Distinguish voltage from charge current and total energy
Use volts for p.d. and separate energy per charge, charge amount, charge-flow rate and transferred energy.
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
- Define component p.d.
- Recognise joules, coulombs and amperes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which is a charge-flow rate unit?
What you will learn
- State volts as the p.d. unit.
- Interpret a p.d. label without assigning unprovided current or total energy.
Keep four quantities distinct
Potential difference between two points is measured in volts, V. A 4 V p.d. means 4 J of work per 1 C passing through the stated component. The p.d. unit is neither an ampere nor a coulomb. The energy transferred for each charge and the amount of charge that passes are separate pieces of information.
Charge is an amount measured in C; current is a flow rate measured in A; work or energy is measured in J; p.d. is energy per charge measured in V. A component can have the same p.d. in two trials but different total transferred energy if different amounts of charge pass.
Read voltage units and assumptions
Voltage prefixes use the normal powers of ten: 250 mV = 0.250 V and 2 kV = 2000 V. A prefix conversion changes the numeric scale, not voltage into a current or energy quantity. The later numerical work–charge lesson uses the same volt meaning explicitly in an equation.
A stated component p.d. does not alone determine its current without an operating relationship or measurement. Nor does it establish a fixed amount of work without passed-charge or compatible power-time data. Name the given quantity and stop at the conclusion supported by the information.
Sources for this lesson
Pause and explain
Which interpretation of a 4 V p.d. is supported?
Worked example
A component has p.d. 4 V. Interpret the work done for each 1 C and identify what is not supplied by this value alone.
Show the worked solution
- 4 V means 4 J per C across the component.
- The total passed charge is not stated.
- The current and total transferred energy are not determined.
Answer 4 J per 1 C; current and total energy are not determined by p.d. alone.
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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.
State the p.d. unit and interpret 4 V for each 1 C through the component.
Give me a hint
Use the volt's energy-per-charge meaning.
Compare my reasoning
- The unit is volt, V.
- One volt means one joule per coulomb.
- 4 V means 4 J per 1 C.
Volts; 4 J of component work per 1 C.
Look for these in your work
- State the unit.
- Keep the component account per charge.
Convert 250 mV and 2 kV into volts.
Give me a hint
Use milli = 10⁻³ and kilo = 10³ for voltage.
Compare my reasoning
- 250 mV = 250 × 10⁻³ V.
- 2 kV = 2 × 10³ V.
- The values are 0.250 V and 2000 V.
0.250 V; 2000 V.
Look for these in your work
- Use both prefixes correctly.
- Retain volt units.
A learner concludes a component at 6 V always transfers exactly 6 J and carries 6 A. Assess both conclusions.
Give me a hint
Identify the missing charge amount and current relationship.
Compare my reasoning
- 6 V means 6 J per C, not a fixed energy total.
- The amount of charge passing is not given.
- No current relationship or measurement is supplied.
Neither follows; 6 V supplies energy per charge, not total energy or current.
Look for these in your work
- Correct both quantity confusions.
- State what further data would be needed.
Common mistakes
- Volts are not amperes or coulombs.
- P.d. is not a fixed total energy.
- A voltage prefix conversion does not change the quantity.
What can you explain now?
What unit measures p.d. between two points?
Compare with the explanation
Volt (V).
Potential difference measures electrical work per unit charge.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Use volt units and separate p.d., current, charge and total energy in a component account.
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.