Unit 9 · Electric Fields and Capacitance
The Complete Cambridge International AS & A Level Physics (9702) Guide · Cambridge International AS & A Level Physics (9702)Unit 9 · Electric Fields and Capacitance
- The Complete Cambridge International AS & A Level Physics (9702) Guide
- Cambridge International AS & A Level Physics (9702)
- 12 sections
Cambridge International does not publish a per-unit weighting for this exam. Exam structure (official): AS Level: Multiple Choice (75 min, 40 questions, 31%) · AS Level Structured Questions (75 min, 46%) · Advanced Practical Skills (120 min, 2 questions, 23%); A Level: Multiple Choice (75 min, 40 questions, 15.5%) · AS Level Structured Questions (75 min, 23%) · Advanced Practical Skills (120 min, 2 questions, 11.5%) · A Level Structured Questions (120 min, 38.5%) · Planning, Analysis And Evaluation (75 min, 2 questions, 11.5%). Be ready to carry the same unit skills and representations into a complete answer on every paper. Converting 220 μF to 220 × 10⁻⁶ F and 4.70 kΩ to 4.70 × 10³ Ω makes RC equal 1.034 s. The opposite prefix powers cancel inside the time constant.
- How Cambridge International AS & A Level Physics (9702) assesses this Cambridge International publishes no per-unit percentage for Cambridge International A Level Physics; Paper 1 accounts for 31% of the AS Level total and 15.5% of the A Level total. · AS Level: Multiple Choice (75 min, 40 questions, 31%) · AS Level Structured Questions (75 min, 46%) · Advanced Practical Skills (120 min, 2 questions, 23%) | A Level: Multiple Choice (75 min, 40 questions, 15.5%) · AS Level Structured Questions (75 min, 23%) · Advanced Practical Skills (120 min, 2 questions, 11.5%) · A Level Structured Questions (120 min, 38.5%) · Planning, Analysis And Evaluation (75 min, 2 questions, 11.5%) · show the model with A graph of voltage (V) against time (s)., A formula card that explains the product rc fixes the timescale in the capacitor’s exponential discharge law., A table linking key concept, why it matters, visible method.
- Key skills Electric Field Lines and Uniform Electric Fields (A Level), Coulomb's Law and the Electric Field of a Point Charge (A Level), Electric Potential and Electric Potential Energy (A Level), Capacitors, Capacitance and Combinations of Capacitors (A Level), Energy Stored in a Capacitor and Capacitor Discharge (A Level)
- How to study for Unit 9 This page turns A graph of voltage (V) against time (s)., A formula card that explains the product rc fixes the timescale in the capacitor’s exponential discharge law., A table linking key concept, why it matters, visible method. into one route: Multiply to obtain τ = 1.034 s.
- The organizing decision Confirm that Ω·F reduces to seconds before using 1.034 s in the exponential denominator.
What Cambridge International AS & A Level Physics (9702) Unit 9 covers
Use this map to connect each assessed skill to the relationship or representation that makes it visible.
Electric Field Lines and Uniform Electric Fields (A Level)
electric field strength; electric potential; dielectric02Coulomb's Law and the Electric Field of a Point Charge (A Level)
electric field strength; electric potential; dielectric03Electric Potential and Electric Potential Energy (A Level)
electric field strength; electric potential; dielectric04Capacitors, Capacitance and Combinations of Capacitors (A Level)
electric field strength; electric potential; dielectric05Energy Stored in a Capacitor and Capacitor Discharge (A Level)
electric field strength; electric potential; dielectricCambridge International does not publish a per-unit weighting for this exam.
Official unit name and weighting: Cambridge International syllabus.
Electric Fields and Capacitance
Connect the published share to the unit model
Converting 220 μF to 220 × 10⁻⁶ F and 4.70 kΩ to 4.70 × 10³ Ω makes RC equal 1.034 s. The opposite prefix powers cancel inside the time constant.
After 1.50 s, the factor e^(−1.50/1.034) reduces 12.0 V to 2.81 V. The negative exponent describes discharge and the voltage approaches zero at long times.
Field is a directed quantity, potential is scalar, and capacitor voltage here is a scalar potential difference. Keeping those roles distinct prevents sign and direction errors in neighboring topics.
Distinguish electric field, a vector, from electric potential, a scalar, before adding contributions. For capacitor networks, reduce the topology with the appropriate series or parallel rule, then apply the energy or exponential relation. Test both the initial value and the long-time limit so that an incorrect sign in the exponent cannot survive.
The decision that organizes this unit
Define the system and choose the route before calculating
Confirm that Ω·F reduces to seconds before using 1.034 s in the exponential denominator.
Multiply to obtain τ = 1.034 s
Mechanism route and repair branches
Relationships to preserve
- Time constant: R is 4.70 × 10³ Ω and C is 220 × 10⁻⁶ F.
- Discharge: Elapsed time is 1.50 s.
- Long-time check: The exponent becomes more negative.
Representations to read
- A graph of voltage (V) against time (s).
- A formula card that explains the product rc fixes the timescale in the capacitor’s exponential discharge law.
- A table linking key concept, why it matters, visible method.
Branches to reject
- Using 4.70 and 220 without their prefixes
- Dropping the negative sign and predicting voltage growth
- Changing the time constant between coordinates
| Key concept | Why it matters | Visible method |
|---|---|---|
| Time constant | R is 4.70 × 10³ Ω and C is 220 × 10⁻⁶ F | Multiply to obtain τ = 1.034 s |
| Discharge | Elapsed time is 1.50 s | Use V = 12.0e^(−t/τ) |
| Long-time check | The exponent becomes more negative | Voltage must approach zero rather than grow |
| Field versus potential | Field has direction; potential is scalar. | Combine contributions with the appropriate vector or scalar rule. |
| Exponential limits | A discharge begins at its initial voltage and tends toward zero. | Test both limits after writing the exponent. |
How Cambridge International AS & A Level Physics (9702) assesses Electric Fields and Capacitance
What a complete response must make visible
Match the task to evidence that a reader can audit, then check the most likely reasoning failure before finalizing the response.
| Task | Evidence to show | Hurdle |
|---|---|---|
| Electric Field Lines and Uniform Electric Fields (A Level) | electric field strength; electric potential; dielectric | Using 4.70 and 220 without their prefixes |
| Coulomb's Law and the Electric Field of a Point Charge (A Level) | electric field strength; electric potential; dielectric | Dropping the negative sign and predicting voltage growth |
| Electric Potential and Electric Potential Energy (A Level) | electric field strength; electric potential; dielectric | Changing the time constant between coordinates |
| Capacitors, Capacitance and Combinations of Capacitors (A Level) | electric field strength; electric potential; dielectric | Using 4.70 and 220 without their prefixes |
| Energy Stored in a Capacitor and Capacitor Discharge (A Level) | electric field strength; electric potential; dielectric | Dropping the negative sign and predicting voltage growth |
Worked example: field, potential, capacitance and exponential change
Read each question as it appears on the exam paper, choose your answer, then tap it to see the feedback.
Practice 1: Capacitor discharge
A charged capacitor discharges through a resistor.
A 220 μF capacitor is charged to 12.0 V and then connected across a 4.70 kΩ resistor. Calculate (a) the time constant and (b) the voltage after 1.50 s.
[4]Show all necessary working.
A. τ = 1.034 s and V = 2.81 V.
✅ Correct! 😊 Correct. Converting μF and kΩ gives RC = 1.034 s, which belongs in the denominator of −t/τ.
B. τ = 1.034 s and V = 50.7 V.
❌ Not quite 😭 This uses a growing exponential. During discharge the exponent is negative, so voltage must fall below 12.0 V.
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C. τ = 0.001034 s and V is effectively zero.
❌ Not quite 😭 This mishandles the opposing kilo and micro prefixes. Their powers combine to make RC about one second, not one millisecond.
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D. τ = 1034 s and V = 12.0 V to three significant figures.
❌ Not quite 😭 This prefix conversion makes the time constant one thousand times too large. It therefore predicts almost no discharge.
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Full explanation
- Step 1Convert C = 220 × 10⁻⁶ F and R = 4.70 × 10³ Ω.
- Step 2Calculate τ = RC = 1.034 s.
- Step 3Use V = V₀e^(−t/τ) with t = 1.50 s.
- Step 4Evaluate V = 12.0e^(−1.50/1.034) = 2.81 V.
Practice 2: Energy stored by a capacitor
A capacitor is charged at constant potential difference.
A 150 μF capacitor is charged to 20.0 V. Calculate the energy stored.
[3]Show all necessary working.
A. E = CV² = 0.060 J.
❌ Not quite 😭 This omits the factor of one half. It gives twice the energy represented by the area under a charge–voltage graph.
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B. E = ½CV² = 0.030 J.
✅ Correct! 😊 Correct. Converting 150 μF to 150 × 10⁻⁶ F gives 0.030 J.
C. E = ½C²V = 2.25 × 10⁻⁷ J.
❌ Not quite 😭 This squares capacitance instead of voltage. The expression does not have the dimensions of energy.
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D. E = ½CV = 1.50 × 10⁻³ J.
❌ Not quite 😭 The voltage must be squared in the energy relation. This expression instead has the dimensions of charge times a number.
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Full explanation
- Step 1Convert the capacitance: C = 150 × 10⁻⁶ F.
- Step 2Use the capacitor-energy relation E = ½CV².
- Step 3Substitute E = ½(150 × 10⁻⁶)(20.0)².
- Step 4Calculate E = 0.0300 J, reported as 0.030 J.
Key terms for Unit 9: Electric Fields and Capacitance
Models, uses, and boundaries
- Electric Fields and Capacitance
- The product RC fixes the timescale in the capacitor’s exponential discharge law. Use this relationship when the product RC fixes the timescale in the capacitor’s exponential discharge law..
- Worked example: field, potential, capacitance and exponential change
- Prefix conversion gives τ = 1.034 s before 1.50 s is placed in the exponent. Use this relationship when prefix conversion gives τ = 1.034 s before 1.50 s is placed in the exponent..
- Electric Fields and Capacitance: change one condition
- Voltages at 2.0 s and 3.0 s compare later points on the same decay curve. Use this relationship when voltages at 2.0 s and 3.0 s compare later points on the same decay curve..
- Electric Fields and Capacitance: test proposed results
- A valid option must start at V₀ and decrease toward zero for positive time. Use this relationship when a valid option must start at V₀ and decrease toward zero for positive time..
Cambridge International AS & A Level Physics (9702) Unit 9 FAQ
How much of Cambridge International AS & A Level Physics (9702) does Unit 9 carry?
Cambridge International does not publish a per-unit weighting for this exam.
What is the first move on a Electric Fields and Capacitance problem?
Multiply to obtain τ = 1.034 s
Which relationships should I preserve?
Time constant: R is 4.70 × 10³ Ω and C is 220 × 10⁻⁶ F. Discharge: Elapsed time is 1.50 s. Long-time check: The exponent becomes more negative.
Which representations should I practice?
Practice moving among A graph of voltage (V) against time (s)., A formula card that explains the product rc fixes the timescale in the capacitor’s exponential discharge law., A table linking key concept, why it matters, visible method..
What error should I check before submitting an answer?
Check for Using 4.70 and 220 without their prefixes; Dropping the negative sign and predicting voltage growth; Changing the time constant between coordinates.
Evidence workshop
Continue from the free model into complete practice
The full unit guide continues with the chapter’s worked examples, figures, scoring tables, and answer checks.
- Worked example: field, potential, capacitance and exponential change
- Electric Fields and Capacitance: change one condition
- Electric Fields and Capacitance: test proposed results
- Check the worked-example evidence for Electric Fields and Capacitance
Full unit practice. Open the complete guide for the full evidence workshop and synthesis.
Related Cambridge International AS & A Level Physics (9702) unit guides
Cambridge International AS & A Level Physics (9702) Exam Guide & Review
The whole exam and its official unit sequence.01Physical Quantities and Units
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Cambridge International does not publish a per-unit weighting for this exam.How to study Cambridge International AS & A Level Physics (9702) Unit 9
Start with the organizing decision
Before solving, restate the decision in operational terms: Confirm that Ω·F reduces to seconds before using 1.034 s in the exponential denominator.. Your first written move should be to Multiply to obtain τ = 1.034 s.
Practice the same idea in several representations
Rotate through A graph of voltage (V) against time (s)., A formula card that explains the product rc fixes the timescale in the capacitor’s exponential discharge law., A table linking key concept, why it matters, visible method.. Use each representation to practice Electric Field Lines and Uniform Electric Fields (A Level), Coulomb's Law and the Electric Field of a Point Charge (A Level), Electric Potential and Electric Potential Energy (A Level), Capacitors, Capacitance and Combinations of Capacitors (A Level), Energy Stored in a Capacitor and Capacitor Discharge (A Level), and explain what stays invariant when the surface form changes.
Turn each error into a repair check
After every attempt, audit the response for Using 4.70 and 220 without their prefixes; Dropping the negative sign and predicting voltage growth; Changing the time constant between coordinates. Then redo only the first step that made the reasoning diverge, keeping units, direction, and model conditions visible.
Confirm current course details in the official Cambridge International syllabus for the 2025-2027 administration.
Where to go next
Return to the Cambridge International AS & A Level Physics (9702) to see every unit and plan your revision.