Magnetism

What is Magnetism?

 Magnetism : The property due to which a substance attracts iron pieces towards it, is called magnetism.

Magnet: The substance having property of magnetism, is called magnet.

Types of Magnets

Types of Magnets

(A) Permanent (Natural & Artificial magnets)

(B) Temporary (Electromagnets)

 (A)   Permanent (Natural & Artificial magnets)

Natural magnets

Definition : Pieces of naturally occurring iron ore lode–stone or magnetic or black iron oxide (Fe2O3), are called natural magnets.

Properties : These have following two properties.

(i) Attractive property: They attract small iron pieces towards them.

(ii) Directive property: When suspended freely, their ends would point in geographical north–south direction. For this reason, the suspended piece is called lode stone or leading stone.

Disadvantage : These have following two demerits :

(i) They have irregular shape.

(ii) They are weak.

Artificial  Magnets

Description : These magnets are made of hard steel or special alloys. The substances of these magnets have many small ‘atomic’ magnets. Ordinarily they are all oriented in random directions (Fig.). Then the substance is unmagnetised.

Unmagnetised
Unmagnetised
Magnetised
Magnetised

When such a piece is put in North–South direction and hammered, the atomic magnets align themselves in the direction of the earth’s field fig. They retain this alignment and the piece becomes a magnet with North (N) and (S) pole near ends. This magnet with two poles, is called a magnetic dipole.

Advantage : These have following two merits

(a) They may be given desired regular shape.

(b) They are strong.

(B)   Electromagnet

Electromagnets are based on the magnetic effect of electric current. An electromagnet is usually prepared by placing a soft iron core in a solenoid, or by winding a large number of turns of an insulated wire (generally the insulated copper wire) on a cylindrical soft iron core. An electromagnet shows magnetic properties only as long as the electric current flows through the solenoid. Thus, electromagnets are temporary magnets.

Difference between electromagnet and permanent magnet

Electromagnet :     

Nature of magnetism : Temporary. An electromagnet shows magnetism only as long as current flows through it.

Polarity : The polarity of an electromagnet can be changed by reversing the direction of the current.

Strength : The strength of an electromagnet can be increased or decreased by increasing or decreasing the current.

Permanent magnet (or Bar magnet)

Nature of magnetism : Permanent (or bar) magnets show permanent magnetism.

Polarity : Polarity of a permanent magnet cannot be changed.

Strength : The strength of a permanent magnet cannot be changed.

Two Poles of a Magnet

Poles of a magnet : When ends of a magnet are dipped in iron filings, the filings stick to its ends only and not to its sides. It means that in magnets, centres of attraction are located near ends only. These centres of attraction near the ends of a magnet are called poles.

When this magnet is freely suspended, the two ends point in north–south direction. The pole near the end pointing towards North (north–seeking end) is called North pole. The pole near the end pointing towards South (south–seeking end) is called South pole.

Interaction between poles : The magnetic poles exert forces on each other. Like pole repel each other, i.e., one north pole will repel another north pole or unlike poles attract each other i.e., north pole attract south pole.