10 min readConcept sheetSection A MCQ (1 mark) and true-or-false parts on the properties of magnets and field lines; Section B very short answer (2 marks) on the properties of field lines

Magnets, magnetic field and field lines

Like poles repel, unlike poles attract, and a freely hung magnet rests north-south. Field lines run from N to S outside the magnet and never cross.

Natural magnetLodestone, an oxide of iron (Fe₃O₄)

Magnetic fieldThe region around a magnet where its force acts, for example on a compass needle

Field linesFrom N to S outside the magnet; never cross; crowded near the poles, where the field is strongest

The EarthBehaves like a giant bar magnet, so a compass points north-south

Magnets and where they are used

Long ago people found stones that attract iron. This natural magnet is lodestone, an oxide of iron (Fe₃O₄). The property of attracting iron is magnetism.
Natural magnets are weak. Strong permanent magnets are made from steel and special alloys of iron, nickel and cobalt.
Magnets come in many shapes: bar, horseshoe, cylindrical, circular (ring) and rectangular.
They hold or lift things, and they work in motors and loudspeakers (electrical to mechanical energy) and in generators and microphones (mechanical to electrical energy).
You find them in door-bells, headphones, phone speakers, fridge doors and toys.
The end of a hanging magnet that points north is its north pole (N); the other end is its south pole (S).

Activity: properties of magnets with iron filings

You need two bar magnets, a magnetic compass, iron filings, an iron pin and a thread.

1
Tie a thread round the middle of a bar magnet and hang it from a hook. Check the direction with a compass. Observe: it always comes to rest north-south.
2
Spread iron filings on paper and bring the magnet near. Observe: most filings cling near the poles and hardly any at the middle.
3
Bring each pole of a second magnet near the hanging magnet. Observe: like poles repel and unlike poles attract.
4
Leave an iron pin touching the magnet for a while. Observe: the pin now attracts filings too. It has become a magnet.
5
Break an old bar magnet into pieces. Observe: each piece has its own N and S pole.
The two poles of a magnet can never be separated.

Five properties of magnets

A magnet attracts iron (and nickel and cobalt).
A freely hung magnet always comes to rest in the north-south direction.
Like poles repel; unlike poles attract.
A piece of iron near a strong magnet becomes a magnet itself.
The two poles of a magnet cannot be separated: every piece of a broken magnet has both poles.

Magnetic field and field lines

Place a small compass near a magnet. The needle turns and settles in one direction, because a force acts on it.
The region around a magnet in which this force acts is its magnetic field.
We show the field with magnetic field lines (lines of force). The tangent to a line at any point gives the direction of the field there.
Outside the magnet, field lines run from the north pole to the south pole. Inside, they continue from S to N, so each line is a closed loop.
Two field lines never cross. At a crossing, a compass needle would have to point two ways at once.
Field lines are crowded near the poles, which shows that the field is strongest there.

The Earth is a magnet

The Earth behaves like a giant bar magnet. Moving molten iron in its hot core makes electric currents, and these create its magnetic field. The Earth's magnetic south pole lies near the geographic north, in the Arctic. So the north pole of a compass needle points north. The magnetic poles are not at the geographic poles. The magnetic axis is tilted to the Earth's axis; your lesson gives the angle as 11.3°, and it slowly changes over the years. This is why a compass is useful for finding direction at sea, in the air and on treks.

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 ship crews and trekkers in the Western Ghats lines up with the Earth's field. Small magnets keep fridge doors and cupboard catches shut, and the speaker in your phone uses a tiny magnet.

Exam angle

1 marks: “A bar magnet hung by a thread at its middle is turned and let go. In which direction does it settle?” In the north-south direction, with its north pole pointing north.

2 marks: “Why do two magnetic field lines never cross each other?” The tangent to a field line gives the direction of the field. At a crossing there would be two directions at one point, and a compass cannot point two ways at once.

2 marks: “A bar magnet is broken into two pieces. What happens to its poles? Give one other property of a magnet.” Each piece becomes a complete magnet with its own N and S pole; poles cannot be separated. Another property: like poles repel and unlike poles attract.

Remember it

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