Vardaan Learning Institute

MAGNETISM

Created by Team Vardaan with ❤️ • Class IX

★ WHAT WE WILL COVER IN THIS CHAPTER

This chapter explores the fundamental concepts of Magnetism, focusing on how magnets influence materials and how the Earth itself acts as a giant magnet. We will cover:

Introduction to Natural & Artificial Magnets: Lodestone, bar magnets, and their properties.
Induced Magnetism: How ordinary iron becomes a temporary magnet.
Magnetic Field Lines: Visualizing the invisible force field around magnets.
Earth's Magnetic Field: Understanding Earth's magnetic elements and evidence.
Neutral Points: Identifying locations where magnetic forces cancel out.
Electromagnets & Applications: Construction, properties, and the working of an Electric Bell.

1. Introduction

Fig. 10.1: Setting of a freely suspended magnet

The first known magnets were pieces of lodestone, an ore of iron oxide ($Fe_3O_4$), found in Magnesia (Asia Minor). This ore was found to possess two key properties:

Attractive Property: It attracts small pieces of iron, nickel, and cobalt.
Directive Property: It sets itself along a definite direction (Geographic North-South) when suspended freely.

Natural magnets are often irregular and weak. Thus, we use Artificial Magnets (Bar magnets, Horse-shoe magnets, Magnetic needles) made from iron or steel, which are stronger and have convenient shapes.

Properties of Magnets

Directive Property: A freely suspended magnet always rests in the geographic North-South direction. The end pointing towards geographic North is called the North Pole.
Attractive Property: Attraction is maximum at the poles (ends) and minimum at the centre (neutral zone).
Like poles repel, unlike poles attract.
Poles exist in pairs: Magnetic monopoles do not exist. Breaking a magnet creates two smaller complete magnets.

Test for Magnetism: A magnet can attract both an unmagnetised iron piece and an opposite pole of another magnet. However, repulsion only occurs between two like poles. Thus, Repulsion is the surest test of magnetism.

2. Induced Magnetism

Fig. 10.2: Induced magnetism (Iron filings attracted vs falling)

Definition: The temporary magnetism acquired by a magnetic substance (like soft iron) when it is kept near or in contact with a magnet is called Induced Magnetism.

When an unmagnetised iron bar is placed near a magnet, it acquires magnetic properties. The end of the bar near the magnet develops opposite polarity (e.g., if near South pole, it becomes North), while the farther end develops similar polarity.

"Induction Precedes Attraction" - IMPORTANT!

A magnet does not attract a piece of iron directly. First, it induces opposite polarity on the near end of the iron piece (Induction). Since unlike poles attract, the induced opposite pole is then attracted to the magnet. Thus, induction always takes place before attraction.

Chain of Nails Experiment

Fig. 10.3: Magnetic Induction forming a chain of nails

If a bar magnet is brought near small iron nails, they form a chain as shown in Fig 10.3. The magnet induces magnetism in the first nail, which induces it in the second, and so on. If the top magnet is removed, all nails fall, proving that induced magnetism is temporary.

3. Magnetic Field Lines

Fig. 10.4: Arrangement of iron filings near a magnet

The space around a magnet in which its magnetic influence can be experienced is called its Magnetic Field. It is a vector quantity having both magnitude and direction.

Properties of Magnetic Field Lines:

They are closed, continuous curves.
Direction: From North to South outside the magnet, and South to North inside the magnet.
The tangent at any point on a field line represents the direction of the magnetic field at that point.
Two field lines can NEVER intersect. (See Fig 10.5 below).
Crowded lines indicate a strong field; spaced out lines indicate a weak field.
They behave like stretched elastic strings, trying to contract longitudinally (attraction) and expand laterally (repulsion of like poles).

Fig. 10.5: Two magnetic field lines intersecting (Why it's impossible)

Why can't two lines intersect? If they did, at the point of intersection, there would be two tangents representing two different directions of the magnetic field, which is impossible.

4. Earth's Magnetic Field

Fig. 10.6: Inclination of magnetic needle at different places

The Earth acts as a huge magnet with its magnetic North pole situated near the geographic South pole and vice-versa. This is why a compass points North (it's attracted to Earth's magnetic South pole located there).

Evidence of Earth's Magnetism:

A freely suspended magnet always rests in North-South direction.
An iron rod buried in the earth along the N-S direction becomes a weak magnet after some time.
Neutral points are obtained on plotting field lines of a magnet (where Earth's field cancels magnetic field).
A magnetic needle rests at different angles with the horizontal (Angle of Dip) at different places.

Magnetic Elements of Earth

Magnetic Meridian: A vertical plane passing through the magnetic axis of a freely suspended magnet.
Geographic Meridian: A vertical plane passing through the geographic North-South axis.
Angle of Dip (Inclination): The angle which the axis of a freely suspended magnetic needle makes with the horizontal.
- At Magnetic Equator, Dip = $0^\circ$ (Needle is horizontal).
- At Magnetic Poles, Dip = $90^\circ$ (Needle is vertical).

Earth's magnetic field lines are parallel and equidistant in a limited space, representing a Uniform Magnetic Field.

Fig. 10.7: Plotting compass usage

5. Non-Uniform Magnetic Fields

The magnetic field around a bar magnet is non-uniform (the lines are curved). The lines are crowded near poles where the field is strong and spaced out where it is weak.

Fig. 10.8: Patterns of Non-Uniform Magnetic Fields

6. Plotting Field Lines & Neutral Points

Neutral Points (X): Points where the net magnetic field is zero. This happens when the magnetic field produced by the magnet is equal in magnitude and opposite in direction to the horizontal component of the Earth's magnetic field ($B_H$).

Case 1: Magnet with North Pole pointing Geographic North

Fig. 10.9: Field lines (N pointing North) & Neutral Points (East-West)

In this position, the neutral points lie on the Equatorial line (axis perpendicular to the magnet's length) on either side of the magnet.

Case 2: Magnet with South Pole pointing Geographic North

Fig. 10.10: Field lines (S pointing North) & Neutral Points (North-South)

In this position, the neutral points lie on the Axial line (axis along the magnet's length) on either side of the magnet.

7. Electromagnets

An electromagnet is a temporary magnet made by passing current through a coil wound around a soft iron core. It behaves as a magnet only as long as current flows.

Factors increasing strength of Electromagnet:

Increasing the Number of turns ($N$) in the solenoid.
Increasing the Current ($I$).
Using a Soft Iron Core (which has high magnetic permeability).
Reducing the air gap between poles (e.g., in U-shaped magnets).

(a) I-Shaped (Bar) Electromagnet

Fig. 10.14: I-shaped (Bar) Electromagnet

It consists of a thin soft iron bar around which an insulated copper wire is wound in the form of a solenoid. When current is passed, the bar gets magnetised.
Polarity Rule: Looking at a face, if current is Clockwise, it becomes a South Pole. If Anticlockwise, it becomes a North Pole.

(b) U-Shaped (Horse-Shoe) Electromagnet

Fig. 10.15: Horse-shoe Electromagnet

It consists of a U-shaped soft iron core. The wire is wound on the two arms such that the current flows in opposite directions in the two arms (when viewed from the ends). This creates a North pole at one end and a South pole at the other. It is stronger than a bar electromagnet due to the smaller air gap.

Comparison: Electromagnet vs Permanent Magnet

Electromagnet (Temporary)	Permanent Magnet (Steel)
Made of Soft Iron.	Made of Steel.
Magnetism is temporary (lost when current stops).	Magnetism is permanent.
Strength can be varied (by changing $I$ or $N$).	Strength is fixed.
Polarity can be reversed.	Polarity is always fixed.

8. Application: The Electric Bell

Fig. 10.16: Electric bell and its wiring

Principle: It works on the magnetic effect of current (Electromagnetism).

Construction & Working

Parts: Electromagnet (Horse-shoe), Soft iron armature, Hammer, Gong, Contact screw, Spring strip.
Working Cycle:
1. When the switch is pressed, current flows, and the electromagnet gets magnetised.
2. It attracts the soft iron armature, causing the hammer to strike the gong (DING!).
3. As the armature moves towards the magnet, its contact with the adjustment screw is broken.
4. The circuit breaks, current stops, and the electromagnet loses its magnetism.
5. The metallic spring strip pulls the armature back to its original position.
6. Contact is re-established with the screw, completing the circuit again.
7. The cycle repeats rapidly, making the bell ring continuously as long as the switch is pressed.

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