7 min readConcept sheet1-mark MCQ and assertion-reason, 2-mark properties of field lines, and the bar magnet diagram

Magnetic field and field lines

A magnetic field is the region around a magnet where its force can be felt. Field lines run from north to south outside the magnet and never cross.

Magnetic fieldRegion around a magnet where its force can be detected; it has both size and direction

Direction of a field lineThe way the north pole of a compass needle points

Path of field linesN to S outside the magnet; S to N inside it

StrengthWhere lines are closer together, the field is stronger

Magnets, compasses and fields

A compass needle is a small bar magnet. The end that points north is its north pole; the other end is its south pole. Like poles repel and unlike poles attract. The region around a magnet where its force can be felt is its magnetic field. A magnetic field has both strength and direction. We show it with magnetic field lines. The direction of a field line at any point is the direction in which the north pole of a compass would point there.

Activity: seeing field lines with iron filings and a compass

1
Fix a sheet of white paper on a drawing board and place a bar magnet in the middle.
2
Sprinkle iron filings evenly around the magnet and tap the board gently. Observation: the filings form curved lines from one pole to the other, crowding near the poles.
3
Remove the filings. Place a small compass near the north pole and mark the two ends of its needle with dots.
4
Move the compass so that its south end sits on the dot where its north end was. Mark the new position of the north end.
5
Repeat until you reach the south pole, then join the dots with a smooth curve. Draw more lines from other starting points.
6
Observation: each line runs from N to S outside the magnet, and the lines never cross.

Properties of magnetic field lines

Outside the magnet they go from the north pole to the south pole. Inside the magnet they go from south to north.
They are closed curves.
They are closer together where the field is stronger, as near the poles.
No two field lines ever cross. At a crossing, a compass would have to point in two directions at once, which is impossible.

How to draw the field lines of a bar magnet

Draw a rectangle for the magnet and label its ends N and S.
Draw 6 to 8 smooth curves that leave the N end and bend round to enter the S end, on both sides of the magnet.
Keep the lines close together near the poles and further apart away from them.
Put an arrow on every line pointing from N towards S outside the magnet.
Make sure no two lines touch or cross. Title it 'Field lines around a bar magnet'.

In real life

A compass used by trekkers in the Western Ghats and by ship crews is a tiny magnet that lines up with the Earth's magnetic field. Fridge-door seals and cupboard catches stay shut with small magnets.

Exam angle

2 marks: “Why do two magnetic field lines never intersect each other?” At a crossing point the compass needle would point in two directions at once, which is impossible. The field has only one direction at each point.

3 marks: “List three properties of magnetic field lines and draw the field lines around a bar magnet.” N to S outside and S to N inside; closed curves; closer where the field is stronger; never cross. Diagram with N and S labelled and arrows on every line.

1 marks: “What does the closeness of magnetic field lines show?” The relative strength of the field: the closer the lines, the stronger the field.

Remember it

North Out, South In (outside the magnet), and field lines never cross.