Exploring Magnets
Magnetic and Non-Magnetic Materials
A magnet is an object that attracts materials made of iron, and a few other metals, towards it. Magnets were first discovered as a natural rock called magnetite (an ore of iron) found long ago near a place called Magnesia — which is how magnets got their name.
Materials are of two kinds, based on how a magnet affects them:
- Magnetic materials — those that are attracted by a magnet. Iron, nickel and cobalt are magnetic. Most steel objects (steel contains iron) are also attracted.
- Non-magnetic materials — those that are not attracted by a magnet. Wood, plastic, paper, rubber, cloth, glass, copper and aluminium are non-magnetic.
You can test any object by bringing a magnet close to it: if it is pulled towards the magnet, it is magnetic. This simple property is very useful — for example, in scrapyards a large magnet is used to lift and separate iron objects from a heap of mixed waste, leaving the non-magnetic materials behind.
Magnets are made in different shapes for different uses: a bar magnet, a horseshoe (U-shaped) magnet, a cylindrical magnet and a ring magnet are common ones.
Magnetic materials are those attracted by a magnet — mainly iron and steel (which contains iron).
- Iron nail — magnetic.
- Steel spoon — magnetic (steel contains iron).
- Plastic ruler, rubber band, copper wire — non-magnetic.
Only magnetic materials are attracted by a magnet.
- The magnet is brought near the heap.
- Iron and steel objects are pulled to the magnet and lifted away.
- Non-magnetic materials (plastic, glass, wood) are left behind.
If a magnet does not attract a metal, that metal is non-magnetic.
- The coin is not made mainly of iron, nickel or cobalt.
- It is likely made of a non-magnetic metal such as aluminium or a copper alloy.
Key Points
- A magnet attracts iron and a few other metals; natural magnet = magnetite.
- Magnetic materials (iron, nickel, cobalt, steel) are attracted; non-magnetic materials (wood, plastic, copper, aluminium, glass) are not.
- Test an object by bringing a magnet near it — attraction means it is magnetic.
- Common shapes: bar, horseshoe, cylindrical and ring magnets.
Poles of a Magnet
If you dip a bar magnet into a tray of iron filings and lift it out, you will see that the filings cling mostly at the two ends and very little in the middle. These two ends, where the magnet’s pull is strongest, are called the poles of the magnet.
Every magnet has two poles — a north pole (N) and a south pole (S). The pull of a magnet is strongest at the poles and weakest in the middle.
A remarkable property: when a magnet is hung freely so that it can swing, it always comes to rest pointing in the north–south direction. The end that points towards the Earth’s geographic north is called the north-seeking pole (north pole), and the other is the south-seeking pole (south pole). This is the basis of the compass.
You cannot get a magnet with only one pole. If you break a bar magnet into two pieces, you do not get a separate N piece and a separate S piece — instead, each smaller piece becomes a complete magnet with its own north and south pole.
The strength of a magnet is not the same everywhere.
- The pull is strongest at the two ends, called the poles.
- So more filings cling at the ends and few in the middle.
A single pole cannot exist on its own.
- Each broken piece becomes a complete magnet.
- So each piece has two poles — one north and one south.
The poles are named by the direction they seek.
- The end pointing to geographic north is the north-seeking pole.
- This is simply called the north pole (N).
Key Points
- The poles are the two ends of a magnet where its pull is strongest.
- Every magnet has a north pole and a south pole; a single pole cannot exist alone.
- A freely hung magnet rests in the north–south direction.
- Breaking a magnet gives two smaller magnets, each with both poles.
Attraction and Repulsion
When the poles of two magnets are brought near each other, they either pull together or push apart, depending on which poles face each other. This gives us the basic law of magnetic poles:
- Like poles repel — two north poles (N–N), or two south poles (S–S), push away from each other.
- Unlike poles attract — a north pole and a south pole (N–S) pull towards each other.
This pushing and pulling happens even without the magnets touching, because a magnet has an invisible region of influence around it called its magnetic field.
There is an important difference between attraction and repulsion. A magnet attracts a piece of ordinary iron (which is not itself a magnet) and attracts the opposite pole of another magnet. But repulsion happens only between two magnets. So if object A pushes object B away, both A and B must be magnets. This makes repulsion the sure test for finding out whether an object is truly a magnet.
Apply the law of poles.
- Both are north poles — these are like poles.
- Like poles repel.
Think about what each behaviour means.
- A magnet attracts both ordinary iron and the opposite pole of another magnet — so attraction alone does not prove the other object is a magnet.
- Repulsion happens only between two magnets.
- So if two objects repel, both must be magnets.
Use the sure test for magnets.
- They repel, and only two magnets can repel.
- So both P and Q are magnets, with like poles facing each other.
Key Points
- Like poles repel (N–N, S–S); unlike poles attract (N–S).
- Magnets act without touching, through their magnetic field.
- Attraction can occur with ordinary iron, but repulsion happens only between two magnets.
- Therefore repulsion is the sure test for a magnet.
The Magnetic Compass
Because a freely suspended magnet always points north–south, people long ago used magnets to find direction. This led to the invention of the magnetic compass — a small, light, magnetised needle that is balanced on a pivot so it can turn freely inside a case marked with directions.
The compass needle is itself a tiny magnet. When left free, its north pole points towards the Earth’s geographic north, so you can read off all the directions — north, south, east and west. Sailors, travellers and explorers used the compass to find their way long before modern devices existed.
You can even make a temporary magnet by stroking a steel needle many times in the same direction with one pole of a bar magnet. The needle becomes magnetised and, if floated on a piece of cork in water, behaves like a compass.
Magnets must be cared for, because they can lose their magnetism if they are dropped, hammered, heated strongly, or stored carelessly. Bar magnets should be stored in pairs with unlike poles together and a small soft-iron piece (a ‘keeper’) across the ends; a horseshoe magnet is stored with a single keeper across its poles.
The needle is a small, freely turning magnet.
- A free magnet always settles in the north–south direction.
- So the needle’s north pole points to the Earth’s north, giving a fixed direction.
Stroking transfers magnetism to the steel.
- Stroke the needle many times with one pole of the bar magnet.
- Always stroke in the same direction, lifting the magnet away each time.
- The needle becomes a magnet.
Rough treatment weakens a magnet.
- Dropping or hammering it.
- Heating it strongly (or storing it carelessly).
Key Points
- A magnetic compass is a freely turning magnetised needle that shows direction.
- The needle’s north pole points to geographic north.
- A steel needle can be magnetised by stroking in one direction with a magnet.
- Magnets lose magnetism if dropped, hammered or heated; store bar magnets in pairs with keepers.