In order to obtain magnetic field lines around a bar magnet, a student performed an experiment using a magnetic compass and a bar magnet. The magnet was placed on a sheet of white paper fixed on a drawing board. Using magnetic needle he obtained on the paper a pattern of magnetic field lines (as shown below) around the bar magnet.
Answer the following sub-parts based on the experiment and the magnetic field pattern shown.
Generated by claude-sonnet-4-6 · 2026-06-15 06:50 · grounding stimulus
Model Answer
(a) By convention, a free north pole (isolated) would move from the north pole of a magnet to its south pole through the external region. Since field lines show the direction of force on a free north pole, they are drawn emerging from N and merging at S.
(b) Greater the closeness (crowding) of field lines, stronger is the magnetic field in that region. Where field lines are widely spaced, the field is weaker.
(c)(i) If two field lines intersected, a compass needle placed at that point would point in two directions simultaneously, which is impossible. Hence, no two field lines can ever intersect.
(ii) Uniform magnetic field — parallel, equidistant, straight field lines all pointing in the same direction:
$$\longrightarrow \quad \longrightarrow \quad \longrightarrow$$
$$\longrightarrow \quad \longrightarrow \quad \longrightarrow$$
$$\longrightarrow \quad \longrightarrow \quad \longrightarrow$$
Source: Chapter – Magnetic Effects of Electric Current, Section – Magnetic Field and Field Lines
---
Explanation
- (a) The convention follows the direction of force on an imaginary free north pole — examiners look for this specific reason.
- (b) Key phrase: closer lines = stronger field; this is a direct one-liner the examiner expects.
- (c)(i) The standard reason is about the compass needle pointing in two directions — always state this clearly.
- (c)(ii) Parallel, equally spaced, straight arrows in the same direction represent a uniform field. Even a rough sketch with arrows scores full marks if properly drawn.