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MAGNETISM

Physics

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 artificia l 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.

  1. 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.
  2. 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.

  1. 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;

  1. a) Hammering them hard with their poles facing E-W direction
  2. b) Heating them strongly
  3. c) Placing a magnet inside a solenoid and passing an a.c. current through it for a short time.

 

Caring for magnets

  1. a) Magnets should be stored in pairs with unlike poles adjacent to each other attached to pieces of soft iron called keepers.
  2. b) Magnets should not be hammered especially with their poles facing E-W direction.
  3. c) Magnets should not be heated strongly or dropped roughly on hard surfaces.
  4. d) Magnets should not be placed near alternating currents.
  5. e) Magnets should be kept dry and clean since rust can make them lose their magnetism.

 

Uses of magnets

  1. Used in making other magnets
  2. Used in making loud speakers
  3. Used in making moving coil meters
  4. Used in making telephone speakers.

 

Domain theory of magnetism.

In ferromagnetic substances small atomic magnets form large groups called domains. These atomic magnets face one direction where the direction varies from one domain to another. In an un-magnetized crystal the directions of these domains are different hence their resultant magnetism is zero.

 

When a magnetic material is placed in a magnetic field the atomic magnets rotate and eventually all domains face the same direction. When this happens then the material becomes magnetized. When a material is magnetized we say it is saturated. This means that the magnetism of the material cannot be increased by any other method and this is the domain theory of magnetism.

 

See also:

CELLS AND SIMPLE CIRCUITS

ELECROSTATICS I

RECTILINEAR PROPAGATION AND REFLECTION AT PLANE SURFACES. INTRODUCTION

HEAT TRANSFER

THERMAL EXPANSION

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