Force on a current-carrying conductor
Cambridge A-Level Physics (9702) · Unit 20: Magnetic fields · 7 flashcards
Force on a current-carrying conductor is topic 20.2 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 20 — Magnetic fields , alongside Concept of a magnetic field, Force on a moving charge and Magnetic fields due to currents. In one line: The force F on a current-carrying conductor is given by F = BILsinθ, where B is the magnetic flux density, I is the current, L is the length of the conductor in the field, and θ is the angle between the conductor and the magnetic field.
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 — 2 definitions, 3 key concepts and 2 calculations — covering the precise wording mark schemes reward. Use the 2 definition cards 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.
Formula that relates the force on a current-carrying conductor to the magnetic field, current, and length
The force F on a current-carrying conductor is given by F = BILsinθ, where B is the magnetic flux density, I is the current, L is the length of the conductor in the field, and θ is the angle between the conductor and the magnetic field.
What the Cambridge 9702 syllabus says
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- understand that a force might act on a current-carrying conductor placed in a magnetic field
- recall and use the equation F = BIL sin θ, with directions as interpreted by Fleming’s left-hand rule
- define magnetic flux density as the force acting per unit current per unit length on a wire placed at right- angles 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 Force on a current-carrying conductor
- › Magnetic field strength B decreases with distance; ensure concentric circles representing the field lines get further apart as distance from the wire increases.
- › Attribute Hall voltage to the deflection of charge carriers by a magnetic field, not to the principles of induction.
- › Always include the 'at right angles' or 'perpendicular' condition when defining B = F / (Il).
- › Always follow the logical sequence: changing flux linkage induces an e.m.f., which then causes a current if the circuit is complete.
- › Always specify that the magnetic force acts on a moving charged particle or a current-carrying conductor.
Under what conditions does a force act on a current-carrying conductor placed in a magnetic field?
A force acts on a current-carrying conductor in a magnetic field when the conductor is not parallel to the magnetic field lines. The force is maximized when the conductor is perpendicular to the field.
State the formula that relates the force on a current-carrying conductor to the magnetic field, current, and length.
The force F on a current-carrying conductor is given by F = BILsinθ, where B is the magnetic flux density, I is the current, L is the length of the conductor in the field, and θ is the angle between the conductor and the magnetic field.
Explain Fleming's left-hand rule and how it is used.
Fleming's left-hand rule gives the direction of the force on a current-carrying conductor in a magnetic field. With your Thumb, First finger, and Second finger at right angles, the First finger points in the direction of the Field, the Second finger points in the direction of the Current, and the Thumb points in the direction of the Force.
Define magnetic flux density (B).
Magnetic flux density (B) is defined as the force acting per unit current per unit length on a wire placed at right angles to the magnetic field. Its unit is the Tesla (T).
A 5 cm wire carrying a current of 3 A is placed perpendicular to a magnetic field of 0.6 T. Calculate the force on the wire.
Using F = BILsinθ, where θ = 90°, F = (0.6 T)(3 A)(0.05 m)(sin 90°) = 0.09 N. Therefore, the force on the wire is 0.09 N.
Describe how the magnitude of the force on a current-carrying wire changes as the angle between the wire and magnetic field varies from 0 to 90 degrees.
The force is zero when the wire is parallel (0 degrees) to the magnetic field (sin 0° = 0). The force increases as the angle increases, reaching a maximum when the wire is perpendicular (90 degrees) to the field (sin 90° = 1).
A wire of length 0.2m, carrying a current of 2A, experiences a force of 0.08N when placed in a magnetic field. If the wire is perpendicular to the field, what is the magnetic flux density?
Using F = BILsinθ, and rearranging for B: B = F / (ILsinθ). Since θ = 90°, sin θ = 1. Therefore B = 0.08N / (2A * 0.2m) = 0.2T. The magnetic flux density is 0.2 Tesla.
Review the material
Read full revision notes on Force on a current-carrying conductor — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 20 — Magnetic fields
Force on a current-carrying conductor 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 Force on a current-carrying conductor deck
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