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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Identify the formula for the magnitude of the electric field in a uniform field: E = V / d.
- S2
Convert the separation distance to meters: d = 2.0 cm = 0.02 m.
- S3
Substitute the values into the formula: E = 100 V / 0.02 m.
- S4
Calculate the result: E = 5000 N/C.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Identify the formula for the magnitude of the electric field in a uniform field: E = V / d.
- S2
Convert the separation distance to meters: d = 2.0 cm = 0.02 m.
- S3
Substitute the values into the formula: E = 100 V / 0.02 m.
- S4
Calculate the result: E = 5000 N/C.
Final answer: 5000 N/C
Common mistakes
- Confusing electric field lines with magnetic field lines. — 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. — 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. — 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. — 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. — 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. — 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. — 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
- Full worked solution (all marking points)4 marks