CHAPTER ONE
MAGNETISM
Introduction
Magnets are substances that are able to attract and hold items. Lodestone is the only known
natural magnet which was discovered by the Chinese 2,000 years ago. Other magnets produced
artificially by man are called artificial magnets.
Magnets and non-magnetic materials
Magnetic materials are those that are strongly attracted by magnets while non-magnetic
ones are those that are not affected by magnets. Iron, steel, cobalt and nickel are magnetic
substances, while wood, glass and copper are examples of non-magnetic substances.
Substances that are repelled by magnets are said to be diamagnetic whereas those which are
strongly attracted i.e. iron, nickel, cobalt are called ferromagnetic materials. The materials
that are so lightly attracted such that the magnet seems to have no effect on them are called
paramagnetic materials (mostly non-magnetic materials). Ferrites are a mixture of iron oxide
and barium oxide are the most newly developed magnetic materials. Ceramic magnets or
magnadur magnets are made from ferrites and are very strong.
Properties of magnets
1. They are double poled substances with both the North and South poles.
2. Like poles repel and unlike poles attract. Repulsion is a sure method of determining whether
two substances are magnets.
3. The greatest magnetic force is concentrated around the poles of a magnet.
Magnetic field patterns.
Magnetic field is the space around a magnet where magnetic field (force) is observed.
Plotting field patterns
A line of force gives the direction of the magnetic field at each point along it. Their closeness
is a measure of the strength of the magnetic field or of the force that would be exerted by the
bar magnet.
Examples of field patterns.
,
,The points marked ‘X’ are called neutral points where there is no magnetic field at such points.
Watches (non-digital), electron beams in cathode ray tubes and TV sets are shielded from external
magnetic fields by placing a soft-iron cylinder around the neck of the tube or watch.
Making magnets
The following are methods used to make magnets.
a) Magnetic induction – this is a process by which magnets are made by placing
ferromagnetic materials in a magnetic field. Materials like iron lose their magnetism
easily and are said to be soft while others like steel gain magnetism slowly but retain it
longer and are therefore said to be hard and are used to make permanent magnets.
, b) Magnetizing by stroking – the object to be magnetized is placed on a bench then a bar
magnet is dragged along the length of the bar from one end to the other. This is repeated
several times and the object becomes magnetized. This method is known as
single -stroke method.
c) Magnetizing using an electric current – this is the use of magnetic effect of an electric
current through a solenoid (insulated wire of many turns).
Demagnetizing
Demagnetizing is the process of removing magnetic properties of a magnet. The following
methods are which a magnet can lose its magnetism;
a) Hammering them hard with their poles facing E-W direction
b) Heating them strongly
c) Placing a magnet inside a solenoid and passing an a.c. current through it for a short time.
MAGNETISM
Introduction
Magnets are substances that are able to attract and hold items. Lodestone is the only known
natural magnet which was discovered by the Chinese 2,000 years ago. Other magnets produced
artificially by man are called artificial magnets.
Magnets and non-magnetic materials
Magnetic materials are those that are strongly attracted by magnets while non-magnetic
ones are those that are not affected by magnets. Iron, steel, cobalt and nickel are magnetic
substances, while wood, glass and copper are examples of non-magnetic substances.
Substances that are repelled by magnets are said to be diamagnetic whereas those which are
strongly attracted i.e. iron, nickel, cobalt are called ferromagnetic materials. The materials
that are so lightly attracted such that the magnet seems to have no effect on them are called
paramagnetic materials (mostly non-magnetic materials). Ferrites are a mixture of iron oxide
and barium oxide are the most newly developed magnetic materials. Ceramic magnets or
magnadur magnets are made from ferrites and are very strong.
Properties of magnets
1. They are double poled substances with both the North and South poles.
2. Like poles repel and unlike poles attract. Repulsion is a sure method of determining whether
two substances are magnets.
3. The greatest magnetic force is concentrated around the poles of a magnet.
Magnetic field patterns.
Magnetic field is the space around a magnet where magnetic field (force) is observed.
Plotting field patterns
A line of force gives the direction of the magnetic field at each point along it. Their closeness
is a measure of the strength of the magnetic field or of the force that would be exerted by the
bar magnet.
Examples of field patterns.
,
,The points marked ‘X’ are called neutral points where there is no magnetic field at such points.
Watches (non-digital), electron beams in cathode ray tubes and TV sets are shielded from external
magnetic fields by placing a soft-iron cylinder around the neck of the tube or watch.
Making magnets
The following are methods used to make magnets.
a) Magnetic induction – this is a process by which magnets are made by placing
ferromagnetic materials in a magnetic field. Materials like iron lose their magnetism
easily and are said to be soft while others like steel gain magnetism slowly but retain it
longer and are therefore said to be hard and are used to make permanent magnets.
, b) Magnetizing by stroking – the object to be magnetized is placed on a bench then a bar
magnet is dragged along the length of the bar from one end to the other. This is repeated
several times and the object becomes magnetized. This method is known as
single -stroke method.
c) Magnetizing using an electric current – this is the use of magnetic effect of an electric
current through a solenoid (insulated wire of many turns).
Demagnetizing
Demagnetizing is the process of removing magnetic properties of a magnet. The following
methods are which a magnet can lose its magnetism;
a) Hammering them hard with their poles facing E-W direction
b) Heating them strongly
c) Placing a magnet inside a solenoid and passing an a.c. current through it for a short time.
