Magnetism Class 9 ICSE — Physics Chapter 10

Magnetism Class 9 ICSE Physics Chapter 10 builds from the simplest observations. The chapter opens with lodestone, the natural magnet that first showed how iron could be attracted. From there it explains induced magnetism, magnetic field lines, and the earth’s own magnetic field. The second half covers electromagnets, permanent magnets, and the electric bell. You can find this chapter in the Class 9 ICSE Physics collection.

Download Magnetism Class 9 ICSE Physics Chapter 10 PDF

Download the Magnetism Class 9 ICSE Physics chapter PDF to revise offline or print for practice. It is the full chapter as printed in the book — 12 pages, 11 sections, 44 questions.

What is inside Chapter 10

12 pages · 11 sections · 2 worked examples · 44 exercise questions

• EXERCISE 10 (A): 26 questions
• Multiple choice type :: 2 questions
• EXERCISE 10 (B): 14 questions
• Multiple Choice Type: 2 questions

10.1 INTRODUCTION

Lodestone is an iron oxide ore, \mathrm{Fe}_3\mathrm{O}_4, found in large quantities in Magnesia in Asia Minor. It was the first known magnet. The ore showed two properties: it attracted small pieces of iron, and it set along a definite direction when suspended freely. The word “magnet” comes from Magnesia, the place where the ore was found.

Natural magnets are irregular in shape and weak in strength. For practical use, artificial magnets are made from iron in convenient shapes. These include the bar magnet, horseshoe magnet, magnetic needle, and magnetic compass.

A magnet suspended freely by a silk thread always rests in the geographic north-south direction. The end pointing north is called the north pole; the end pointing south is the south pole. Like poles repel each other, while unlike poles attract each other. This single rule drives most of the chapter.

10.2 INDUCED MAGNETISM (MAGNETISM INDUCED BY A BAR MAGNET ON THE MAGNETIC MATERIALS)

Place an unmagnetised iron bar near a bar magnet. The bar acquires the ability to attract iron filings. Remove the magnet, and the filings fall away. The bar loses its magnetism almost completely.

This temporary magnetism is called induced magnetism. The process by which a magnetic material acquires magnetic properties in the presence of another magnet is called magnetic induction.

The near end of the bar acquires polarity opposite to the magnet’s nearby pole. The far end acquires the same polarity as that nearby pole. So a north pole induces south on the near end and north on the far end. A magnetic pole always induces opposite polarity on the near end and similar polarity on the far end.

This explains why iron is attracted to a magnet. The iron first becomes a magnet by induction, then attraction follows. In short, induction precedes attraction.

Iron nails form a chain below a magnet’s pole. Each nail magnetises the next by induction. Remove the magnet, and the whole chain falls. That shows induced magnetism is purely temporary. It lasts only while the inducing magnet stays near.

10.3 LINES OF MAGNETIC FIELD

A compass needle rests in the geographic north-south direction when far from a magnet. Near a magnet, the needle swings and settles in a different direction. The region in which the compass is influenced is called the magnetic field of the magnet.

The field weakens as distance from the magnet increases. Magnetic field is a vector quantity. Its magnitude is the force a magnetic pole experiences at that point. Its direction is the direction in which a compass needle rests there.

Place a magnet under a sheet of stiff paper and sprinkle iron filings on top. Tap gently, and the filings arrange themselves along curved lines. Each filing becomes a tiny magnet by induction and aligns with the field. These curved lines are the magnetic field lines.

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

10.4 PROPERTIES OF MAGNETIC FIELD LINES

Magnetic field lines have seven properties you must know for the Class 9 ICSE syllabus.

• They are closed and continuous curves.
• Outside the magnet, they run from the north pole to the south pole.
• The tangent at any point gives the direction of the field at that point.
• They never intersect one another.
• They are crowded near the poles, where the field is strong, and spread out where the field is weak.
• Parallel and equidistant field lines represent a uniform magnetic field.
• They behave like stretched elastic rubber strings.

The intersection rule matters most. If two field lines crossed at a point, a compass needle placed there would have to point in two directions at once. That is impossible, so field lines never intersect.

Uniform fields appear often in exam questions. The earth’s magnetic field in a limited space is treated as uniform.

10.5 MAGNETIC FIELD OF EARTH (Evidences of existence of earth’s magnetic field)

The earth itself behaves like a huge magnet. Four pieces of evidence support this claim.

• A freely suspended magnetic needle always rests in the geographic north-south direction.
• An iron rod buried along the north-south direction becomes a weak magnet after some days.
• Neutral points appear when field lines of a magnet are plotted, where the net field is zero.
• A needle suspended to rotate vertically makes different angles with the horizontal at different places.

At two places the needle becomes vertical. These are the earth’s magnetic poles. At two other places it becomes horizontal. The line joining these places is the magnetic equator.

The conclusion is that the earth acts like a bar magnet at its centre. Its magnetic south pole lies near the geographic north. This is a common confusion: magnetic south is near geographic north, and magnetic north is near geographic south. The magnetic axis makes an angle of 17^\circ with the earth’s axis of rotation.

The magnetic south pole is in Canada, about 2240 km from the geographic north pole. The magnetic north pole is about 2240 km from the geographic south pole, near Antarctica. These positions shift slowly over long periods.

Within a limited space, the earth’s field lines are parallel, equidistant, and directed from geographic south to geographic north. They are horizontal at the magnetic equator and vertical at the magnetic poles. For the latest syllabus details, see the official CISCE website.

10.6 PLOTTING OF UNIFORM MAGNETIC FIELD LINES OF EARTH

Earth’s magnetic field is uniform within a limited space. You can plot it with a small compass needle.

Fix a sheet of paper on a drawing board. Place the compass at position 1 and mark two pencil dots in front of the needle’s ends. Move the compass so one end of the needle lines up with the second dot. Mark the new position of the other end. Repeat this process, then join all the dots. You get a straight line.

Repeat from different starting points to trace several lines. Each line carries an arrow from the needle’s south pole to its north pole.

The lines turn out parallel and equidistant. They run from geographic south to geographic north. They never intersect. That is how you prove the field is uniform in that space.

10.7 PLOTTING OF NON UNIFORM MAGNETIC FIELD OF A STRONG BAR MAGNET AND NEUTRAL POINTS

A bar magnet produces a non-uniform field. Its field lines are curved, converging near the poles and diverging away from them. Closely spaced lines mean a strong field; widely spaced lines mean a weak field. The field of the magnet combines with the earth’s field during plotting.

Two orientations are studied in this ICSE chapter.

Case 1 — North pole facing geographic north. The magnet’s field and the earth’s field combine. At two points equidistant from the magnet’s centre, in the east and west directions, the two fields cancel. These are neutral points. A compass placed there rests in any direction.

Case 2 — South pole facing geographic north. The neutral points now lie in the north and south directions, again equidistant from the centre.

Remember the pattern as N-N gives E-W, S-N gives N-S. North pole facing north gives neutral points east-west. South pole facing north gives neutral points north-south.

Try this pattern with your own numbers. Suppose a bar magnet of length 10 cm lies along the magnetic meridian with its north pole facing geographic north. The two neutral points appear on the east and west sides, each about 8 cm from the centre of the magnet. At each point, the magnet’s field equals the earth’s horizontal component in magnitude, but points opposite to it. The net field is zero.

10.9 ELECTROMAGNET

An electromagnet is a temporary magnet made from soft iron. It produces a strong magnetic field while current flows through a coil wound around it. It is an artificial magnet. Electromagnets depend on electric current, which you studied in the current electricity chapter of this ICSE book.

Two shapes are common. The I-shaped electromagnet uses a straight soft iron bar with a solenoid wound around it. The U-shaped (horseshoe) type winds the coil on both arms in opposite directions, producing a strong field in the gap between the poles.

The polarity at each face follows a simple rule: clockwise current gives a south pole; anticlockwise current gives a north pole. Remember it as CSAN — Clockwise South, Anticlockwise North.

The strength of an electromagnet increases in two ways. Add more turns to the winding, or increase the current through the coil.

Soft iron has low retentivity, so the magnet loses its magnetism the moment the current stops. That is why it is called temporary. With an alternating current source at 50\ \text{Hz}, the core still gets magnetised. But the polarity at the ends reverses 50 times each second.

10.10 PERMANENT MAGNET

A permanent magnet keeps its magnetism after the magnetising force is removed. It is made from steel because steel has high retentivity. Soft iron, by contrast, has low retentivity and loses magnetism quickly, so it suits electromagnets. Steel holds its magnetism, which is why permanent magnets are built from it.

To make a permanent magnet, wind an insulated copper coil around a steel piece and pass current through it. Once magnetised, the steel does not lose its magnetism easily.

Permanent magnets are used in electric meters such as the galvanometer, ammeter, and voltmeter. They are also used in the magnetic compass.

10.11 COMPARISON OF AN ELECTROMAGNET WITH A PERMANENT MAGNET

The table below gives the six differences the textbook draws between an electromagnet and a permanent magnet.

Examiners often ask for two differences from this table. Pick material and field duration, or strength and polarity reversal.

10.12 ADVANTAGES OF AN ELECTROMAGNET OVER A PERMANENT MAGNET

An electromagnet has three clear advantages over a permanent magnet.

• It can produce a strong magnetic field.
• Its strength can be changed by varying the current or the number of turns.
• Its polarity can be reversed by reversing the current direction.

Electromagnets lift and transport heavy iron scrap, girders, and plates. They load furnaces with iron. They separate iron pieces from ores and debris, such as iron from crushed copper ore. They remove iron pieces from wounds. They are used in scientific research and in devices like the electric bell, telegraph, electric tram, relay, microphone, and loudspeaker.

Electric bell. The bell uses a horseshoe electromagnet, a soft iron armature, a hammer, a gong, a spring strip, an adjusting screw, a switch, and a battery. Pressing the switch sends current through the coil. The electromagnet attracts the armature, and the hammer strikes the gong.

As the armature moves, the spring strip loses contact with the screw. The circuit breaks, the electromagnet demagnetises, and the armature springs back. Contact is restored, current flows again, and the hammer strikes once more. This make-and-break cycle rings the bell continuously while the switch is pressed.

With alternating current, the core still magnetises but its polarity flips 50 times per second. The armature is attracted regardless of polarity, so the bell still rings.

What to practise from Magnetism Class 9 ICSE

This chapter has four practice blocks. Work through them in order after you finish reading. More resources are listed in the Class 9 ICSE books hub.

• EXERCISE 10 (A) — 26 questions covering sections 10.1 to 10.7
• Multiple choice type — 2 questions after Exercise 10 (A)
• EXERCISE 10 (B) — 14 questions on electromagnets, permanent magnets, and the electric bell
• Multiple Choice Type — 2 questions after Exercise 10 (B)

Magnetism Class 9 ICSE frequently asked questions

Why is induced magnetism temporary?

Induced magnetism lasts only while the magnetising magnet stays near. The material’s magnetic domains align under the magnet’s influence. When the magnet is removed, the domains return to random arrangement. Soft iron has low retentivity, so it cannot hold the alignment.

Where are neutral points located when the north pole faces geographic north?

The neutral points lie on either side of the magnet, in the east-west direction, at equal distances from its centre. At these points, the magnet’s field and the earth’s horizontal component cancel each other. The net field is zero, so a compass rests in any direction.

How can I increase the strength of an electromagnet in Class 9 ICSE Physics?

Two methods work. Increase the number of turns in the winding, or increase the current through the coil. Both raise the magnetic field strength of the electromagnet.

Why is soft iron used in an electromagnet but steel in a permanent magnet?

Soft iron has low retentivity, so it magnetises and demagnetises quickly. That suits a temporary magnet. Steel has high retentivity and holds its magnetism, which suits a permanent magnet.

Does an electric bell work with alternating current (AC)?

Yes. The core still magnetises with AC, but the polarity at its ends flips 50 times per second. The armature is attracted regardless of polarity, so the bell continues to ring.

Reference: ICSE Class 9 Physics textbook material, chapter 10: Magnetism — CISCE-aligned.

More from this book

• Chapter 1: Measurements and Experimentation
• Chapter 2: Motion in One Dimension
• Chapter 3: Laws of Motion
• Chapter 4: Pressure in Fluids and Atmospheric Pressure
• Chapter 5: Upthrust in Fluids Archimedes Principle and Floatation
• Chapter 6: Heat and Energy
• Chapter 7: Reflection of Light
• Chapter 8: Propagation of Sound Waves
• Chapter 9: Current Electricity