Page 148 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

MODULE 5: Magnetism, AC Circuits

LESSON 26-29: Terminology used in magnetic circuit,

faraday’s laws of magnetism

Objectives

At the end of this lesson you shall be able to:
• define the terms magneticfeld, magnetic line, properties or magnet
• explain permanent magnet, principle of electro magnet
• state faraday’s laws of electro magnetic induction BH curve
• explain the generation of alternating current and terms used
• explain the star and delta connection
• explain three phase power measurements.

A substance that attracts pieces of iron and steel is called a magnet. This property of the material is called
magnetism. Magnetism is a force field that acts on some material and not on other material. Physical devices
which posses this force are called magnets.
Earths magnetic field
Our earth itself is a huge magnet with its North Pole lies on the geometrical South Pole. Moving molten iron in
Earth’s outer core causes most of Earth’s magnetic field. Magnetic field poles are NOT aligned with geographic
poles.
Properties of magnets
1 Magnets attract objects of iron, cobalt and nickel.
2 The force of attraction of a magnet is greater at its poles than in the middle.
3 Like poles of two magnets repel each other.
4 Opposite poles of two magnets attracts each other.
5 If a bar magnet is suspended by a thread and if it is free to rotate, its South Pole will move towards the North
Pole of the earth and vice versa.
Classification of magnets
Magnets are classified into two groups
1 Natural magnet
2 Artificial magnet
Temporary magnet (Electromagnet)
Permanent magnet
Natural magnet
Natural magnet occurs as an ore of iron called magnetite. Magnetite has magnetic properties and behaves like a
weak magnet. Its called lodestone.
Two thousand years ago people of ancient Greece and China discovered that a lodestone would always align
itself in a longitudinal direction if it was allowed to rotate freely. This property of lodestones allowed for the creation
of compasses.
Electromagnets
An iron core made into a magnet by the passage of electric current through a
coil surrounding it.
Permanent magnet
Different applications need magnets of different shapes and strength. The artificially prepared powerful magnets
of different shapes are called artificial magnets. Nickel, Steel, ALNICO, TUNGSTEN are used to make permanent
magnets.

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