(i) State the rule used to find the direction of the magnetic field produced by a current-carrying straight conductor.
(ii) A horizontal power line carries current from west to east. Using the appropriate rule, determine the direction of the magnetic field at a point (a) directly above the wire and (b) directly below the wire.
(iii) How would the magnetic field change at those points if the current in the power line were doubled?
Generated by claude-sonnet-4-6 · 2026-06-26 01:11 · grounding rag
Model Answer
(i) Right-Hand Thumb Rule:
Imagine holding the current-carrying straight conductor in your right hand such that the thumb points in the direction of current. Then the fingers wrap around the conductor in the direction of the magnetic field lines.
(ii) Current flows from West to East:
Applying the right-hand thumb rule — point the thumb eastward (direction of current). The fingers curl such that the field lines form concentric circles around the wire.
- (a) Directly above the wire: The magnetic field is directed from South to North (i.e., towards the north).
- (b) Directly below the wire: The magnetic field is directed from North to South (i.e., towards the south).
(iii) Effect of doubling the current:
The magnetic field produced by a current-carrying conductor is directly proportional to the current. If the current is doubled, the strength (magnitude) of the magnetic field at both points will also double. The direction of the field remains unchanged.
Source: Chapter 12, Section 12.2.2
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Explanation
- The right-hand thumb rule is the key rule examiners expect — state it precisely with "thumb = current direction, fingers = field direction."
- For part (ii), note the question flips the direction from the textbook example (which uses east-to-west); here current is west-to-east, so directions reverse. Above: northward; below: southward.
- For part (iii), the textbook states field depends directly on current — doubling current doubles field strength; direction is unaffected. These two points (magnitude doubles, direction unchanged) are both needed for full marks.