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
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For calculation questions, marks are typically awarded for correct substitution into the equation (1 mark), correct working (1-2 marks), and the final answer with units (1 mark). For explanation questions, marks are given for clear and accurate descriptions of principles and processes.
What the command words demand
- Calculate
- Perform a numerical calculation using given data and appropriate formulae.
- Determine
- Find the value or direction of a physical quantity using given information.
- Explain
- Provide a detailed account of how something works, including relevant principles and equations.
- Describe
- Give a detailed account of the characteristics or features of a phenomenon or process.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5 minutes for each question in this topic to ensure you have enough time to carefully read the question, perform calculations, and provide detailed explanations.
- Identify the given values: B = 3 T, I = 2 A, l = 0.5 m.0 marks
- Use the equation F = BIl to calculate the force.1 mark
- F = 3 × 2 × 0.52 marks
- F = 3 N1 mark
Final answer: 3 N
Work through every step correctly and you earn all 4 marks.
Another worked example
A current-carrying wire experiences a force of 6 newtons when placed in a magnetic field of 1.5 teslas. If the length of the wire is 2 meters, what is the current?
- Identify the given values: F = 6 N, B = 1.5 T, l = 2 m.0 marks
- Rearrange the equation F = BIl to solve for I: I = F / (Bl).1 mark
- I = 6 / (1.5 × 2)2 marks
- I = 2 A1 mark
Final answer: 2 A
Work through every step correctly and you earn all 4 marks.
Common mistakes
Using the wrong units for magnetic flux density.
Why it happens: Students often confuse the unit of magnetic flux density (tesla) with other units like weber or henry. It is crucial to remember that one tesla is equivalent to one newton per ampere-meter (N A -1 m -1 ).
Fix: 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.
Why it happens: Students may focus solely on calculating the magnitude of the force and neglect to consider its direction. Understanding the direction is crucial for a complete understanding of the phenomenon.
Fix: 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.
Why it happens: Students may use F = BIl without considering the angle between the magnetic field and the current. This can lead to incorrect results if the fields are not perfectly perpendicular.
Fix: 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.
Why it happens: Students may mistakenly use the distance between the magnets instead of the length of the wire when calculating the force. This can lead to significant errors in their results.
Fix: 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.
Why it happens: Students may forget to subtract the initial mass reading from the final reading, leading to an incorrect force measurement.
Fix: 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.
Why it happens: Students may skip this step or plot incorrect graphs, missing out on the opportunity to analyze the relationships between the variables.
Fix: 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
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Calculate Magnetic Force | Use F = BIl to find the force on a current-carrying wire in a magnetic field. | 4 |
| Calculate Current | Rearrange the motor-effect equation F = BIl and calculate the current in the wire. | 4 |
| Calculate Magnetic Flux Density | Rearrange the force equation F = BIL to find the magnetic flux density from given values. | 4 |
| Calculate Magnetic Force | Use F = BIL sinθ to find the force on a current-carrying wire in a magnetic field. | 6 |
| Calculate Magnetic Flux Density | Rearrange F = BIL to find magnetic flux density from force, current, and length. | 4 |
| Total across these question types | 22 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.