A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Force on Charged Particles in a Magnetic Field and Applications — mark scheme explained
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
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 calculations, show all steps clearly and include units. For conceptual questions, provide clear and concise explanations using appropriate physics terminology.
What the command words demand
- Calculate
- Perform a numerical calculation to find a specific value.
- Determine
- Find or establish something with certainty, often involving a calculation or reasoning.
- Explain
- Provide a detailed account of how or why something happens.
- Describe
- Give a detailed account of the characteristics or features of something.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 5-7 minutes for each question in this section to ensure you have enough time to complete the necessary calculations and explanations.
- Identify the given values: B = 0.2 T , Q = +1.6 × 10 -19 C , v = 5 × 10 6 m/s0 marks
- Use the equation F = BQv1 mark
- F = (0.2 T) × (+1.6 × 10 -19 C) × (5 × 10 6 m/s)2 marks
- F = 1.6 × 10 -13 N1 mark
Final answer: 1.6 × 10 -13 N
Work through every step correctly and you earn all 4 marks.
Another worked example
A particle with a charge of -2 × 10 -18 C is moving at 3 × 10 5 m/s in a magnetic field of 0.5 T, perpendicular to its velocity. Determine the direction of the force on the particle using the right-hand rule.
- Identify the given values: B = 0.5 T , Q = -2 × 10 -18 C , v = 3 × 10 5 m/s0 marks
- Use the right-hand rule for a positive charge, but remember to reverse the direction for a negative charge1 mark
- Point your thumb in the direction of the velocity and your fingers in the direction of the magnetic field1 mark
- Your palm will point in the direction of the force on a positive charge. For a negative charge, the force is in the opposite direction1 mark
Final answer: The force is in the opposite direction to that determined by the right-hand rule for a positive charge.
Work through every step correctly and you earn all 3 marks.
Common mistakes
Using the wrong hand for determining the direction of the force on a negative charge.
Why it happens: Students often forget that the right-hand rule is used for positive charges and the left-hand rule for negative charges.
Fix: 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.
Why it happens: Students may overlook the importance of the charge sign in determining the direction of the magnetic force.
Fix: 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.
Why it happens: Students might confuse the formula for the radius with other formulas involving magnetic fields and charged particles.
Fix: Memorize and use the correct formula: r = mv/(BQ) .
Forgetting to convert between 2D and 3D representations of magnetic field problems.
Why it happens: Students may struggle with visualizing the problem in three dimensions, especially when converting from a 2D diagram.
Fix: Practice drawing and interpreting both 2D and 3D diagrams to understand the spatial relationships.
Using the wrong units for magnetic field strength or charge.
Why it happens: Students might use incorrect units, leading to errors in calculations.
Fix: 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.
Why it happens: Students might use the simplified form F = BQv even when the angle is not 90°.
Fix: Use the general form F = BQv sin(θ) when the magnetic field and velocity are not perpendicular.
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 |
|---|---|---|
| Magnetic Force On Charge | Calculate the magnetic force on a moving charged particle using F = BQv. | 4 |
| Magnetic Force Direction | Determine the direction of the magnetic force on a moving charge using the hand rule. | 3 |
| Charged Particle Radius | Calculate the radius of a charged particle's circular path in a magnetic field. | 4 |
| Cyclotron Orbital Period | Calculate the period of a charged particle's circular motion in a magnetic field. | 4 |
| Calculate Magnetic Force | Find the force on a moving charge in a magnetic field at an angle. | 5 |
| Total across these question types | 20 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.