Q1. [3] medium thorough-understanding
A student sets up a circuit with a straight copper wire placed parallel to a compass needle. When current flows, the needle deflects. The student then moves the compass needle farther from the wire without changing the current. What change, if any, will occur in the deflection of the needle, and why?
Generated by claude-sonnet-4-6 · 2026-06-26 01:10 · grounding rag
Model Answer
When the compass is moved farther from the wire, the deflection of the needle will decrease.
A current-carrying wire produces a magnetic field around it. The strength of this magnetic field decreases as the distance from the wire increases. As stated in the textbook, field lines are drawn closer together where the field is stronger — meaning the field weakens with increasing distance. Since the current remains unchanged, the magnetic force acting on the compass needle becomes weaker at a greater distance, resulting in a smaller (reduced) deflection of the needle.
Source: Chapter 12 — Magnetic Effects of Electric Current, Sections 12.1 and 12.2
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Explanation
- The examiner expects you to: (1) state that deflection decreases, (2) link it to the concept that magnetic field strength decreases with distance.
- Key phrase from the chapter: "field lines are shown closer together where the magnetic field is greater" — use this to justify weaker field at greater distance.
- Do not say the field disappears; it weakens. This distinction matters.
- This question tests conceptual understanding of Activity 12.4 and field line properties from Section 12.1.