Q1. [3] medium thorough-understanding
A current-carrying straight wire is held vertically. A compass is placed at point P, close to the wire, and then moved to point Q, which is farther away, while the current remains unchanged. How does the deflection of the compass needle change, and why?
Generated by claude-sonnet-4-6 · 2026-06-26 01:11 · grounding rag
Model Answer
When the compass is moved from P (closer) to Q (farther), the deflection of the needle decreases.
This is because the magnetic field produced by a current-carrying straight wire is inversely proportional to the distance from the wire. As the compass moves farther away, the magnetic field strength at that point becomes weaker. A weaker field exerts a smaller force on the compass needle, causing it to deflect less from its original north-pointing position.
Source: Chapter 12 — Magnetic Effects of Electric Current, Section 12.2
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Explanation
- The key concept examiners look for is the inverse relationship between magnetic field strength and distance from the wire.
- State clearly: deflection decreases at Q (don't leave this implicit).
- Link the reason in two steps: greater distance → weaker field → less deflection. This is the logical chain worth full marks.
- The passage states "the magnetic field produced by a current-carrying straight wire depends inversely on the distance from it" — use this directly in your reasoning.