A current flows through a long straight wire from west to east. A compass is placed first directly above the wire and then directly below it. How does the direction of deflection of the compass needle differ between these two positions, and why?
Generated by claude-sonnet-4-6 · 2026-06-26 01:11 · grounding rag
Model Answer
Current flows west to east. Using the Right-Hand Thumb Rule (thumb pointing east, fingers curl around the wire):
- Above the wire: The magnetic field at that point is directed from south to north (northward). The compass needle deflects so its north pole points north (or towards north).
- Below the wire: The magnetic field at that point is directed from north to south (southward). The compass needle deflects in the opposite direction, with its north pole pointing south.
Reason: The magnetic field lines form concentric circles around the wire. Above and below the wire, the field lines run in opposite directions, so the compass needle deflects in opposite directions at the two positions.
Source: Chapter 12, Section 12.2.2 – Right-Hand Thumb Rule (Example 12.1)
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Explanation
- The key concept is the Right-Hand Thumb Rule: thumb → direction of current, curling fingers → direction of magnetic field.
- Examiners expect you to correctly state that the deflections are opposite to each other and give the rule as the reason.
- Example 12.1 in the textbook is almost identical (east-to-west current); here current is west-to-east, so directions reverse accordingly.
- Avoid vague phrases — state the actual directions (north/south) clearly for full marks.