Magnetic fields due to currents
Cambridge A-Level Physics (9702) · Unit 20: Magnetic fields · 7 flashcards
Magnetic fields due to currents is topic 20.4 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 20 — Magnetic fields , alongside Concept of a magnetic field, Force on a current-carrying conductor and Force on a moving charge.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 7 flashcards — 1 definition and 6 key concepts — covering the precise wording mark schemes reward. Use the definition card to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Some cards in this deck are on other syllabus topics: 20.2 Force on a current-carrying conductor. Each card is labelled with its topic.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Sketch the magnetic field patterns around three current-carrying shapes: a long straight wire, a long solenoid and a flat circular coil.
- Understand that a ferrous core inside a solenoid increases the magnetic field produced by the current.
- Explain why current-carrying conductors exert forces on each other, and determine the directions of these forces.
Also in this deck: 20.2 Force on a current-carrying conductor
- Understand that a current-carrying conductor in a magnetic field may experience a force.
- Recall F = BIL sin θ and use it, finding the directions involved with Fleming's left-hand rule.
- Define magnetic flux density: the force per unit length of wire per unit current, for a wire perpendicular to the magnetic field.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in these topics
- › Magnetic field strength B decreases with distance; ensure concentric circles representing the field lines get further apart as distance from the wire increases.
- › The field inside a long solenoid is uniform, so B is the same at every point well inside it.
- › Always include the 'at right angles' or 'perpendicular' condition when defining B = F / (Il).
Topic 20.4 · Magnetic fields due to currents
Sketch the magnetic field pattern around a long straight wire carrying a current.
The magnetic field lines are concentric circles around the wire, with the wire at the center. The direction of the field is determined by the right-hand grip rule: thumb in direction of current, fingers curl in direction of the magnetic field.
Topic 20.4 · Magnetic fields due to currents
Describe the magnetic field pattern inside a long solenoid.
The magnetic field inside a long solenoid is approximately uniform and parallel to the axis of the solenoid. Outside the solenoid, the field is weak and resembles that of a bar magnet.
Topic 20.4 · Magnetic fields due to currents
What effect does a ferrous core have on the magnetic field strength of a solenoid?
A ferrous core significantly increases the magnetic field strength of a solenoid. This is because the ferrous material is easily magnetized, aligning its magnetic domains with the solenoid's field and enhancing it.
Topic 20.4 · Magnetic fields due to currents
Explain the origin of the force between two parallel current-carrying conductors.
Each conductor creates a magnetic field around it. The other conductor, carrying a current, experiences a force due to this magnetic field. Parallel currents attract, antiparallel currents repel.
Topic 20.2 · Force on a current-carrying conductor
Determine the direction of the force on a current-carrying conductor in a magnetic field.
The direction of the force is given by Fleming's left-hand rule. Thumb = Thrust (Force), First finger = Field (Magnetic), Second finger = Current. Orient your hand to align with the field and current directions, and your thumb will point in the direction of the force.
Topic 20.4 · Magnetic fields due to currents
Sketch the magnetic field pattern for a flat circular coil carrying a current.
The magnetic field lines pass through the center of the coil and form loops. The field is strongest at the center and weakens as you move away from the coil. It resembles the field of a short bar magnet.
Topic 20.4 · Magnetic fields due to currents
How does increasing the current in a solenoid affect its magnetic field strength?
Increasing the current directly increases the magnetic field strength inside the solenoid. The magnetic field strength is proportional to the current.
Review the material
Read full revision notes on Magnetic fields due to currents — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 20 — Magnetic fields
Magnetic fields due to currents sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Magnetic fields due to currents deck
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