A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Force on a Current-Carrying Wire in a Magnetic Field — mark scheme explained
The short answer
The force experienced by a current-carrying wire placed in a magnetic field is an important concept in physics, particularly within the study of electromagnetism.
The question
A wire of length 0.5 meters carries a current of 2 amperes in a magnetic field of 3 teslas. Calculate the force on the wire. [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 = 3 T, I = 2 A, l = 0.5 m.
- S2
Use the equation F = BIl to calculate the force.
- S3
F = 3 × 2 × 0.5
- S4
F = 3 N
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Identify the given values: B = 3 T, I = 2 A, l = 0.5 m.
- S2
Use the equation F = BIl to calculate the force.
- S3
F = 3 × 2 × 0.5
- S4
F = 3 N
Final answer: 3 N
Common mistakes
- Using the wrong units for magnetic flux density. — Always check the units and ensure you are using teslas (T) for magnetic flux density.
- Forgetting to use Fleming’s left-hand rule to determine the direction of the force. — Always apply Fleming’s left-hand rule to find the direction of the force, even if it is not explicitly asked in the question.
- Using the wrong equation when the magnetic field and current are not perpendicular. — Use the more general form of the equation, F = BIl sin(θ), when the magnetic field and current are not perpendicular.
- Confusing the length of the wire with the distance between the magnets in the practical setup. — Ensure you measure and use the correct length of the wire that is within the magnetic field.
- Failing to account for the initial mass reading on the top pan balance. — Always measure and record the initial mass reading with no current, then subtract it from the final reading to find the force exerted by the magnetic field.
- Not plotting graphs of force against B, I, and l in the required practical. — Plot accurate graphs of force against magnetic flux density (B), current (I), and length of the wire (l) to clearly show the linear relationships.
Where the marks go
- Full worked solution (all marking points)4 marks