Page 151 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

• Each molecule of a magnetic substance is a complete magnet.

• In an un-magnetised substance, the molecular magnets are arranged randomly.
• Hence the magnetic effect of any one of them is cancelled by its neighbour.
• So the entire bar does not exhibit any net magnetic property.
• When the substance is magnetized, all the tiny molecular magnets arrange themselves in parallel lines

• When the substance is heated or hammered, the molecular magnets get kinetic energy and the straight line
get disturbed, thus losing the magnetic property.
• Molecular theory of magnetism was given by weber and modified by Ewing.
Magnetic field

The space around the magnet in which a magnetic pole experience a force is called a magnetic field. The
magnetic field is strongest near the pole and goes decreasing as we move away from the magnet.
Magnetic lines of force

The magnetic field around a magnet is represented by imaginary lines is called magnetic lines of force. magnetic
lines of force would emerge from N-pole of the magnet, pass through the surrounding medium and re-enter the
S-pole. Inside the magnet each lines passes from S-pole to N- pole.

Properties of magnetic lines of force
1 Magnetic lines of force start from the North Pole and end at the South Pole.

2 They are continuous through the body of magnet
3 Magnetic lines of force can pass through iron more easily than air.
4 Two magnetic lines of force do not intersect each other.
5 They tend to contract longitudinally.

6 They tend to expand laterally.
Magnetic circuit
Just as a closed path followed by electric current is called electric circuit, similarly closed path followed by magnetic
flux is called magnetic circuit. Magnetic circuit usually consist of material having high permeability.
Magnetic flux
The amount of magnetic field produced by a magnetic source is called magnetic flux. Magnetic flux is denoted by
Greek letter ɸ. If 10 magnetic lines come out of the north pole or enter the south pole of a magnet, then magnetic
flux ɸ = 10 lines or Maxwell’s. The SI unit of magnetic flux is weber.
1 wb=108Maxwell’s

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