A vertical straight wire carries current flowing upward. What is the direction of the magnetic field lines at a point to the east of the wire (as seen from above)? State the rule you used and explain how you arrived at your answer.
Generated by claude-sonnet-4-6 · 2026-06-26 01:09 · grounding rag
Model Answer
Rule Used: Right-Hand Thumb Rule
If we hold the wire in the right hand with the thumb pointing upward (direction of current), the fingers curl around the wire showing the direction of magnetic field lines.
As seen from above, the field lines form concentric circles around the wire in the anticlockwise direction. At a point to the east of the wire, the tangent to this anticlockwise circle points toward the south.
∴ The direction of the magnetic field at a point to the east of the wire is toward the south.
Source: Chapter 12, Section 12.2.2 — Right-Hand Thumb Rule
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Explanation
- Examiners expect: (1) naming the rule, (2) applying it correctly, (3) stating the final direction clearly.
- Visualise looking down from above: current comes upward (out of the page). By the right-hand thumb rule, fingers curl anticlockwise. At the east side of the wire, the anticlockwise tangent points south.
- A common mistake is saying "north" — remember anticlockwise rotation means the east point has a southward tangent.
- Always state the rule by name and briefly explain the hand orientation for full marks.