AQA A-Level Physics Paper 2, June 2025: Question 15

1 mark · Medium difficulty · Multiple Choice

Identify which point has an electric field strength of zero near two opposite point charges of different magnitudes.

Practise this question

Question

Diagram showing a horizontal line with two point charges, Q and q, and four labelled points marked with crosses: A to the left of Q, B and C between Q and q, and D to the right of q. The text states that Q and q have opposite signs, the magnitude of charge Q is greater than the magnitude of charge q, and asks at which point A, B, C, or D the electric field strength is zero.

Mark scheme

Show the mark scheme Mark scheme table showing question number 15 with correct answer D, repeating the diagram showing points A, Q, B, C, q, D, with assessment objective AO2.

How to answer it

Resultant Electric Field of Two Point Charges

📋 What this question tests

This question assesses your understanding of Coulomb's law, the vector nature of electric field strength (E), and how fields combine via vector addition. You must be able to deduce where the resultant electric field is zero by analyzing both field directions and the inverse-square dependence on distance.

Question 15 • 1 Mark

Locating the Zero Field Point

Analysis of opposing charges with unequal magnitudes

✅ Correct Answer

D

The resultant electric field strength is zero at point D (to the right of the smaller charge q).

Award: 1 mark for option D (AO2)

💡 Key Knowledge

  • Vector Quantity: Electric field strength E = Q / (4πε₀r²) has direction: away from positive (+), towards negative (−).
  • Zero Condition: For resultant E = 0 , the individual field vectors must be equal in magnitude and opposite in direction.
  • Opposite Charges: Between opposite charges, fields point in the same direction, so they reinforce each other and can never cancel.

📐 Step-by-Step Elimination and Deduction

  1. Step 1: Check the region between the charges (Points B and C)
    Assume Q is positive (+) and q is negative (−):
    • Field due to Q points to the right (away from +).
    • Field due to q points to the right (towards −).
    Both fields act in the same direction, meaning they reinforce each other. Therefore, resultant E cannot be zero anywhere between Q and q. This eliminates B and C.
  2. Step 2: Check outside the charges (Points A and D)
    Outside the charges, the field vectors point in opposite directions, meaning cancellation is possible. To have equal magnitudes:

    |Q| / (rQ)² = |q| / (rq)²  ⇒  (|Q| / |q|) = (rQ / rq)²

  3. Step 3: Factor in charge magnitudes (|Q| > |q|)
    Since |Q| > |q| , we must have rQ > rq for the fields to balance.
    The neutral point must be further from the larger charge Q and closer to the smaller charge q:
    • At A: Point A is closer to the larger charge Q ( rQ < rq ). Here, EQ dominates massively; they can never cancel.
    • At D: Point D is closer to the smaller charge q and farther from the larger charge Q ( rQ > rq ). Here, the greater distance weakens Q's field enough to equal q's field. Hence, the field can cancel to zero.

🧠 Exam Technique (Speed Rule)

Commit this two-step rule to memory for multiple-choice questions:

  • Like charges (+ / + or − / −): Zero field point is between them (closer to smaller charge).
  • Opposite charges (+ / −): Zero field point is outside them on the side of the smaller charge.

Since |Q| > |q|, the smaller charge is q. The point outside next to q is D.

❌ Common Traps & Errors

  • Confusing Field with Potential: Electric potential (V) is a scalar and CAN be zero between opposite charges. Electric field (E) is a vector and CANNOT be zero between opposite charges.
  • Ignoring Magnitude: Picking A because it's also "outside", forgetting that point A is closer to the already stronger charge, making balance impossible.
  • Picking the Midpoint: Selecting B or C believing charges always cancel out somewhere in the middle.

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.