AQA A-Level Physics Paper 2, June 2023: Question 18

1 mark · Easy difficulty · Multiple Choice

Identify the correct equilibrium orientation of a polar molecule in a downward external electric field.

Practise this question

Question

Question 18 asking: 'A polar molecule is in an external electric field. Which diagram shows the orientation of the polar molecule?' Four options (A to D) depict downward-pointing uniform electric field lines and an oval representing a polar molecule with positive (+) and negative (-) charges. In A, the molecule is oriented vertically with negative charge at the top and positive charge at the bottom. In B, it is horizontal with positive on the left and negative on the right. In C, it is vertical with positive at the top and negative at the bottom. In D, it is horizontal with negative on the left and positive on the right.
Question text

18 A polar molecule is in an external electric field.

Which diagram shows the orientation of the polar molecule?

[1 mark]

A

B

C

D

Mark scheme

Show the mark scheme Mark scheme for question 18 shows the correct answer is A, with a reproduction of diagram A where the polar molecule has the negative charge at the top and positive charge at the bottom, aligned with downward-pointing field lines.

18 A

How to answer it

Orientation of a Polar Molecule in a Uniform Electric Field

📌 What this question tests

This multiple-choice question tests your understanding of dielectrics and electric fields, specifically:

  • The standard convention for the direction of electric field lines (from positive to negative potential).
  • The nature of electrostatic forces on opposite charges ( F = qE ).
  • How an electric field exerts a couple on a permanent or induced dipole, rotating it into an equilibrium orientation aligned with the field.
Question 18 • 1 Mark

Alignment of an Electric Dipole in an External Field

Identifying the stable equilibrium orientation of a polar molecule

✅ Correct Answer

A — The molecule aligns parallel to the field lines with the negative end pointing upwards and the positive end pointing downwards.

Mark Scheme Breakdown:
• 1 mark for selecting option A.

💡 Key Knowledge

  • Field Line Direction: Electric field lines always point in the direction that a positive test charge would move (from positive/high potential to negative/low potential).
  • Force on Charges:
    • Positive charge ( +q ) experiences a force in the same direction as the field (downwards).
    • Negative charge ( -q ) experiences a force opposite to the field (upwards).
  • Torque / Alignment: The couple acting on the dipole rotates it until it aligns parallel with the field lines in a minimum energy state (stable equilibrium).

📐 Step-by-Step Physical Reasoning

  1. Identify Field Direction: The arrows on the uniform field lines point downwards. This means the top region acts as positive potential, and the bottom region acts as negative potential.
  2. Determine Electrostatic Forces:
    • Top (+) source attracts the negative end ( − ) and repels the positive end ( + ).
    • Bottom (−) destination attracts the positive end ( + ) and repels the negative end ( − ).
  3. Evaluate Orientations:
    • Options B & D: The molecule is perpendicular to the field. This experiences maximum torque and is not an equilibrium position.
    • Option C: Positive is at the top, negative at the bottom. The forces pull the ends outward in opposite directions, but this is an unstable equilibrium; any slight displacement causes it to flip.
    • Option A: Negative at top, positive at bottom. The forces on each end pull them in their preferred directions, creating a stable equilibrium.

🧠 Exam Technique

  • Draw the Plates: When dealing with field lines, mentally or lightly sketch parallel plates. Since arrows point downwards, imagine a positive plate at the top and a negative plate at the bottom.
  • Use "Opposites Attract":
    • The − charge is drawn toward the top (+) plate.
    • The + charge is drawn toward the bottom (−) plate.
  • This instantly confirms that the negative end must face up and the positive end must face down, pointing straight to A in under 15 seconds.

❌ Common Errors & Traps

  • Choosing C (Reversed Polarity): Confusing which way field lines point or forgetting that electric field lines point away from positive. Students mistakenly think "+" should lead the direction of the arrow. While the force on a positive charge is in the field direction (downwards), that means the positive end must end up at the bottom.
  • Choosing B or D (Perpendicular Alignment): Misinterpreting the dipole alignment as perpendicular, confusing electric field alignment with the Hall effect or magnetic force rules where directions are at right angles.

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.