Science (086) — CBSE Class 10 board question

(a) [Diagram to be drawn in answer sheet]

The magnetic field lines should be drawn as follows:

• Point 1 (near P): Concentric circles around the wire where it passes through hole P.
• Point 2 (centre of loop): Straight parallel lines through the centre of the loop (all pointing in the same direction).
• Point 3 (near Q): Concentric circles around the wire where it passes through hole Q (direction opposite to those at P).

(b) The intensity of the magnetic field at the centre of a circular loop depends on:

1. The magnitude of current flowing through the loop (field increases with current).
2. The number of turns of the coil (field is n times for n turns).

(c) Right-Hand Thumb Rule — To determine the direction of the magnetic field produced by a current-carrying straight conductor.

Source: Chapter 12, Sections 12.2.1 and 12.2.3

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• (a) The key point: at the centre of the loop, the arcs appear as straight lines (all contributions add up in the same direction). Near P and Q, the wire is straight, so field lines are concentric circles. Examiners want the contrast between straight-line field at centre vs. concentric circles near holes.
• (b) From the textbook: field depends on current AND number of turns. A common error is writing "radius/distance" — while valid in general, the textbook specifically highlights current and number of turns for the centre of a circular loop.
• (c) "Right-Hand Thumb Rule" is specifically mentioned for straight conductors. Don't confuse with Fleming's Left-Hand Rule (for force on conductor) or the Right-Hand rule for circular loops.