Force on a moving charge
Cambridge A-Level Physics (9702) · Unit 20: Magnetic fields · 8 flashcards
Force on a moving charge is topic 20.3 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 Magnetic fields due to currents. In one line: F = BQv sin θ, where F is the force (N), B is the magnetic flux density (T), Q is the charge (C), v is the velocity (m/s), and θ is the angle between the velocity 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 8 flashcards — 2 definitions, 5 key concepts and 1 calculation — 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.
Write the formula for the force on a charge moving in a magnetic field and define each term
F = BQv sin θ, where F is the force (N), B is the magnetic flux density (T), Q is the charge (C), v is the velocity (m/s), and θ is the angle between the velocity 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.
- determine the direction of the force on a charge moving in a magnetic field
- recall and use F = BQv sin θ
- understand the origin of the Hall voltage and derive and use the expression VH = BI / (ntq), where t = thickness
- understand the use of a Hall probe to measure magnetic flux density
- describe the motion of a charged particle moving in a uniform magnetic field perpendicular to the direction of motion of the particle
- explain how electric and magnetic fields can be used in velocity selection
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 moving charge
- › 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.
State the direction of the force on a positive charge moving in a magnetic field.
The force is perpendicular to both the velocity of the charge and the magnetic field direction. Can be determined using Fleming's Left-Hand Rule (thumb = force, first finger = field, second finger = conventional current).
Write the formula for the force on a charge moving in a magnetic field and define each term.
F = BQv sin θ, where F is the force (N), B is the magnetic flux density (T), Q is the charge (C), v is the velocity (m/s), and θ is the angle between the velocity and the magnetic field.
What is the Hall voltage, and how does it arise?
The Hall voltage (VH) is the voltage difference created across a conductor perpendicular to both an electric current and a magnetic field. It arises due to the Lorentz force deflecting moving charges to one side of the conductor.
Give the formula for Hall Voltage and define each term.
VH = BI / (ntq), where VH is the Hall voltage, B is the magnetic flux density, I is the current, n is the charge carrier density, t is the thickness of the conductor, and q is the elementary charge.
Describe how a Hall probe is used to measure magnetic flux density.
A Hall probe is placed in the magnetic field. The Hall voltage produced is proportional to the magnetic flux density. By calibrating the Hall probe with known magnetic fields, the unknown field can be determined. V ∝ B
Describe the motion of a charged particle moving in a uniform magnetic field when its velocity is perpendicular to the field.
The particle will move in a circular path. The magnetic force provides the centripetal force required for circular motion.
A proton moves at 3.0 x 10^6 m/s perpendicularly through a uniform magnetic field of 0.50 T. Calculate the magnetic force on the proton.
F = BQv sin θ = (0.50 T)(1.60 x 10^-19 C)(3.0 x 10^6 m/s) sin 90° = 2.4 x 10^-13 N.
Explain how electric and magnetic fields can be used for velocity selection of charged particles.
By applying perpendicular electric and magnetic fields, only particles with a specific velocity will pass through undeflected. The electric force (F = QE) and magnetic force (F = BQv) are balanced when v = E/B.
Review the material
Read full revision notes on Force on a moving charge — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 20 — Magnetic fields
Force on a moving charge 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 moving charge deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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