Define potential difference and calculate transferred energy
Interpret potential difference as energy transferred per unit charge and apply V = W/Q.
Section 9 · Electricity
AS Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 9. Circuit calculations and component sketches are original study practice; full timed assessment, supervised practical competence and human subject-teacher review remain outstanding.
Before you begin
- Use charge, time, energy, power, area and consistent SI units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does 1 V mean?
What you will learn
- Define potential difference across a component
- Recall and use V = W/Q
- Relate energy transfer to charge and potential difference
Potential difference measures energy per charge
The potential difference across a component is the energy transferred from or to electrical stores per unit charge passing through it. One volt is one joule per coulomb.
Use V = W/Q, where W is transferred energy in joules and Q is charge in coulombs. A component with a 6 V potential difference transfers 6 J per coulomb under the stated direction and operating conditions.
Keep component energy transfer distinct from charge flow
A higher potential difference means more energy transferred per coulomb, not necessarily a greater current. Current also depends on circuit resistance and other components.
If energy is transferred to a component, it may become thermal, light, kinetic or another form. The equation gives the amount per charge; the component model tells you where that energy goes.
Pause and explain
A 12 V component has 0.50 C passing through it. How much energy is transferred?
Worked example
A component transfers 18 J of energy when 3.0 C passes. Find its potential difference.
Show the worked solution
- Use V = W/Q.
- V = 18/3.0.
- State joules per coulomb as volts.
Answer V = 6.0 V.
Try a topic test
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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.
A 9.0 V battery transfers energy to 2.0 C of charge. Find the energy transfer.
Give me a hint
Rearrange V = W/Q to W = VQ.
Compare my reasoning
- W = VQ.
- W = 9.0 × 2.0.
- Give the result in joules.
18 J.
Look for these in your work
- Equation rearranged
- Product calculated
- Energy unit stated
A motor transfers 240 J from electrical energy stores when 20 C passes. Find the potential difference across the motor.
Give me a hint
Potential difference is energy divided by charge.
Compare my reasoning
- V = W/Q.
- V = 240/20.
- Give the answer in volts.
12 V.
Look for these in your work
- Correct definition used
- Ratio calculated
- Volt unit stated
Two components each have 4.0 V across them. Component X transfers 8.0 J and component Y transfers 20 J. Compare the charge through each during those transfers.
Give me a hint
Use Q = W/V separately for each component.
Compare my reasoning
- For X: Q = 8.0/4.0 = 2.0 C.
- For Y: Q = 20/4.0 = 5.0 C.
- The same energy per coulomb does not mean the same total charge passed.
X has 2.0 C and Y has 5.0 C passing during the stated transfers.
Look for these in your work
- Equation rearranged
- Both charges calculated
- Potential difference interpreted
Common mistakes
- Calling potential difference the same thing as current
- Using charge in amperes rather than coulombs
- Assuming a voltage alone specifies current without resistance information
What can you explain now?
What does 1 V mean?
Compare with the explanation
1 J of energy transferred per coulomb
Volt is joule per coulomb.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Why does potential difference describe energy per charge rather than total energy?
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.