SS3 First Term Physics Lesson Note – Magnetic Field

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WEEKS 7 & 8

MAGNETIC FIELD

CONTENTS

• Magnets and its properties
• Magnetization and demagnetization
• Temporary and permanent magnets
• Theory of magnetization
• Magnetic flux
• Earth magnetic field

MAGNET AND ITS PROPERTIES

A magnet is any material that is capable of attracting other pieces of the same material as well as pieces of iron. A substance is said to be ferromagnetic if it is attracted by a magnet. Examples are iron, cobalt, Nickel, and certain alloys. Substances which cannot be attracted by a magnet are called non-magnetic material e.g. brass, wood, copper, and glass.

Properties of magnets

1. The ends of a magnet where the attracting power is greatest are called the poles.
2. A bar magnet suspended freely in a vertical plane called magnetic meridian comes to rest with its axis in the North-South direction. The part which points northwards is called the north seeking pole or North Pole while the opposite pole is called the South Pole
3. Like poles of magnet repel each other while unlike poles attract each other.
4. The polarity of a magnet can be tested by bringing both poles in turn nearer to the known pole of a suspended magnet. Repulsion indicates similar polarity. Attraction could be due to two unlike poles or a pole and a piece of un-magnetized material. Hence, repulsion is the only sure test for polarity.

MAGNETIZATION AND DEMAGNETIZATION

Magnetization is a process whereby a material is made to become magnetic. This can be achieved through any of the following methods:

Electrical method – A cylindrical coil wound with several turns of insulated copper wire is connected in series with a six- or twelve-volt electric battery and switch. A coil of this type is called a solenoid. A steel bar is placed inside the coil and the current is switched on for some time. On removing and testing the steel, it will be found to have been

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