This is the magnetization of a material when it is brought close to or touching a magnet.

The figure below illustrates magnetic induction.

• The first nail becomes magnetized with a north pole at the end touching the South Pole. Other nails can now be attached as shown.
• When an object is attracted to a magnet, it also becomes an induced magnet. The pole induced at the end touching the magnet is opposite to the inducing pole.

Therefore a material which cannot become a magnet cannot also be attracted to a magnet

Some materials can keep their induced magnetism for long while others lose their magnetism as soon as they are taken away from the magnet.

Magnetic and non-magnetic materials

1. Magnetic materials

These are materials that can be attracted to a magnet and therefore can be magnetized.

There are two types of magnetic materials;

1. Hard magnetic materials

They are not easily magnetized and do not easily lose their magnetism once magnetized.

They are used in making permanent magnets. Examples of hard magnetic materials include steel and alcomax.

2. Soft magnetic materials

They are easily magnetized and easily demagnetized. They are used in making electromagnets. Examples of some magnetic materials include soft iron, cobalt and mumetal.

2. Non-magnetic materials

These are materials that cannot be attracted to a magnet and therefore cannot be magnetized. Examples include brass, copper, wood, plastic, glass, etc.

Magnetic fields

A magnetic field is the space around a magnet or a current-carrying conductor where the magnetic force is felt.

A magnetic field is mapped by magnetic field lines carrying arrows. The arrows indicate the direction of the magnetic force. The total number of magnetic field lines in a given area is called magnetic flux.

MAPPING MAGNETIC FIELD LINES

Magnetic field lines around a magnet are invisible but experimental techniques can be used to reveal them.

Plotting magnetic field lines using iron filings

The magnet is placed under a cardboard paper. The iron filings are sprinkled on the cardboard paper. On tapping gently, the iron filings align themselves along lines which correspond to the field lines of the magnet.

The iron filing method is good for highly curved magnetic fields but the field has to be strong enough to displace the iron filings.

Plotting magnetic field lines using the plotting compass

A plotting compass consists of a tiny magnetic needle, supported by a spindle through its centre so that it can turn freely. The pointed end is the North Pole while the other end is the South Pole. Therefore when placed in a magnetic field, the pointed end is always pointing towards the South Pole of the field.

The figure below shows how the magnetic field around a bar magnet can be mapped using a plotting compass.

• The compass is placed near one end of the magnet and penciled dots are made on the paper on which the magnet is placed, to mark the positions of the ends of the needle.
• The compass is then moved so that its needle lines up with the previous dot made, and so on. The dots are then connected with a line.
• By plotting different field lines, it is possible to build up the pattern of the magnetic field around the magnet.