Chapter Magnetism - Class 9 ICSE Board

Magnetism

Introduction to Magnetism

Magnetism is a force of attraction or repulsion that acts at a distance due to the motion of electric charges. The area around a magnet within which its magnetic force can be detected is called the magnetic field.

Types of Magnets

Permanent Magnets: Objects that produce their own persistent magnetic field. Examples include bar magnets and horseshoe magnets.
Temporary Magnets: Materials that behave like a magnet when they are in a strong magnetic field but lose their magnetism when the field disappears. Examples include soft iron nails.

Magnetic Poles

North Pole (N): The end of the magnet that points towards the Earth's North Pole when freely suspended.
South Pole (S): The end of the magnet that points towards the Earth's South Pole.

Key Point: Like poles repel each other (N-N or S-S) and unlike poles attract each other (N-S).

Magnetic Field Lines

Imaginary lines that represent the magnetic field around a magnet.
They emerge from the North Pole and merge at the South Pole.
The density of these lines indicates the strength of the magnetic field; closer lines represent stronger fields.
Magnetic field lines never intersect.

Earth's Magnetism

The Earth itself acts like a giant magnet with a magnetic North and South Pole.
The Earth's magnetic field protects us from harmful solar radiation by deflecting charged particles.

Making a Magnet

Stroking Method: Rubbing a piece of iron or steel with a magnet in one direction repeatedly.
Electrical Method: Using an electric current to magnetize a piece of iron or steel (Electromagnet).

Demagnetization

Ways to demagnetize a magnet include:

Heating it.
Hammering it.
Placing it in a coil carrying alternating current (AC).

Applications of Magnetism

Compass: A device that uses a magnetic needle to show direction.
Electric Motors: Use magnets to convert electric energy into mechanical energy.
Magnetic Storage: Hard drives and credit card strips use magnetic fields to store data.

History of Magnets

Ancient Discovery

The story of magnets begins in ancient times with the discovery of a naturally occurring mineral called magnetite (Fe₃O₄), also known as lodestone:

Early Discoveries:

Around 2500 years ago, in ancient Greece, a shepherd named Magnes is said to have discovered magnetite when he noticed his iron-tipped staff being pulled towards certain rocks.
The region where he found these rocks was called Magnesia, which is why the mineral was named magnetite.

China and the Compass:

Around 2000 years ago, the Chinese discovered that magnetite could be used to make a simple compass.
They observed that a piece of magnetite, when floated on water or suspended on a string, would always align itself in a north-south direction.

Medieval Europe:

By the Middle Ages, European sailors were using magnetic compasses for navigation. This helped them to navigate the seas even on cloudy days when the stars were not visible.

The Science Behind Magnetism

William Gilbert (1600):

An English physician, William Gilbert, is often called the father of magnetism. He conducted experiments and wrote a book titled "De Magnete" (On the Magnet).
Gilbert proposed that the Earth itself was a giant magnet, which explained why compasses point north.

Hans Christian Ørsted (1820):

Danish physicist Hans Christian Ørsted discovered the connection between electricity and magnetism. He found that an electric current passing through a wire creates a magnetic field around it.

James Clerk Maxwell (1860s):

Scottish physicist James Clerk Maxwell formulated a set of equations that described how electric and magnetic fields are generated and altered by each other and by charges and currents.

Modern Applications

Today, the principles of magnetism are used in numerous applications, from simple compasses to advanced technologies like MRI machines and maglev trains.

Summary Magnetite: Discovered in ancient Greece, known as lodestone.

Chinese Compass: Used magnetite for navigation.
William Gilbert: Proposed Earth as a giant magnet.
Hans Christian Ørsted: Discovered the link between electricity and magnetism.
James Clerk Maxwell: Formulated the fundamental equations of electromagnetism.

Two Important Properties of Magnets

1. Magnetic Poles

A magnet has two ends called poles: the North Pole (N) and the South Pole (S). Here are the key points about magnetic poles:

Attraction and Repulsion: Like poles repel each other (N-N or S-S), while unlike poles attract each other (N-S).
Magnetic Strength: The magnetic force is strongest at the poles of a magnet.
Nature of Poles: Magnetic poles always come in pairs. You cannot have a magnet with only one pole. If you break a magnet, each piece will have its own North and South Pole.

2. Magnetic Field

The region around a magnet where magnetic forces can be observed is called the magnetic field. Here are some key aspects of the magnetic field:

Magnetic Field Lines: These are imaginary lines used to represent the magnetic field. They start from the North Pole and end at the South Pole.
Direction of Field Lines: Outside the magnet, they go from North to South. Inside the magnet, they go from South to North, forming closed loops.
Strength of the Magnetic Field: The closer the field lines, the stronger the magnetic field. This is why the field is strongest at the poles.

Summary

Magnetic Poles: Magnets have a North and a South Pole, with like poles repelling and unlike poles attracting each other.
Magnetic Field: The area around a magnet where magnetic forces act, represented by field lines going from North to South outside the magnet and South to North inside the magnet.

Induced Magnetism

Induced Magnetism occurs when a magnetic material (like iron) becomes a magnet temporarily due to the presence of a nearby magnet. This process can be explained with the following concepts:

Magnetic Induction:

When a magnetic material (e.g., a piece of iron) is brought near a magnet, it starts to behave like a magnet. This happens because the magnetic field of the permanent magnet aligns the domains (tiny magnetic regions) in the material.
The region where this occurs is called the magnetic field of the permanent magnet.

Attraction Preceded by Induction:

Before the magnetic material is attracted to the magnet, it first gets magnetized due to the influence of the magnetic field.
For instance, when you bring a nail close to a bar magnet, the end of the nail nearest to the magnet gets magnetized, with poles induced in such a way that attraction occurs.

Temporary Nature of Induced Magnetism:

Induced magnetism is temporary because the material only acts as a magnet as long as it is within the magnetic field of the permanent magnet.
Once the magnetic material is removed from the magnetic field, it loses its magnetism as the alignment of the domains returns to a random state.

Illustration with an Example

Let's take an iron nail and a bar magnet to understand this better:

Magnetic Induction: When the iron nail is brought near the bar magnet, the domains within the iron align such that the end of the nail closest to the magnet becomes an opposite pole to the nearest pole of the bar magnet (e.g., if the magnet's North Pole is near the nail, the nearest end of the nail will become a South Pole).
Attraction: Due to the induced poles, the nail is attracted to the bar magnet. This attraction happens because the opposite poles (induced South in the nail and North in the bar magnet) attract each other.
Temporary Effect: When the nail is taken away from the bar magnet, the domains in the iron lose their alignment, and the nail ceases to be a magnet.

Key Points to Remember

Induced magnetism only occurs in materials that are magnetic (such as iron, cobalt, nickel).
The strength of the induced magnetism depends on the strength of the magnetic field and the properties of the material.
Induced magnetism is lost once the material is removed from the magnetic field.

Lines of Magnetic Field

Magnetic field lines are a visual representation of the magnetic field around a magnet. They help us understand the direction and strength of the magnetic field. some key points to remember about magnetic field lines:

Direction: Magnetic field lines emerge from the North Pole of a magnet and enter the South Pole. Inside the magnet, they travel from the South Pole back to the North Pole, forming continuous loops.

No Intersection: Magnetic field lines never intersect each other. This is because at any given point in space, the magnetic field has only one direction.
Closeness Indicates Strength: The closer the magnetic field lines are to each other, the stronger the magnetic field in that region. Conversely, where the lines are spread out, the magnetic field is weaker.
Uniformity: Inside a bar magnet, the field lines are nearly parallel, indicating a uniform magnetic field.
Visualizing with Iron Filings: If you sprinkle iron filings around a magnet, they align themselves along the magnetic field lines, giving a visible pattern of the magnetic field.

Examples and Applications

Bar Magnet: If you place a bar magnet under a sheet of paper and sprinkle iron filings on top, you will see the iron filings align themselves along the magnetic field lines, showing the characteristic pattern of lines emerging from the North Pole and entering the South Pole.
Earth's Magnetic Field: The Earth itself acts like a giant bar magnet with magnetic field lines extending from the magnetic North Pole to the magnetic South Pole. This is why a compass needle aligns with the Earth's magnetic field, pointing towards the magnetic poles.

Why Are Magnetic Field Lines Important?

Understanding Magnetic Effects: They help us visualize and understand how magnetic forces work at a distance.
Designing Magnetic Devices: Engineers use the concept of magnetic field lines to design devices like electric motors, transformers, and magnetic storage systems.
Navigation: The concept is crucial for navigation, as compasses rely on the Earth's magnetic field.

Magnetic Field of the Earth

Concepts of the Earth's Magnetic Field

The Earth itself behaves like a gigantic bar magnet, with a magnetic field surrounding it. This magnetic field extends from the Earth's interior out into space and interacts with the solar wind (a stream of charged particles emanating from the Sun).

Magnetic Poles: The Earth has a magnetic North Pole and a magnetic South Pole, which are not exactly aligned with the geographic poles. The magnetic North Pole is actually in the southern hemisphere in terms of magnetism, and the magnetic South Pole is in the northern hemisphere.
Magnetic Field Lines: The magnetic field lines emerge from the Earth's magnetic South Pole, loop around, and enter the magnetic North Pole. These lines form a protective shield around the Earth, known as the magnetosphere.
Magnetic Inclination and Declination:

Magnetic Inclination: The angle that a magnetic needle makes with the horizontal plane at any point on the Earth's surface.
Magnetic Declination: The angle between geographic north and the north to which a compass needle points. This varies from place to place on Earth's surface.

Evidences of the Existence of the Earth's Magnetic Field

Compass Navigation:

A compass needle, which is a small bar magnet, aligns itself with the Earth's magnetic field. This has been used for navigation for centuries, as the needle points towards the magnetic North Pole.

Auroras (Northern and Southern Lights):

Auroras are visible evidence of the Earth's magnetic field. These are natural light displays in the sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). They occur when charged particles from the Sun are directed by the Earth's magnetic field into the atmosphere, where they collide with gas molecules and produce light.

Magnetic Materials in Ancient Rocks:

Rocks formed from molten lava contain iron particles that align with the Earth's magnetic field as the lava cools and solidifies. By studying the orientation of these particles in ancient rocks, scientists have gathered evidence of the Earth's magnetic field's presence and changes over geological time.

Satellites and Space Probes:

Instruments on satellites and space probes measure the Earth's magnetic field directly from space. These measurements confirm the existence and detailed structure of the Earth's magnetosphere.

Magnetometers:

Sensitive instruments called magnetometers measure the strength and direction of the magnetic field at various points on the Earth's surface. These measurements provide a detailed map of the Earth's magnetic field.

Effect on Cosmic Rays:

The Earth's magnetic field deflects charged particles from space, known as cosmic rays. The decrease in the number of cosmic rays reaching the Earth's surface, compared to what would be expected without a magnetic field, provides evidence of the magnetic field's protective role.

Summary

The Earth acts as a giant magnet with a magnetic field that influences many natural phenomena. Evidence for this field comes from compass navigation, auroras, rock magnetism, space measurements, and the deflection of cosmic rays. Understanding the Earth's magnetic field helps us comprehend how it protects life on Earth and assists in navigation.

Neutral Points

Neutral Points: Neutral points are specific locations in the magnetic field of a magnet where the magnetic field due to the magnet cancels out the Earth's magnetic field. At these points, the net magnetic field is zero.

How Neutral Points Are Formed:

When a bar magnet is placed in the Earth's magnetic field, two neutral points can be observed on either side of the magnet along the equatorial line (the line perpendicular to the magnetic axis of the magnet and passing through its center).
At these points, the magnetic field strength due to the bar magnet is equal in magnitude but opposite in direction to the Earth's magnetic field. This results in the cancellation of both fields, leading to no net magnetic field at these points.

Experiment to Find Neutral Points:

Place a bar magnet on a table.
Use a compass to trace the magnetic field lines around the magnet.
Move the compass along the equatorial line of the magnet.
Identify the points where the compass needle does not point in any specific direction but rather stays aligned with the Earth's magnetic field. These are the neutral points.

Importance of Neutral Points:

Neutral points help in studying the interaction between the magnetic field of a magnet and the Earth's magnetic field.
They provide insights into the strength and direction of magnetic fields.

Magnetic Needles and Angles with the Horizontal

Magnetic Needles: A magnetic needle is a small, lightweight magnet, often in the form of a compass needle, which can rotate freely on a pivot.

Behavior of Magnetic Needles:

A magnetic needle aligns itself with the Earth's magnetic field lines.
The angle made by the magnetic needle with the horizontal plane varies at different locations on the Earth's surface.

Explanation of Angles:

The Earth’s magnetic field is not uniform and varies from place to place.
The angle that the magnetic needle makes with the horizontal plane is called the angle of dip or magnetic inclination.

Angle of Dip (Magnetic Inclination):

At the magnetic equator, the magnetic needle rests horizontally, making a 0° angle with the horizontal plane.
As one moves towards the magnetic poles, the angle of dip increases.
At the magnetic poles, the magnetic needle points straight downwards, making a 90° angle with the horizontal plane.

Reasons for Varying Angles:

The Earth’s magnetic field has both horizontal and vertical components.
Near the equator, the horizontal component is stronger, causing the needle to lie flat.
Near the poles, the vertical component is stronger, causing the needle to stand up.

Practical Use:

Understanding the angle of dip is essential for navigation, especially in aviation and maritime contexts.
It helps in calibrating compasses to ensure accurate directional readings.

Summary

Neutral Points: Specific points where the magnetic field of a magnet cancels out the Earth's magnetic field.
Magnetic Needles and Angles: The angle a magnetic needle makes with the horizontal plane varies based on location due to the Earth's non-uniform magnetic field.

Plotting of Uniform and Non-Uniform Magnetic Field Lines

1. Understanding Magnetic Field Lines

Magnetic field lines are a visual representation of the magnetic field around a magnet. They show the direction and strength of the magnetic field:

Direction: From the North Pole to the South Pole outside the magnet.
Strength: Represented by the density of the lines; closer lines mean a stronger field.

2. Uniform Magnetic Field

A uniform magnetic field has parallel, equally spaced magnetic field lines. This means the strength and direction of the magnetic field are the same at every point.

Example of a Uniform Magnetic Field:

Inside a solenoid (a coil of wire), when electric current flows through it, the magnetic field inside is uniform.

How to Plot Uniform Magnetic Field Lines:

Materials Needed: A solenoid, iron filings, a sheet of paper.
Steps:

Place the solenoid flat on a table.
Put the sheet of paper on top of the solenoid.
Sprinkle iron filings evenly on the paper.
Gently tap the paper. The filings will align themselves along the magnetic field lines.
You will observe that inside the solenoid, the iron filings form parallel, equally spaced lines, showing a uniform magnetic field.

3. Non-Uniform Magnetic Field

A non-uniform magnetic field has magnetic field lines that are not parallel and have varying distances between them. This means the strength and direction of the magnetic field change from point to point.

Example of a Non-Uniform Magnetic Field:

Around a bar magnet.

How to Plot Non-Uniform Magnetic Field Lines:

Materials Needed: A bar magnet, iron filings, a sheet of paper.
Steps:

Place the bar magnet flat on the table.
Put the sheet of paper on top of the magnet.
Sprinkle iron filings evenly on the paper.
Gently tap the paper. The filings will align themselves along the magnetic field lines.
You will observe that the iron filings are denser near the poles of the magnet and spread out as you move away from the poles, showing a non-uniform magnetic field.

4. Earth's Magnetic Field

The Earth itself acts like a giant bar magnet, with its magnetic field resembling that of a bar magnet. The magnetic field lines emerge from the magnetic South Pole and enter at the magnetic North Pole, creating a non-uniform magnetic field around the Earth.

How to Visualize Earth's Magnetic Field:

Concept: Imagine the Earth as a huge bar magnet with its magnetic South Pole near the geographic North Pole and magnetic North Pole near the geographic South Pole.
Magnetic Field Lines:

They emerge from the magnetic South Pole.
They curve around the Earth and enter the magnetic North Pole.
This creates a non-uniform magnetic field, stronger at the poles and weaker at the equator.

Summary

Uniform Magnetic Field: Parallel, equally spaced lines, such as inside a solenoid.
Non-Uniform Magnetic Field: Curved lines with varying distances, such as around a bar magnet.
Earth's Magnetic Field: Similar to a bar magnet, with a non-uniform field stronger at the poles and weaker at the equator

Conclusion of the Chapter: Magnetism

In this chapter, we explored the fascinating world of magnetism. Here are the key takeaways that we learned:

Nature of Magnetism: Magnetism is a force exerted by magnets when they attract or repel each other or other materials. This force is due to the motion of electric charges within the magnets.
Types of Magnets: We identified two main types of magnets: permanent magnets, which maintain their magnetic properties indefinitely, and temporary magnets, which act like magnets only when in the presence of a magnetic field.
Magnetic Poles: Magnets have two poles - North and South. Like poles repel, and unlike poles attract. These poles are the points where the magnetic force is the strongest.
Magnetic Field Lines: Magnetic field lines are used to visualize the magnetic field around a magnet. They emerge from the North Pole and enter the South Pole. The closer these lines are, the stronger the magnetic field.
Earth's Magnetism: Earth behaves like a giant magnet with its magnetic field, which has a North and South magnetic pole. This magnetic field protects us from solar radiation.
Making and Demagnetizing Magnets: Magnets can be created through methods such as stroking with another magnet or using electric currents. They can also be demagnetized by heating, hammering, or exposure to alternating current.
Applications of Magnetism: Magnetism has a wide range of applications in our daily lives, from compasses and electric motors to magnetic storage devices and medical imaging technologies.
Electromagnets: These are temporary magnets created by passing an electric current through a coil of wire wrapped around a core. Electromagnets are essential in many devices and technologies.

Summarizing Key Points

Magnetism is an essential force in nature and technology.
Magnetic fields and poles are fundamental concepts.
Earth itself is a giant magnet.
Practical applications of magnetism are vast and diverse.
Understanding electromagnets opens doors to advanced technologies.

Final Thought

Understanding magnetism not only helps us grasp fundamental physical principles but also enables us to appreciate the practical applications that make modern technology possible. From the simplest compass to advanced medical imaging, magnetism plays a crucial role in our daily lives.

Quick Review

Let's ensure we have covered the critical aspects:

Magnetism is caused by moving electric charges.
Permanent and temporary magnets have different characteristics.
Magnetic poles are where the magnetic force is strongest, and field lines help visualize the magnetic field.
Earth's magnetism protects us from solar radiation.
Magnets can be made and demagnetized through specific methods.
Electromagnets are temporary magnets that are incredibly useful in technology.

 Question: What is a magnet?

Answer: A magnet is an object that produces a magnetic field and attracts materials like iron, nickel, and cobalt.

 Question: Name the two types of poles in a magnet.

Answer: The two types of poles in a magnet are the North Pole and the South Pole.

 Question: What happens when two like poles of a magnet are brought close to each other?

Answer: Two like poles of a magnet repel each other.

 Question: What is the region around a magnet where its force is felt called?

Answer: The region around a magnet where its force is felt is called the magnetic field.

 Question: What materials are commonly used to make magnets?

Answer: Common materials used to make magnets include iron, steel, nickel, and cobalt.

 Question: What is the shape of the Earth's magnetic field?

Answer: The Earth's magnetic field has a shape similar to that of a bar magnet, with magnetic poles near the geographic poles.

 Question: What is the purpose of a magnetic compass?

Answer: A magnetic compass is used to determine direction by aligning itself with the Earth's magnetic field.

 Question: What is the law of magnetic poles?

Answer: The law of magnetic poles states that like poles repel each other, while opposite poles attract each other.

 Question: How can you demagnetize a magnet?

Answer: You can demagnetize a magnet by heating it, hammering it, or by placing it in a strong alternating magnetic field.

 Question: What is the property of magnetism observed in materials like iron and steel?

Answer: The property of magnetism observed in materials like iron and steel is called ferromagnetism.

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