Draw a diagram to show the pattern of magnetic field lines on a horizontal sheet of paper due to a straight conductor passing through its centre and carrying current vertically upwards. Mark on it (i) the direction of current in the conductor and (ii) the corresponding magnetic field lines. State right hand thumb rule and check whether the directions marked by you are in accordance with this rule or not.
Generated by claude-sonnet-4-6 · 2026-06-15 06:50 · grounding rag
Model Answer
Diagram:
```
↑ (current upward, marked with dot •)
← ⊙ →
(anti-clockwise concentric circles)
```
The magnetic field lines form concentric circles around the conductor, directed anti-clockwise when current flows vertically upward (out of the page, shown by ⊙).
(i) Direction of current: upward / out of the page (⊙)
(ii) Magnetic field lines: concentric circles, anti-clockwise
Right-Hand Thumb Rule: If a current-carrying straight conductor is held in the right hand such that the thumb points in the direction of current, then the curled fingers show the direction of magnetic field lines around the conductor.
Verification: With the thumb pointing upward (current direction), the fingers curl anti-clockwise — this matches the directions marked in the diagram. Hence, the directions are in accordance with the right-hand thumb rule.
Source: Chapter 12, Section 12.2.2
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Explanation
- The diagram must show ⊙ (dot) at the centre to indicate current coming out/upward, and concentric circles going anti-clockwise.
- Examiners award 1 mark for the correct diagram, 1 mark for stating the rule correctly, and 1 mark for the verification.
- The key phrase in the rule is: "thumb → current direction; curled fingers → field direction."
- Do not confuse anti-clockwise (current upward) with clockwise (current downward).