ICSE Class 9 Physics Chapter 10 Magnetism

Magnetism

Syllabus

(i) Induced magnetism; Magnetic field of earth, neutral points in magnetic fields.

Scope - Magnetism: magnetism induced by bar magnets on magnetic materials; induction precedes attraction, lines of magnetic field and their properties, evidences of existence of earth's magnetic field, magnetic compass; uniform magnetic field and non-uniform field of a bar magnet placed along magnetic north-south; neutral point; properties of magnetic field lines.

(ii) Introduction fo electromagnets and its uses.

A - Induced Magnetism and Neutral Points

10.1 Introduction

The first known magnets were the pieces of lodestone, an ore of iron oxide (Fe₃O₄) found in large quantities in Magnesia, in Asia Minor. This ore was found to possess two properties: (i) it attracts small pieces of iron, and (ii) it sets itself along a definite direction when it is suspended freely. The Chinese, earlier than 2500 B.C., used these pieces to guide their boats. The pieces of lodestone found in nature were later on called the natural magnets. The word magnet has been derived from magnesia.

The natural magnets are found in quite irregular and odd shapes. They are not magnetically strong enough for use. Therefore, for different uses, artificial magnets are prepared from iron in different convenient sizes and shapes such as bar magnet, horse shoe magnet, magnetic needle, magnetic compass, etc.

If a magnet is suspended with a silk thread such that it is free to rotate in a horizontal plane, it sets itself always pointing in the geographic north-south direction as shown in Fig. 10.1. Depending on the direction in which an end of the magnet rests, its polarity is named as north or south.

Two like poles (both north poles or both south poles) repel each other, while two unlike poles (one north pole and the other south pole) attract each other.

10.2 Induced Magnetism (Magnetism Induced By A Bar Magnet on the Magnetic Materials)

When an unmagnetised bar AB of a magnetic material such as soft iron or steel, is placed near or in contact with a magnet as shown in Fig 10.2 (a), the bar AB becomes a magnet i.e., it acquires the property of attracting iron filings when they are brought near its ends. If the magnet is now removed, it is seen that nearly all the iron filings which have clung to it, fall down i.e., the bar AB loses its magnetism (Fig. 10.2(b)). Thus, the bar of a magnetic material behaves like a magnet so long it is kept near or in contact with a magnet. The magnetism so produced is called induced magnetism.

The temporary magnetism acquired by a magnetic material, when it is kept near (or in contact with) a magnet, is called induced magnetism.

The process in which a piece of magnetic material acquires the magnetic properties temporarily in presence of another magnet near it, is called the magnetic induction.

If polarity at the ends A and B of the bar AB is tested with a compass needle, it is found that the polarity developed at the end A is north (opposite to the polarity of the magnet near the end A) and the polarity at the end B is south (i.e., similar to the polarity at the end of the magnet near the end A). Thus,

A magnetic pole induces an opposite polarity on the near end and a similar polarity on the farther end of the iron bar.

Induction precedes attraction

Now we can explain how an ordinary piece of iron is attracted towards a magnet. When a piece of iron is brought near one end of a magnet (or one end of a magnet is brought near the piece of iron), the nearer end of the piece acquires an opposite polarity by magnetic induction. Since unlike poles attract each other, therefore iron piece is attracted towards the end of the magnet. Thus, piece of iron first becomes a magnet by induction and then it is attracted. In other words, induction precedes attraction.

Induced magnetism is temporary

If one pole of a bar magnet is brought near the small iron nails, they form a chain of nails as shown in Fig. 10.3. The reason is that the bar magnet by induction magnetises an iron nail which gets attracted by the magnet and clings to it. This magnetised nail magnetises the other nail near it by magnetic induction and attracts it. This process continues till force of attraction of magnet on first nail is sufficient to balance the total weight of all the nails in chain.

Now holding the uppermost nail in position by fingers, if magnet is removed, we find that all nails fall down. The reason is that on removing the magnet, the uppermost nail loses its magnetism, so all other nails also lose their magnetism, they get separated from each other and they all fall down due to force of gravity. This shows that the magnetism acquired by induction is purely temporary. It lasts so long as the magnet causing induction remains in its vicinity.

10.3 Lines of Magnetic Field

If a magnetic compass is placed on a table, it is found that its needle rests in geographic north-south direction. But when it is placed near a magnet, the needle swings and then rests in some other direction. As the compass is placed at different positions around a magnet, the direction in which the needle rests, changes such that its one end always points towards the nearer pole of the magnet. This behaviour of needle is due to the influence of the magnet near it. The region in which the compass gets influenced is called the magnetic field of the magnet.

The space around a magnet in which the needle of a compass rests in a direction other than the geographic north-south direction, is called magnetic field of the magnet.

As the distance of point from the magnet increases, the effect of its magnetic field decreases.

Magnetic field is a vector quantity. The magnitude of magnetic field at a point is measured by the force which a magnetic pole placed at that point, experiences, while the direction of magnetic field is the direction in which the needle of compass rests when it is placed at that point.

If we place a magnet below a sheet of stiff paper (or a glass plate) and spread some iron filings uniformly over the glass plate, then on tapping the glass plate gently, the iron filings arrange themselves along the curved lines as shown in Fig. 10.4. The reason is that due to magnet, each piece of iron filing gets magnetised by magnetic induction and experiences a force due to the magnet and arrange itself along the curved line. These curved lines are called the magnetic field lines.

A magnetic field line is a continuous curve in a magnetic field such that tangent at any point on it gives the direction of the magnetic field at that point.

10.4 Properties of Magnetic Field Lines

The magnetic field lines have following properties:

(1) They are closed and continuous curves.

(2) Outside the magnet, they are directed from the north pole towards the south pole of the magnet.

(3) The tangent at any point on a field line gives the direction of magnetic field at that point.

(4) They never intersect one another. If two field lines intersect, there would be two directions of the magnetic field at that point which is not possible. Fig 10.5 shows two magnetic field lines PQ and PR intersecting such that each other at a point P. It would mean that if a compass needle is placed at the point P, north pole of its needle will point in two directions PQ and PR simultaneously which is not possible.

(5) They are crowded near the poles of the magnet where the magnetic field is strong and are far separated near the middle of the magnet and far from the magnet, where the magnetic field is weak.

(6) Parallel and equi-distant field lines represent a uniform magnetic field. The earth's magnetic field in a limited space is uniform.

(7) They behave like the stretched elastic rubber strings.

10.5 Magnetic Field of Earth (Evidences of existence of earth's magnetic field)

Our earth itself has a magnetic field and it behaves like a magnet. The existence of earth's magnetic field is based on the following facts:

(i) A freely suspended magnetic needle always rests in geographic north-south direction.

When a magnetic needle (or a magnet) is suspended such that it is free to rotate in a horizontal plane, it always rests indicating the geographic north-south direction. But the north pole of a magnet will point towards the geographic north only when there is a magnetic south pole attracting it. Similarly the south pole of the magnet will point towards the geographic south when there is a magnetic north pole attracting it. Therefore we can assume a magnet inside the earth which must have its south pole in the geographic north and north pole in the geographic south.

(ii) An iron rod buried inside the earth along north-south direction becomes a magnet.

If an iron rod is buried few metres inside the earth keeping it along north-south direction, after some days it is found that the rod becomes a weak magnet. It is possible only if the earth itself behaves like a magnet.

(iii) Neutral points are obtained on plotting the field lines of a magnet.

If a magnet is placed in a horizontal plane with its north pole facing towards the geographic north and the magnetic field lines are plotted, we obtain two neutral points, one on either side of the magnet, on its broad side-on position. Similarly, if the magnet is placed in a horizontal plane with its north pole facing towards the geographic south and the magnetic field lines are plotted, we obtain two neutral points, one on either side of the magnet on its end-on position. At each neutral point, the resultant magnetic field is zero i.e., if a compass needle is placed at a neutral point, it rests in any direction. The reason for zero resultant magnetic field at the neutral point is that the magnetic field produced by the magnet is neutralised by some other equal and opposite magnetic field. This other magnetic field is actually the horizontal component of the earth's magnetic field.

(iv) A magnetic needle rests making different angles with horizontal when suspended at different places of the earth.

If a magnetic needle is suspended such that it is free to rotate in a vertical plane and it is taken around the earth through its geographic poles, we find that at two places, magnetic needle becomes normal to the earth surface i.e., it becomes vertical. These points are called the earth's magnetic poles. At two places it becomes parallel to the earth surface i.e., it becomes horizontal. These points lie on earth's magnetic equator. At other places, it rests making different angles with the horizontal as shown in Fig. 10.6. It implies that the earth itself has a magnetic field.

Two places where the magnetic needle becomes vertical are called the magnetic poles. The line joining the poles where the magnetic needle becomes horizontal, is called the magnetic equator.

Conclusion: On the basis of above facts, it can be concluded that the earth behaves as if a huge bar magnet is present at its centre with its magnetic south pole in the geographic north and the magnetic north pole in the geographic south. Actually, the geographic poles do not coincide with the earth's magnetic poles, but they are somewhat displaced. The magnetic axis of the earth makes an angle of 17° with the axis of rotation of the earth. The magnetic south pole of earth is in Canada at a distance nearly 2240 km from the geographic north pole at 70-75° north latitude and 96° west longitude, while the earth's magnetic north pole is at a distance nearly 2240 km from the geographic south pole at 73° south latitude and 155° east longitude. It has been experimentally observed that the positions of these poles are not stationary, but they gradually change over a long period of time.

Magnetic Field Lines of Earth

In a limited space, the magnetic field lines of earth are parallel and equidistant to each other as shown in Fig. 10.7(b). They are always directed from the geographic south to the geographic north. They are horizontal at the magnetic equator and vertical at the magnetic poles, but at any other point, they are inclined to the horizontal.

The magnetic field lines of the earth are normal to earth surface near the magnetic poles and parallel to earth surface near the magnetic equator.

Teacher's Note

Magnetic navigation has guided human exploration for centuries - from ancient mariners using lodestone to modern compasses that help us find our way.

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