A-Level · Physics · AQA · Mark scheme decoded

AQA A-Level Physics: Force on Charged Particles in a Magnetic Field and Applications — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Physics specificationlast verified 2 July 2026

The short answer

In this section, we will explore the force experienced by charged particles when they move through a magnetic field, specifically focusing on the equation F = BQv when the magnetic field is perpendicular to the velocity of the particle.

The question

A proton with a charge of +1.6 × 10 -19 C is moving at 5 × 10 6 m/s in a magnetic field of 0.2 T, perpendicular to its velocity. Calculate the force on the proton. [Paraphrased for study — not reproduced from any exam paper.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

S1
Identify the given values: B = 0.2 T , Q = +1.6 × 10 -19 C , v = 5 × 10 6 m/s
S2
Use the equation F = BQv
S3
F = (0.2 T) × (+1.6 × 10 -19 C) × (5 × 10 6 m/s)
S4
F = 1.6 × 10 -13 N

Model answer

Worked through, with each step tagged to the mark it earns.

S1
Identify the given values: B = 0.2 T , Q = +1.6 × 10 -19 C , v = 5 × 10 6 m/s
S2
Use the equation F = BQv
S3
F = (0.2 T) × (+1.6 × 10 -19 C) × (5 × 10 6 m/s)
S4
F = 1.6 × 10 -13 N
Final answer: 1.6 × 10 -13 N

Common mistakes

Using the wrong hand for determining the direction of the force on a negative charge. — Always use the right-hand rule for positive charges and the left-hand rule for negative charges.
Forgetting to include the charge sign when calculating the force direction. — Always consider the sign of the charge when using the right-hand or left-hand rule.
Using the wrong formula for the radius of the circular path. — Memorize and use the correct formula: r = mv/(BQ) .
Forgetting to convert between 2D and 3D representations of magnetic field problems. — Practice drawing and interpreting both 2D and 3D diagrams to understand the spatial relationships.
Using the wrong units for magnetic field strength or charge. — Always check that you are using the correct units: Tesla (T) for magnetic field strength and Coulombs (C) for charge.
Forgetting to include the sine of the angle when the magnetic field and velocity are not perpendicular. — Use the general form F = BQv sin(θ) when the magnetic field and velocity are not perpendicular.

Where the marks go

Full worked solution (all marking points)4 marks