AQA A-Level Physics Paper 2, June 2023: Question 16

1 mark · Easy difficulty · Multiple Choice

Determine the direction of the uniform electric field that causes an electron to deflect upwards in a vertical plane.

Practise this question

Question

A diagram shows a shaded square area labelled 'region of uniform electric field'. A curved arrow representing the 'path of electron' enters from the left edge moving horizontally to the right and curves upward, exiting through the top-right corner. Below the diagram, candidates are asked for the direction of the electric field, with four options: A vertically down the plane, B vertically up the plane, C horizontally into the plane, and D horizontally out of the plane.
Question text

16 The diagram shows the path of an electron in a uniform electric field.

The electron moves in a vertical plane.

The direction of the electric field is

[1 mark]

A vertically down the plane.

B vertically up the plane.

C horizontally into the plane.

D horizontally out of the plane.

Mark scheme

Show the mark scheme Mark scheme table row for question 16 showing the key A with description 'vertically down the plane.'

16 A vertically down the plane.

How to answer it

Electron Deflection in a Uniform Electric Field

📋 What this question tests

This question assesses your understanding of electric field direction convention, the nature of the electrostatic force acting on charged particles (specifically negatively charged electrons), and how a uniform field causes parabolic trajectory motion in a 2D plane.

Multiple Choice • Question 16

Direction of the Electric Field

AQA A-Level Physics — Electric Fields

✅ Correct Answer: A

A: vertically down the plane.

Mark Scheme: 1 mark for selecting option A.

💡 Key Knowledge

  • Definition of Electric Field Direction: By universal definition, the direction of an electric field vector (E) is the direction of the force that acts on a positive test charge.
  • Force on a Negative Charge: An electron carries a negative charge ( q = -e ). The electrostatic force equation is F = qE .
  • Because q is negative, the force acting on an electron is in the opposite direction to the electric field lines: F = -eE .

📐 Step-by-Step Deduction

  1. Observe the trajectory: The electron enters travelling generally towards the right and curves upwards.
  2. Determine the direction of acceleration and force: Since the electron curves upwards in the vertical plane, the resultant electrostatic force acting on it must be directed vertically upwards ( F ↑ ).
  3. Relate force to electric field direction: Electrons have a negative charge ( -1.60 × 10⁻¹⁹ C ). A negative charge experiences an electrostatic force in the direction opposite to the electric field.
  4. Conclude electric field direction: Since the force is upwards, the electric field must point vertically downwards ( E ↓ ).

🧠 Exam Technique & Visualisation

Imagine parallel capacitor plates:

  • If an electron is repelled/attracted such that it curves upwards, the top plate must be positive (+) and the bottom plate must be negative (−).
  • Electric field lines always run from positive to negative (from the top plate down to the bottom plate).
  • Therefore, the field lines must point vertically downwards.

❌ Common Examiner Pitfalls

  • Selecting B (vertically up): The most common error. Students correctly identify that the force is upward, but forget that an electron is negative, leading them to assume field direction equals force direction.
  • Selecting C or D (into/out of plane): Confusing electric fields with magnetic fields. Magnetic fields produce forces perpendicular to both the field and velocity ( F = Bqv ), which often involve 3D directions into/out of the plane. Electric fields produce acceleration directly along the field axis.

Topics

Physics · 3.7 Fields and their consequences (A-level only)

Question and mark scheme from the AQA A-Level Physics examination, Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.