AQA A-Level Physics Paper 2, June 2025: Question 11

1 mark · Medium difficulty · Multiple Choice

Identify the correct electric field line pattern between two charges $+Q$ and $-q$ where the magnitude of $+Q$ is greater than $-q$.

Practise this question

Question

Question 11 asks to identify the correct diagram of electric field lines between two metal spheres with charges +Q and -q, where the magnitude of +Q is greater than the magnitude of -q. Four diagrams labeled A to D show different arrangements of field lines between the two spheres. In A, field lines point inward toward +Q and outward from -q. In B, lines point outward from +Q, some terminate on -q, and others continue outward away from the charges. In C and D, the field lines between the two charges do not connect, behaving like like charges rather than opposite charges.

Mark scheme

Show the mark scheme Mark scheme for question 11 showing the correct answer as option B, accompanied by the corresponding diagram B of electric field lines pointing from +Q toward -q and into infinity, with assessment objective AO2 indicated.

How to answer it

Electric Field Lines Around Unequal Opposite Charges

📋 What this question tests

This question assesses your conceptual understanding of electric field patterns (Specification Section 3.7.3: Electric Fields). Specifically, it evaluates whether you can deduce:

  • The correct direction of electric field lines based on the definition of an electric field.
  • The shape of the field between opposite (attractive) charges versus like (repulsive) charges.
  • How relative charge magnitude impacts field line density and asymmetry.

Question 11 (Multiple Choice)

Analysis of Field Line Diagrams for +Q and -q where |+Q| > |-q| [1 mark]

✅ Correct Answer: B

Diagram B is the only correct representation:

  • Field lines start on positive charges (+Q) and end on negative charges (-q).
  • Lines connect across the region between opposite charges due to electrostatic attraction.
  • Because |+Q| > |-q|, more field lines leave +Q than enter -q; the excess lines radiate outwards to infinity.

📐 Systematic Elimination Process

  1. Check interaction type: Charges are +Q and -q (opposite charges). They must attract, meaning field lines must bridge continuously between the two spheres.
    Eliminates C and D immediately (which show repulsion).
  2. Check arrow direction: By definition, electric field lines point in the direction of the force on a small positive test charge. Hence, arrows must point away from +Q and towards -q.
    Eliminates A (where arrows enter +Q and exit -q).
  3. Verify charge magnitude asymmetry: Since |+Q| > |-q| , the field line density must be greater around +Q , and not all lines from +Q terminate on -q . Diagram B displays this exact asymmetry.

💡 Key Knowledge

  • Field Direction: Electric field strength E is defined as force per unit positive charge ( E = F / q ). Arrows must always travel positive → negative.
  • Line Density & Magnitude: The number of field lines entering or leaving a charge is directly proportional to the magnitude of that charge.
  • Neutral Point: Because the charges are opposite and unequal, there is no neutral point ( E = 0 ) between them; a neutral point only exists outside on the side of the weaker charge ( -q ).
  • Normal to Surface: Field lines must meet conducting spherical surfaces at 90° (normal to the surface).

🧠 Exam Technique

  • Two-second check for MCQ: Look at the central region first. Continuous connecting lines mean opposite charges. Diverging/parted lines mean like charges.
  • Arrow test: Always check the arrowheads on the central horizontal line. It must go directly from + to - .
  • Time saver: In Paper 1/Paper 2 Section A, spend no more than 1.5 minutes per MCQ. Eliminating two obviously wrong options (C and D) instantly narrows your odds to 50/50.

❌ Common Misconceptions & Traps

  • Confusing field lines with electron flow: Students often think arrows point towards positive charges because "electrons are attracted to positive". Remember: field lines are defined by positive test charges!
  • Selecting C or D: Mistaking opposite charges for identical charges. Diagrams C and D show the characteristic pattern of two charges repelling each other (a neutral point between them).
  • Overlooking Diagram A's reversed arrows: Diagram A has the correct geometric shape and correct line density, but every single arrowhead is backwards. Always double-check arrow directions carefully.
Assessment Objective: AO2 (Application of knowledge of electric fields and field line representations) • Award: 1 mark for option B.

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 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.