A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Electric Fields and Their Representation — mark scheme explained
The short answer
Electric fields are a fundamental concept in physics, representing the region around an electric charge where other charges experience a force. This explainer will cover how electric fields are represented using field lines, the definition of electric field strength, and the magnitude of the electric field in both uniform and radial configurations.
The question
A uniform electric field exists between two parallel plates separated by 2.0 cm with a potential difference of 100 V. Calculate the magnitude of the electric field.
[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 of values into the formula, showing intermediate steps, and providing the final answer with appropriate units. For conceptual questions, marks are given for clear and accurate explanations that demonstrate understanding of the underlying principles.
What the command words demand
- Calculate
- Perform a numerical calculation using the given data and appropriate formula.
- Determine
- Find the value of a quantity or solve a problem using logical reasoning and calculations.
- Explain
- Provide a clear and concise explanation, including relevant concepts and principles.
- Describe
- Give a detailed account of a phenomenon or process without necessarily providing an explanation.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 2-3 minutes per mark for calculation questions and 1-2 minutes per mark for conceptual questions to ensure you have enough time to show all necessary steps and provide detailed explanations.
- Identify the formula for the magnitude of the electric field in a uniform field: E = V / d.1 mark
- Convert the separation distance to meters: d = 2.0 cm = 0.02 m.1 mark
- Substitute the values into the formula: E = 100 V / 0.02 m.1 mark
- Calculate the result: E = 5000 N/C.1 mark
Final answer: 5000 N/C
Work through every step correctly and you earn all 4 marks.
Another worked example
A point charge of +3 μC is placed at a distance of 1.0 m from another point charge. Calculate the magnitude of the electric field at this distance.
- Identify the formula for the magnitude of the electric field in a radial field: E = 1 / (4πε 0 ) × Q / r 2 .1 mark
- Substitute the values into the formula: E = 1 / (4π × 8.85 × 10 -12 ) × 3 × 10 -6 C / (1.0 m) 2 .2 marks
- Calculate the result: E ≈ 26977 N/C.2 marks
Final answer: 26977 N/C
Work through every step correctly and you earn all 5 marks.
Common mistakes
Confusing electric field lines with magnetic field lines.
Why it happens: Students sometimes mix up the concepts of electric and magnetic fields, leading to confusion about the direction and representation of field lines.
Fix: Review the definitions and properties of both electric and magnetic fields. Electric field lines start from positive charges and end on negative charges, while magnetic field lines form closed loops around a magnet.
Using the wrong formula for the magnitude of the electric field in different configurations.
Why it happens: Students may use the formula for a uniform field (E = V / d) when they should be using the formula for a radial field (E = 1 / (4πε 0 ) × Q / r 2 ).
Fix: Ensure you identify the type of electric field configuration before applying the appropriate formula. Practice problems involving both uniform and radial fields to reinforce this distinction.
Forgetting to convert units when calculating the magnitude of the electric field.
Why it happens: Students often forget to convert distances from centimeters to meters, leading to incorrect calculations.
Fix: Always check and convert units before substituting values into formulas. Practice converting between different units to build confidence.
Misinterpreting the direction of the electric field for a negative charge.
Why it happens: Students may incorrectly assume that the electric field lines point away from a negative charge, similar to positive charges.
Fix: Remember that electric field lines always point away from positive charges and towards negative charges. Visualize the field lines around different types of charges to reinforce this concept.
Confusing force with acceleration when calculating the trajectory of a charged particle in a uniform electric field.
Why it happens: Students may use the formula for force (F = QE) instead of the formula for acceleration (a = F / m) when determining the motion of a charged particle.
Fix: Understand the relationship between force, mass, and acceleration. Practice problems that involve calculating both force and acceleration to solidify this understanding.
Forgetting to include the permittivity of free space (ε 0 ) in calculations for a radial field.
Why it happens: Students may overlook the constant ε 0 when using the formula E = 1 / (4πε 0 ) × Q / r 2 .
Fix: Always include the value of ε 0 in your calculations for radial fields. Practice problems that involve this constant to ensure you remember it.
Misinterpreting the work done formula (W = QV) as a direct relationship between electric field and potential difference.
Why it happens: Students may incorrectly assume that W = QV directly gives the electric field strength without considering the distance d.
Fix: Understand that the work done formula is derived from the relationship Fd = QV, which simplifies to E = V / d. Practice problems that involve both work and electric field calculations to reinforce this concept.
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 Electric Field | Find the electric field strength between parallel plates from voltage and separation. | 4 |
| Calculate Electric Field | Use the radial field equation to find the field strength from a point charge. | 5 |
| Calculate Acceleration | Find the force on a charge in an electric field, then use it to calculate acceleration. | 4 |
| Calculate Electric Force | Find the electric field between parallel plates, then calculate the force on a charge. | 5 |
| Calculate Electric Field | Find the electric field magnitude and direction produced by a point charge at a given distance. | 5 |
| Total across these question types | 23 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.