Unit 10 · Magnetic Fields and Alternating Currents
The Complete Cambridge International AS & A Level Physics (9702) Guide · Cambridge International AS & A Level Physics (9702)Unit 10 · Magnetic Fields and Alternating Currents
- 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. At 90°, sin θ equals one, so 0.450 T × 3.20 A × 0.180 m gives 0.2592 N. The three factors describe field strength, current and active wire length.
- 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 force (N) against angle (degrees)., A formula card that explains the sine factor accounts for the angle between current and magnetic field., A table linking key concept, why it matters, visible method.
- Key skills Magnetic Fields and the Force on a Current-Carrying Conductor (A Level), Force on a Moving Charge, the Hall Effect and Velocity Selection (A Level), Magnetic Fields due to Currents (A Level), Electromagnetic Induction: Faraday's and Lenz's Laws (A Level), Characteristics of Alternating Currents: Peak and r.m.s. Values (A Level), Rectification and Smoothing (A Level)
- How to study for Unit 10 This page turns A graph of force (N) against angle (degrees)., A formula card that explains the sine factor accounts for the angle between current and magnetic field., A table linking key concept, why it matters, visible method. into one route: Use F = BIL sin θ.
- The organizing decision Use the stated conventional-current direction, not electron motion, when applying the cross-product direction rule.
What Cambridge International AS & A Level Physics (9702) Unit 10 covers
Use this map to connect each assessed skill to the relationship or representation that makes it visible.
Magnetic Fields and the Force on a Current-Carrying Conductor (A Level)
magnetic flux density; electromagnetic induction; root-mean-square value02Force on a Moving Charge, the Hall Effect and Velocity Selection (A Level)
magnetic flux density; electromagnetic induction; root-mean-square value03Magnetic Fields due to Currents (A Level)
magnetic flux density; electromagnetic induction; root-mean-square value04Electromagnetic Induction: Faraday's and Lenz's Laws (A Level)
magnetic flux density; electromagnetic induction; root-mean-square value05Characteristics of Alternating Currents: Peak and r.m.s. Values (A Level)
magnetic flux density; electromagnetic induction; root-mean-square value06Rectification and Smoothing (A Level)
magnetic flux density; electromagnetic induction; root-mean-square valueCambridge International does not publish a per-unit weighting for this exam.
Official unit name and weighting: Cambridge International syllabus.
Magnetic Fields and Alternating Currents
Connect the published share to the unit model
At 90°, sin θ equals one, so 0.450 T × 3.20 A × 0.180 m gives 0.2592 N. The three factors describe field strength, current and active wire length.
The cross-product rule fixes a force perpendicular to both conventional current and magnetic field. A correct numerical magnitude without that direction leaves the vector result incomplete.
Changing the angle produces a sine curve: zero force for parallel alignment and the 0.2592 N maximum when the wire is perpendicular to the field.
Establish the directions of magnetic field, conventional current or velocity, and the surface normal before selecting a force, flux or induction relation. State the direction rule explicitly, calculate in SI units, and connect the result to induced opposition, phase or energy transfer where the question requires it.
The decision that organizes this unit
Define the system and choose the route before calculating
Use the stated conventional-current direction, not electron motion, when applying the cross-product direction rule.
Use F = BIL sin θ
Mechanism route and repair branches
Relationships to preserve
- Magnitude: The active wire length is 0.180 m.
- Maximum: Current is perpendicular to the 0.450 T field.
- Direction: Force follows the current–field cross product.
Representations to read
- A graph of force (N) against angle (degrees).
- A formula card that explains the sine factor accounts for the angle between current and magnetic field.
- A table linking key concept, why it matters, visible method.
Branches to reject
- Omitting the length or using degrees as a multiplier
- Giving only the force magnitude
- Using the angle itself as a multiplier
| Key concept | Why it matters | Visible method |
|---|---|---|
| Magnitude | The active wire length is 0.180 m | Use F = BIL sin θ |
| Maximum | Current is perpendicular to the 0.450 T field | Set sin 90° = 1 |
| Direction | Force follows the current–field cross product | State the perpendicular orientation |
| Direction construction | Field, conventional current or velocity, and force form a directional relationship. | Establish all directions before calculating. |
| Angle limit | Parallel alignment gives zero force and perpendicular alignment gives the maximum. | Use these limits to check the sine factor. |
How Cambridge International AS & A Level Physics (9702) assesses Magnetic Fields and Alternating Currents
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 |
|---|---|---|
| Magnetic Fields and the Force on a Current-Carrying Conductor (A Level) | magnetic flux density; electromagnetic induction; root-mean-square value | Omitting the length or using degrees as a multiplier |
| Force on a Moving Charge, the Hall Effect and Velocity Selection (A Level) | magnetic flux density; electromagnetic induction; root-mean-square value | Giving only the force magnitude |
| Magnetic Fields due to Currents (A Level) | magnetic flux density; electromagnetic induction; root-mean-square value | Using the angle itself as a multiplier |
| Electromagnetic Induction: Faraday's and Lenz's Laws (A Level) | magnetic flux density; electromagnetic induction; root-mean-square value | Omitting the length or using degrees as a multiplier |
| Characteristics of Alternating Currents: Peak and r.m.s. Values (A Level) | magnetic flux density; electromagnetic induction; root-mean-square value | Giving only the force magnitude |
| Rectification and Smoothing (A Level) | magnetic flux density; electromagnetic induction; root-mean-square value | Using the angle itself as a multiplier |
Worked example: magnetic force, induction, transformers and rms
Read each question as it appears on the exam paper, choose your answer, then tap it to see the feedback.
Practice 1: Force on a current-carrying wire
A straight wire is perpendicular to a uniform magnetic field.
A wire of active length 0.180 m carries a conventional current of 3.20 A perpendicular to a 0.450 T magnetic field. Calculate the magnetic force and state its direction in relation to the current and field.
[3]Show all necessary working.
A. F = 0 because sin 90° = 0.
❌ Not quite 😭 The trigonometric value is wrong: sin 90° = 1. A perpendicular wire experiences the maximum force.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
B. F = 0.259 N, parallel to the conventional current.
❌ Not quite 😭 The magnitude is correct, but magnetic force is perpendicular to both current and field, not parallel to the wire.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
C. F = 0.259 N, perpendicular to both conventional current and field in the cross-product direction.
✅ Correct! 😊 Correct. F = BIL because θ = 90°, and the direction follows the conventional-current cross product.
D. F = 2.59 N in the electron-flow direction.
❌ Not quite 😭 This is ten times too large and applies the direction rule to electron motion. The stated current is conventional current.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
Full explanation
- Step 1Use F = BIL sin θ for a straight current-carrying wire.
- Step 2Because the wire is perpendicular to the field, sin 90° = 1.
- Step 3Calculate F = (0.450)(3.20)(0.180) = 0.2592 N ≈ 0.259 N.
- Step 4Apply the cross-product rule using conventional current; force is perpendicular to current and field.
Practice 2: Root-mean-square voltage
An alternating voltage is sinusoidal.
A sinusoidal supply has peak voltage 24.0 V. Determine its root-mean-square voltage.
[2]Show all necessary working.
A. Vᵣₘₛ = V₀/√2 = 17.0 V.
✅ Correct! 😊 Correct. Dividing the sinusoidal peak value by √2 gives 16.97 V, which rounds to 17.0 V.
B. Vᵣₘₛ = √2V₀ = 33.9 V.
❌ Not quite 😭 This converts an rms value to a peak value, the reverse of what is required. It also makes the rms value exceed the peak.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
C. Vᵣₘₛ = V₀/2 = 12.0 V.
❌ Not quite 😭 Halving gives neither the mean nor rms value of a sinusoid. The rms relation contains √2.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
D. Vᵣₘₛ = V₀ = 24.0 V.
❌ Not quite 😭 Peak and rms voltage are different measures. Equality would incorrectly imply the voltage remains at its maximum throughout the cycle.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
Full explanation
- Step 1For a sinusoidal alternating voltage, Vᵣₘₛ = V₀/√2.
- Step 2Insert the peak value V₀ = 24.0 V.
- Step 3Calculate Vᵣₘₛ = 24.0/√2 = 16.97 V.
- Step 4Report the rms voltage as 17.0 V.
Key terms for Unit 10: Magnetic Fields and Alternating Currents
Models, uses, and boundaries
- Magnetic Fields and Alternating Currents
- The sine factor accounts for the angle between current and magnetic field. Use this relationship when the sine factor accounts for the angle between current and magnetic field..
- Worked example: magnetic force, induction, transformers and rms
- Perpendicular alignment reduces F = BIL sin θ to the product BIL. Use this relationship when perpendicular alignment reduces F = BIL sin θ to the product BIL..
- Magnetic Fields and Alternating Currents: change one condition
- Comparing 90° with 135° uses two coordinates on the same magnetic-force curve. Use this relationship when comparing 90° with 135° uses two coordinates on the same magnetic-force curve..
- Magnetic Fields and Alternating Currents: test proposed results
- A viable response includes both the newton magnitude and the cross-product direction. Use this relationship when a viable response includes both the newton magnitude and the cross-product direction..
Cambridge International AS & A Level Physics (9702) Unit 10 FAQ
How much of Cambridge International AS & A Level Physics (9702) does Unit 10 carry?
Cambridge International does not publish a per-unit weighting for this exam.
What is the first move on a Magnetic Fields and Alternating Currents problem?
Use F = BIL sin θ
Which relationships should I preserve?
Magnitude: The active wire length is 0.180 m. Maximum: Current is perpendicular to the 0.450 T field. Direction: Force follows the current–field cross product.
Which representations should I practice?
Practice moving among A graph of force (N) against angle (degrees)., A formula card that explains the sine factor accounts for the angle between current and magnetic field., A table linking key concept, why it matters, visible method..
What error should I check before submitting an answer?
Check for Omitting the length or using degrees as a multiplier; Giving only the force magnitude; Using the angle itself as a multiplier.
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: magnetic force, induction, transformers and rms
- Magnetic Fields and Alternating Currents: change one condition
- Magnetic Fields and Alternating Currents: test proposed results
- Check the worked-example evidence for Magnetic Fields and Alternating Currents
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
Cambridge International does not publish a per-unit weighting for this exam.02Kinematics and Dynamics
Cambridge International does not publish a per-unit weighting for this exam.03Forces, Energy and Deformation of Solids
Cambridge International does not publish a per-unit weighting for this exam.04Waves and Superposition
Cambridge International does not publish a per-unit weighting for this exam.05Electricity and D.C. Circuits
Cambridge International does not publish a per-unit weighting for this exam.06Particle Physics
Cambridge International does not publish a per-unit weighting for this exam.07Circular Motion, Gravitational Fields and Oscillations
Cambridge International does not publish a per-unit weighting for this exam.08Thermal Physics
Cambridge International does not publish a per-unit weighting for this exam.09Electric Fields and Capacitance
Cambridge International does not publish a per-unit weighting for this exam.11Quantum and Nuclear Physics
Cambridge International does not publish a per-unit weighting for this exam.12Medical Physics, Astronomy and Cosmology
Cambridge International does not publish a per-unit weighting for this exam.How to study Cambridge International AS & A Level Physics (9702) Unit 10
Start with the organizing decision
Before solving, restate the decision in operational terms: Use the stated conventional-current direction, not electron motion, when applying the cross-product direction rule.. Your first written move should be to Use F = BIL sin θ.
Practice the same idea in several representations
Rotate through A graph of force (N) against angle (degrees)., A formula card that explains the sine factor accounts for the angle between current and magnetic field., A table linking key concept, why it matters, visible method.. Use each representation to practice Magnetic Fields and the Force on a Current-Carrying Conductor (A Level), Force on a Moving Charge, the Hall Effect and Velocity Selection (A Level), Magnetic Fields due to Currents (A Level), Electromagnetic Induction: Faraday's and Lenz's Laws (A Level), Characteristics of Alternating Currents: Peak and r.m.s. Values (A Level), Rectification and Smoothing (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 Omitting the length or using degrees as a multiplier; Giving only the force magnitude; Using the angle itself as a multiplier. 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.