Define potential difference across a component
Describe p.d. as work done per unit charge passing through a component and identify the two-point boundary.
Section P4.2.3 · Electromotive force and potential difference
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Distinguish charge and energy.
- Define source e.m.f.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which is the source's complete-circuit work-per-charge quantity?
What you will learn
- Define potential difference by component work per unit charge.
- Distinguish a component account from the source's complete-circuit account.
Name the component and its terminals
Potential difference, p.d., across a component is the work done by a unit charge passing through that component. It describes energy transferred per charge between its two terminals, rather than the amount of charge used up. Identify both points so the account has a clear boundary.
For a lamp, the electrical energy transferred can go into light and internal energy of the lamp and surroundings. The charge is not consumed by producing those outcomes. A voltage account says energy per charge; it does not say how much charge passes each second.
Compare source and component accounts
E.m.f. names electrical work supplied by a source per unit charge around a complete circuit. Component p.d. names work per unit charge across a specified part of that circuit. These definitions can share the volt unit while referring to different energy-account boundaries.
A p.d. can be specified across two points even when no sustained current flows through a disconnected or open path. For example, an open source's terminals can have p.d. A voltage alone therefore does not guarantee that a lamp is taking energy at a particular rate; the connected path and current also matter.
Sources for this lesson
Pause and explain
A lamp transfers energy while charge passes through it. What does its p.d. express?
Worked example
A lamp has a stated p.d. meaning 3 J is transferred for each 1 C passing through it. What quantity is described?
Show the worked solution
- The account concerns a named component.
- It expresses work done per unit charge through the lamp.
- It is the potential difference across the lamp.
Answer The lamp's p.d., an energy-transfer-per-charge quantity.
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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 potential difference across a component.
Give me a hint
Include work per unit charge and the chosen component boundary.
Compare my reasoning
- Identify the component's two terminals.
- Consider work done as charge passes through it.
- Express that work per unit charge.
Work done by a unit charge passing through the component.
Look for these in your work
- Include the per-charge meaning.
- Keep the component boundary explicit.
Explain how source e.m.f. and lamp p.d. have related units but different boundaries.
Give me a hint
Compare source-supplied work with work transferred through a named component.
Compare my reasoning
- Both describe electrical work per charge.
- E.m.f. is the complete-circuit source account.
- Lamp p.d. is the component's terminal-to-terminal account.
Both are work per charge; e.m.f. is the source account and lamp p.d. the component account.
Look for these in your work
- Describe both quantities.
- Do not confuse their boundaries.
An open path has a stated p.d. across its gap. Does this alone prove sustained current or a lamp's operating power?
Give me a hint
Check whether charge flow and a complete load path are given.
Compare my reasoning
- A p.d. can exist across an open gap.
- The simple open path has no sustained conducting flow.
- A voltage alone does not establish operating power.
No; p.d. alone does not establish sustained current or operating power.
Look for these in your work
- Allow voltage without sustained flow.
- Identify the missing operating information.
Common mistakes
- Identify two points across the component.
- Charge is not used up by the component.
- Voltage alone is not a current or power value.
What can you explain now?
What does component p.d. describe?
Compare with the explanation
Work done per unit charge passing through the component.
The definition concerns the component's energy-transfer account between its terminals.
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Make it stick
Define p.d. using the component work-per-charge boundary and distinguish it from e.m.f. and current.
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