Page 149 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Application of permanent magnet
Permanent magnets are used in microphone, loud speaker, magnetic compass, PMDC motor, PMMC measuring
instruments etc.
Application of electromagnet

Electromagnets are used in electric bell, motor, maglev train, relay, MRI scanning etc.
Advantages
• Their strength can be increased or decreased as per requirement.
• They can be switched ON and OFF when required
• We can easily change the polarity of the magnet.

• Electromagnet can develop stronger magnetic field than permanent magnet.
Sure test of magnetism
A magnet can attract the opposite pole of other magnets well as the magnetic substance (iron cobalt nickel). So,
if A magnet attracts another object, we cannot say that the other object is a magnet. But if the magnet repels the
object, then we can surely say that the other object is a magnet. Because “Like poles of a magnet repel each
other”.

Magnetic substances
Magnetic substances are categorized into three groups as follows:
1 Ferromagnetic substances
Those substances which are strongly attracted by a magnet are known as ferromagnetic substances.

e.g., iron, nickel, cobalt, steel and their alloys
2 Paramagnetic substances
Those substances which are slightly attracted by a magnet are known as ferromagnetic substances.
E.g., air, aluminium, manganese, platinum, copper etc

3 Diamagnetic substances
Those substances which are slightly repelled by a magnet are known as ferromagnetic substances.
e.g., bismuth, graphite, glass, paper, wood, water etc Bismuth is the strongest diamagnetic substance.

Ferromagnetic Paramagnetic Diamagnetic
Type magnetic material
Substances Substances Substances
Magnetic susceptance, K positive positive negative

Relative permeability, μr Much greater than unity Slightly greater than unity Slightly less than unity

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CITS : Power - Electrician & Wireman - Lesson 26-29