Study Notes: Fun with Magnets

Table of Contents
1. Introduction
2. How Magnets Were Discovered
3. Magnetic and Non-magnetic Materials
4. Poles of a Magnet
5. Finding Directions using a Magnet
6. Make Your Own Magnet
7. Attraction and Repulsion between Magnets
8. A Few Cautions about Magnets
View more

Introduction

Imagine walking into a scrapyard full of metal junk. A large crane moves over a pile and, as if by magic, pieces of iron begin to stick to it. This happens because the crane carries a magnet.

Magnets are used in many ordinary places: on refrigerator doors as fridge magnets, in boxes that close without a lock, and in machines that lift heavy iron scrap. Magnets hold things together or cause movement without any visible contact, using an invisible influence called the magnetic force.

How Magnets Were Discovered

Long ago, people noticed certain naturally occurring stones that could attract pieces of iron. One story from ancient Greece tells of a shepherd called Magnes whose iron-tipped staff stuck to such a stone. The stone that attracted iron was later called magnetite. Because of this association the words magnet and magnetite came into use; the name may also be linked to the place Magnesia where such rocks were found.

Later, people learned to make artificial magnets from iron and other materials and to make magnets in shapes such as bar magnets and horseshoe magnets.

Magnetic and Non-magnetic Materials

Materials can be divided into two groups according to whether they are affected by a magnet.

• Magnetic materials: These are materials that are attracted to a magnet. Common examples are iron, nickel and cobalt. Small objects made of these metals will move toward a magnet.
• Non-magnetic materials: These materials are not attracted to a magnet. Examples include wood, plastic, glass and cloth. These materials do not stick to a magnet even when brought very close.

Poles of a Magnet

A magnet has two special regions called poles. These are the places where the magnetic force is strongest.

When you sprinkle iron filings around a magnet they arrange themselves along the magnetic lines of force and collect more densely near the ends of the magnet. These ends are the poles.

The two poles are named the North-seeking pole (or North pole) and the South-seeking pole (or South pole).

The shape of a magnet does not change the fact that it has two poles. Whether a magnet is a straight bar, horseshoe, ring or any other shape, there will still be regions where the magnetic effect is strongest; these are its poles.

Finding Directions using a Magnet

A magnet that is free to rotate will come to rest pointing along the North-South direction. The end that points towards the geographic north is called the north-seeking end and the other end is the south-seeking end.

Historical examples show early use of this property. For instance, an account from China mentions Emperor Hoang Ti and a chariot that always pointed south, a device used for direction finding.

The compass is a practical device based on this behaviour. A small magnetised needle in a compass turns until it points to the north-south line, helping travellers and sailors to find direction. The north end of the needle is often coloured to make it easy to read.

Make Your Own Magnet

A simple way to convert a piece of iron into a magnet is by stroking it with an already magnetised bar magnet. The experiment below describes the commonly used stroking method.

1. Take a rectangular piece of iron (or an iron nail, needle or blade) and place it on a table.
2. Hold a bar magnet and place one of its poles near one end of the iron piece.
3. Without lifting the bar magnet from the surface of the iron piece, move it along the length of the iron piece until you reach the other end.
4. Lift the magnet, return the same pole to the starting point, and repeat the motion in the same direction along the iron piece.
5. Repeat this stroking action about 30-40 times, keeping the same pole and the same direction of movement each time.
6. Check whether the iron piece has become magnetised by bringing a pin or some iron filings close to it.
7. If it is not yet magnetic, continue stroking more times, maintaining a consistent pole and direction of motion.
8. This method works for converting small iron objects such as nails, needles or blades into temporary magnets.

Attraction and Repulsion between Magnets

• Every magnet has two poles: a north pole and a south pole.
• Opposite poles attract: a north pole and a south pole pull towards one another.
• Like poles repel: two north poles or two south poles push away from each other.
• If two magnets are placed with opposite poles facing each other they will move together because of attraction. If they are placed with like poles facing each other they will move apart because of repulsion.
• This behaviour can be observed using suspended magnets or small toy models. For example, two toy cars each carrying a bar magnet will move towards one another if opposite poles face each other and will move apart if like poles face.

A Few Cautions about Magnets

• Magnets can lose their magnetic properties if they are heated, hammered or dropped from a height.
• Magnets may become weaker with time if not stored properly.
• Bar magnets are often stored in pairs, keeping unlike poles on the same side and using a thin piece of wood between them as a simple separation method.
• Putting a piece of soft iron (called a keeper) across the ends of bar magnets helps to preserve their strength.
• For horseshoe magnets, keeping a piece of iron across the poles gives protection and helps maintain magnetism.
• Keep magnets away from sensitive electronic items such as cassette tapes, mobile phones, televisions, music systems, compact discs (CDs), and computers because strong magnets can interfere with or damage magnetic storage and electronic components.