AQA A-Level Physics Paper 2, June 2025: Question 14

1 mark ยท Medium difficulty ยท Multiple Choice

Determine the direction of the electric force on an electron placed in an electric field shown by curved equipotential lines.

Practise this question

Question

Multiple-choice question showing three concentric curved equipotential lines labelled +10 V, +15 V, and +20 V from left to right. A black dot representing an electron is located between the +15 V and +20 V lines. Four arrows labelled A, B, C, and D emerge from the electron: arrow A points perpendicularly towards lower potential, arrow C points perpendicularly towards higher potential, while arrows B and D point tangentially along the curvature parallel to the equipotentials.

Mark scheme

Show the mark scheme Mark scheme table for question 14 showing the correct answer as 'C' corresponding to assessment objective AO1.

How to answer it

Direction of Electric Force on an Electron

๐Ÿ“Œ What this question tests

This question tests your understanding of equipotential lines, the relationship between potential gradient and electric field direction, and how the sign of a charge dictates the direction of electrostatic force ( F = qE ).

Question 14: Electric Force on a Negative Charge

Multiple Choice (1 Mark) โ€” Assessment Objective: AO1

โœ… Correct Answer: C

The electron experiences an electrostatic force directed along arrow C (towards the higher potential of +20 V, perpendicular to the equipotential line).

Mark Scheme: 1 mark for option C.

๐Ÿ’ก Key Knowledge

  • Equipotential surfaces: Lines connecting points of equal potential. Moving along an equipotential line requires zero work ( ฮ”W = qฮ”V = 0 ).
  • Field lines vs. Equipotentials: Electric field lines always cross equipotential surfaces at 90ยฐ (right angles). Therefore, the force must be along arrow A or C.
  • Field Direction: Electric field lines point down the potential gradient, from higher potential to lower potential ( E = -ฮ”V/ฮ”x ).
  • Force on charges: Positive charges accelerate towards lower potential (in the direction of E); negative charges (electrons) accelerate towards higher potential (opposite to E).

๐Ÿ“ Step-by-Step Logical Deduction

  1. Identify the geometry: The electron sits on the +15 V equipotential line. The force must act perpendicular to this line.
    โ†’ Arrows B and D run parallel (tangent) to the equipotential, so they are immediately ruled out.
  2. Determine the Electric Field direction: The electric field E always points in the direction of the greatest rate of decrease of potential (from +20 V towards +10 V).
    โ†’ Therefore, the electric field vector E points along arrow A.
  3. Apply the force equation ( F = qE ):
    Because an electron has a negative charge ( q = -e ), the force acts in the direction opposite to the electric field vector.
    โ†’ Opposite to arrow A is arrow C.
  4. Alternative intuitive check: Negatively charged particles are naturally attracted towards regions of higher (more positive) electric potential. Therefore, the electron is pulled towards +20 V (arrow C).

๐Ÿง  Exam Technique & Shortcuts

Use the "Attraction to Positive" Rule:

Whenever an electron is placed between different potentials, ask yourself: "Where would a negative particle want to go?"

A negative charge always moves towards the region of more positive potential to reduce its electric potential energy ( Eโ‚š = qV ). Moving to +20 V makes qV more negative (lower energy).

โŒ Common Traps & Mistakes

  • Selecting Option A: The most common error. Students correctly deduce that electric field lines go from high potential to low potential, but forget the electron is negatively charged. Option A is the force on a proton.
  • Selecting Option B or D: Confusing field lines with equipotentials. No electrostatic force acts tangent to an equipotential surface.

Topics

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

Question and mark scheme from the AQA A-Level Physics examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.