.m.f. and distinguish it from mechanical force, current and loaded terminal p.d.
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.