Chapter 12 · Question 1

What is a magnetic field? Explain how magnetic field lines can be drawn around a bar magnet using a compass needle. List the properties of magnetic field lines.

Answer Revealed

Direct Answer:

A magnetic field is the region surrounding a magnet within which the force of the magnet can be detected. To draw field lines, place a bar magnet on paper and mark its boundary. Place a compass near the north pole — the south pole of the compass points towards it. Mark both ends of the needle, then move the compass so that its south pole occupies the spot previously held by its north pole. Repeat until reaching the south pole, then join the marks with a smooth curve. Properties: (i) field lines emerge from the north pole and merge at the south pole outside the magnet; inside the magnet, they go from south to north, forming closed curves; (ii) the degree of closeness of field lines indicates the relative field strength — they are most crowded near the poles; (iii) no two field lines intersect each other, because at the point of intersection the compass needle would have to point in two directions, which is impossible.

Simple Explanation

A magnetic field is the invisible zone around a magnet where its force can be felt. To map it, place a compass near the north pole of a bar magnet. The compass needle aligns itself with the field — mark where the needle points, then move the compass so its back end sits on the old mark. Keep doing this until you reach the south pole of the magnet, then connect the dots. The lines you draw have three key rules: they always go north to south outside the magnet and form closed loops, they are packed tightest near the poles (where the field is strongest), and they never cross — because a compass can only point one way at a time.

Exam-Ready Structure

The concept of a magnetic field and its representation through field lines is the foundational tool for understanding all magnetic phenomena. 1. Magnetic field: The region surrounding a magnet in which the force of the magnet can be detected is called its magnetic field. A magnetic field is a vector quantity — it has both direction and magnitude. The direction of the magnetic field at a point is taken as the direction in which the north pole of a compass needle moves when placed at that point. 2. Drawing field lines (Activity 12.3): (a) Place a bar magnet on a sheet of white paper fixed to a drawing board and mark its boundary. (b) Place a small compass near the north pole of the magnet. The south pole of the compass needle points towards the north pole of the magnet, while the north pole of the compass is directed away from it. (c) Mark the position of both ends of the compass needle. (d) Move the compass to a new position such that its south pole occupies the position previously occupied by its north pole. (e) Proceed step by step in this manner until the south pole of the magnet is reached. (f) Join the points marked on the paper with a smooth curve — this curve represents a magnetic field line. (g) Repeat the procedure to draw multiple lines. 3. Properties of magnetic field lines: (a) Outside the magnet, field lines emerge from the north pole and merge at the south pole. Inside the magnet, the direction is from the south pole to the north pole. Thus, magnetic field lines are closed curves. (b) The relative strength of the magnetic field is shown by the degree of closeness of the field lines. The field is stronger at the poles where the field lines are crowded. (c) No two field lines intersect each other. If they did, it would mean that at the point of intersection the compass needle would point in two different directions simultaneously, which is not possible. (d) The deflection of the compass needle increases as it is moved along a field line towards the poles.

Key Points

Magnetic field: the region around a magnet where its magnetic force can be detected
Field line direction outside magnet: north pole to south pole; inside: south to north pole (closed curves)
Field line closeness indicates field strength — most crowded near the poles
No two magnetic field lines ever intersect (compass cannot point in two directions at once)
Compass needle deflection increases as it moves towards the poles along a field line

What is a magnetic field? Explain how magnetic field lines can be drawn around a bar magnet using a compass needle. List the properties of magnetic field lines.