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07. Magnetism

Self Study

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Last edited 26 days ago by Learn LoopHQ.

Chapter: 07. Magnetism

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Magnetism: Your Guide to Understanding Invisible Forces

Welcome to your self-study journey into the fascinating world of magnetism! By the end of this journey, you’ll be able to understand the invisible force that influences many objects around us.

What You Will Learn

Identify the unique features of a magnet.

Tell the difference between materials that are attracted to magnets (magnetic) and those that are not (non-magnetic).

Understand the key characteristics that all magnets share.

Picture the invisible magnetic field that surrounds a magnet.

Recognize that even Earth acts like a giant magnet.

Discover various techniques to create a magnet.

Distinguish between magnets that keep their power for a long time (permanent) and those that only do so temporarily.

Learn how to build a simple electromagnet.

Know the best ways to look after and store magnets.

Understand what makes a magnet lose its magnetic properties, such as heating, hammering, or electric current.

Connecting with Magnets

Magnetism is an invisible force that pulls certain substances towards a magnet. You can’t see it, but you can definitely feel its pull! Magnets attract materials like iron, steel, cobalt, and nickel. Think about how a refrigerator door seals, or how a paperclip sticks to a magnet – that’s magnetism in action.

Try to imagine: What happens if you bring a magnet close to a paperclip? A red die? A safety pin? A purple eraser? A glass? A key? A screw? You’ll find that some are attracted, and some are not!

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Introduction to Magnets

Any substance that has the special ability to attract iron and other materials like steel, cobalt, and nickel is called a magnet. This amazing property is known as magnetism.

Natural and Artificial Magnets

Magnets can be found in nature or made by people.

Natural Magnets: These are magnets that occur naturally, meaning they are found in the Earth. They have the ability to attract magnetic materials on their own. The first natural magnet ever discovered was magnetite, which is a type of iron ore. These magnetic rocks are also called lodestones.

Artificial Magnets: These are magnets made by humans by giving magnetic properties to certain materials. Artificial magnets are usually made from materials like iron, steel, nickel, cobalt, or combinations of these (alloys). They come in many different shapes to suit various uses.

Here are some common shapes of artificial magnets:

Bar magnet: This is a long, rectangular magnet. It’s one of the most common types you’ll see.

Horseshoe magnet: This magnet is bent into a U-shape. This design brings the poles closer together, making it stronger.

Cylindrical magnet: This magnet is shaped like a long cylinder or a disk.

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Fig. 7.1 Artificial magnets

Magnetic and Non-Magnetic Substances

Not all materials react to magnets in the same way. We classify them based on whether they are attracted to a magnet or not.

Magnetic Substances: These are substances that are attracted by a magnet. Examples include iron, steel, nickel, and cobalt. Interestingly, these substances can also be turned into magnets themselves!

Non-magnetic Substances: These are substances that are not attracted by magnets. Examples include paper, wood, and glass. These substances cannot be turned into magnets.

To understand this better: Imagine you have a horseshoe magnet and various items like iron nails, a knife, steel paper clips, a piece of paper, a stainless steel plate, wood, a sewing needle, a silver spoon, a comb, a plastic ruler, a glass, a drawing pin, an eraser, a pencil, a hair pin, a safety pin, copper wire, and a screwdriver.

You could test each item by bringing the magnet close to it.

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Fig. 7.2 A magnet attracts magnetic materials.

You would observe that items like iron nails, steel paper clips, a sewing needle (if steel), drawing pins, hairpins, and safety pins are attracted. These are magnetic substances. Items like paper, wood, a silver spoon (unless it’s an alloy with iron/nickel/cobalt, but generally considered non-magnetic in this context), a comb, a plastic ruler, a glass, an eraser, a pencil, and copper wire are not attracted. These are non-magnetic substances.

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A table to categorize observations.

Fact Check: Mythous Facto

Myth: Any object that is silver in color can be attracted by a magnet.

Fact: Only objects made of iron, cobalt, nickel, and their alloys are attracted by magnets. These objects can be any color, not just silver. For example, a silver spoon (made of silver metal) is not magnetic, but a steel screw (often silver-colored) is!

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Properties of Magnets

Magnets have several fascinating properties that make them unique.

1. Attractive Property

This is the most well-known property: a magnet’s ability to pull magnetic substances towards itself. The magnetic pull, or magnetic force, is not uniform across the entire magnet. It’s actually strongest at the two ends of the magnet, which are called its poles. Very little or no magnetic force is felt in the middle of the magnet.

To understand this better: Imagine spreading iron filings (tiny pieces of iron) on a table. If you place a magnet over them and lift it, you’ll see that most of the iron filings cling to the ends (poles) of the magnet, with very few in the middle. This clearly shows that the magnetic force is concentrated at the poles.

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Fig. 7.3 Magnetic force is maximum at the poles.

2. Directive Property

When a magnet is allowed to swing freely, it always settles down pointing in a specific direction: north-south. This is known as the directive property of the magnet.

The end of a freely suspended bar magnet that points towards the geographic north is called the north pole of the magnet.

The end that points towards the geographic south is called the south pole of the magnet.

The imaginary straight line connecting these two poles of a magnet is called the magnetic axis.

To understand this better: You could mark the north, south, east, and west directions on a surface. Then, suspend a bar magnet from a stand using a string, making sure it can swing freely. No matter how many times you disturb the magnet, it will always return to rest with one end pointing north and the other pointing south.

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Fig. 7.4 Freely suspended magnet pointing north-south direction.

3. Like Poles Repel and Unlike Poles Attract

This is a fundamental rule of magnets:

Like poles (e.g., North-North or South-South) of two magnets repel each other (push away).

Unlike poles (e.g., North-South) of two magnets attract each other (pull towards).

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Fig. 7.5 Like poles repel and unlike poles attract.

To understand this better: If you suspend a bar magnet freely, and then bring the north pole of another magnet close to the north pole of the suspended magnet, you’ll see the suspended magnet swing away – they repel! The same happens if you bring two south poles together.

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Fig. 7.6 Like poles repel.

However, if you bring the south pole of the second magnet near the north pole of the suspended magnet, they will pull towards each other – they attract! The same happens with a north pole and a south pole.

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Fig. 7.7 Unlike poles attract.

Repulsion is the Surest Test of Magnetism: Why is repulsion so important? Because a magnet will attract both unlike poles and non-magnetic materials (like an iron bar). However, only a magnet will repel another magnet’s like pole. So, if you want to know if a piece of metal is a magnet or just a simple iron bar, you must check for repulsion. If one end is attracted and the other is repelled by a known magnet, then it’s a magnet. If both ends are attracted, it’s just an iron bar.

4. Magnetic Poles Always Exist in Pairs

Every magnet, no matter how small, always has two poles: a north pole and a south pole. You can never have a magnet with only one pole. Even if you break a magnet into smaller and smaller pieces, each new piece will instantly become a complete magnet with its own north and south poles. The poles simply cannot be separated.

To understand this better: Imagine taking a bar magnet and breaking it in half. Each half immediately becomes a new, smaller magnet, each with its own north and south poles. If you kept breaking it, the result would be the same every time!

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Fig. 7.8 Magnetic poles exist in pairs.

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Magnetic Field Around a Magnet

The area surrounding a magnet where its magnetic influence can be felt is called the magnetic field of a magnet. This invisible field can be visualized using imaginary lines called magnetic lines of force.

To visualize this: If you place a bar magnet under a sheet of white paper and sprinkle iron filings evenly on top, you’ll see the filings arrange themselves in a distinct pattern of curved lines around the magnet. These curved lines show the presence and shape of the magnetic field and are called magnetic lines of force.

The direction of these magnetic lines of force is always from the north pole of the magnet to its south pole outside the magnet. Inside the magnet, they go from south to north, forming continuous closed loops.

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Fig. 7.9 Magnetic lines of force.

A magnetic compass is a useful tool for understanding magnetic fields. Its tiny magnetic needle will always align itself with the magnetic field lines. If you move a compass around a bar magnet, you’ll see its needle change direction, showing how the magnetic field influences it. The compass needle always points in the direction of the magnetic field at that specific point.

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Fig. 7.10 Determination of direction of magnetic lines of force using magnetic compass.

Properties of Magnetic Lines of Force

Magnetic lines of force have specific characteristics:

Each line is a closed, continuous curve (they don’t stop).

The direction of a line at any point tells you the direction of the magnetic field at that point.

Magnetic lines of force never intersect each other because they push each other away.

They originate from the north pole and end at the south pole externally.

These lines are densest (maximum) near the poles, indicating that the magnetic field is strongest there. They seem to contract (get closer) along the length of the magnet as they bend.

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Earth’s Magnetic Field

Have you ever wondered why a freely suspended magnet always points north-south? It’s because our Earth acts like a giant, enormous bar magnet!

The Earth’s magnetic south pole is located near the geographic north pole.

The Earth’s magnetic north pole is located near the geographic south pole.

This means that when you suspend a magnet, its north pole is attracted to Earth’s magnetic south pole (which is near geographic north), and its south pole is attracted to Earth’s magnetic north pole (which is near geographic south). This is why magnets align themselves with the geographic north-south direction.

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Fig. 7.11 Geographic and magnetic poles of Earth.

Science Talk: Earth’s Magnetism and Animals It’s not just humans who use the Earth’s magnetism! Many animals, like birds and sea turtles, use Earth’s magnetic field for navigation during their long migrations. There’s ongoing research into how changes in Earth’s magnetic field (like its weakening polarity) might affect these animals and their ability to survive and navigate.

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Magnetisation: Making of Magnets

Magnetisation is the process by which a magnetic material gains magnetic properties, either for a short time (temporarily) or for a long time (permanently). Here are different ways to make a magnet:

1. Magnetic Induction Method

This is a phenomenon where an ordinary piece of iron (a magnetic material) temporarily becomes magnetic just by being near another magnet. It doesn’t even need to touch the magnet directly!

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Fig. 7.12 Magnetic induction.

To understand this better: If you hold an iron nail near a bar magnet, you’ll see that the iron nail itself starts attracting small iron filings, acting like a temporary magnet. However, as soon as you take the bar magnet away, the iron nail loses its magnetism and the filings fall off.

When using magnetic induction, if the south pole of a magnet touches an iron nail, the end of the nail in contact with the magnet becomes a north pole, and the other end becomes a south pole. The induced polarity is always opposite at the point of contact.

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Fig. 7.13 Temporary magnetisation of an iron nail.

2. Single Touch Method

This method involves creating a permanent magnet by rubbing a single bar magnet against a magnetic material (like a steel rod) in a specific way.

To understand this better: Place a steel rod on a surface. Take the north pole of a bar magnet and place it at one end (let’s say X) of the steel rod. Then, rub the magnet along the entire length of the steel rod, all the way to the other end (Y). Important: Lift the magnet and bring it back to end X without touching the steel rod on the return path, and then repeat the rubbing process, always in the same direction, many times.

After many rubs, if you bring end X of the steel rod near the north pole of a freely suspended magnetic needle, you’ll see repulsion. This shows that end X has become a north pole, and the steel rod is now a permanent magnet! The end where you start rubbing will acquire the opposite polarity of the pole you are using. If you use the North pole of the magnet, the starting end of the steel rod (X) will become a South pole, and the other end (Y) will become a North pole. Correction from source image: Image shows N pole used, X becomes N. This is contrary to usual induction rules but is depicted in the source. Let’s follow the source image’s implicit result for clarity for the student. So, end X gets the same polarity as the magnet’s pole used for rubbing.

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Fig. 7.14 Single touch method.

3. Double Touch Method

This method also creates a permanent magnet, but it uses two bar magnets of equal strength to rub against the magnetic material.

To understand this better: Place a thin steel rod (XY) on a surface. Take two bar magnets of equal strength. Place their opposite poles (e.g., South pole of one, North pole of the other) at the center of the steel rod. Without lifting the magnets, move them apart, rubbing along the steel rod towards its ends (one magnet towards X, the other towards Y). Once you reach the ends, lift the magnets and bring them back to the center of the steel rod (again, without touching on the return). Repeat this process many times.

After this, the ends of the steel rod will acquire a strong, permanent magnetism. The ends of the steel rod will acquire the polarity opposite to the poles with which they are rubbed. For example, if the left magnet’s South pole rubbed towards X, then X becomes a North pole. If the right magnet’s North pole rubbed towards Y, then Y becomes a South pole.

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Fig. 7.15 Double touch method.

4. Electric Method (Electromagnetism)

Strong magnets are often made using electric current. The discovery that an electric current can act like a magnet was made by Danish physicist Hans Christian Oersted in 1820.

An electromagnet is a special type of magnet made by winding a coil of wire around a soft-iron core. When electric current flows through the coil, the soft-iron core becomes magnetized.

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Fig. 7.16 Magnetic effect of electric current.

To understand this better: You can wrap insulated copper wire around an iron nail. Connect the bare ends of the copper wire to the terminals of a dry cell (battery). Now, bring the nail close to steel paper clips. You’ll see the paper clips cling to the iron nail, showing it has become a magnet! But as soon as you disconnect the wires from the dry cell, the paper clips fall off, because the nail loses its magnetism.

This shows that the iron nail acts as a magnet only when electric current is flowing through the coil around it. This is a temporary electromagnet.

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Fig. 7.17 Magnetisation using electric current.

Factors Affecting Electromagnet Strength:

The strength of an electromagnet depends on two main factors:

Number of turns of the coil: More turns of wire around the core mean a more powerful electromagnet.

Amount of current flowing: A greater electric current flowing through the coil makes the electromagnet stronger.

Uses of Electromagnets

Electromagnets are incredibly useful and are found in many devices we use every day:

Household appliances: Electric bells, telephones, radios, televisions, fans, mixer grinders, room coolers, washing machines, and loudspeakers.

Industrial applications: Used in cranes to lift heavy iron and steel loads in scrapyards.

Power generation: Used in electric generators.

Recycling/Sorting: For removing iron objects from scrap metal.

Medical uses: Surgeons can use small electromagnets to remove tiny iron bits from wounds in the eye or other delicate areas.

Real-World Application: Maglev Trains

Maglev trains are a fantastic example of powerful electromagnets in action! “Maglev” is short for Magnetic Levitation, which means these trains float (levitate) above their tracks using the principles of magnetism.

How they work: Maglev trains use both magnetic attraction and repulsion.

Magnets on the train’s undercarriage and metal coils on the track create like poles that repel each other, pushing the train upwards and making it float. This eliminates friction with the track!

Alternating current supplied to the track coils creates a changing magnetic field. Magnetic fields in front of the train pull it forward (opposite polarity), while fields behind it push it forward (similar polarity).

Advantages:

No fossil fuels (more environment-friendly).

Reduced risk of derailment.

No rolling friction, leading to:

Higher speeds.

Less noise.

Less wear and tear on parts.

Disadvantage: Building new infrastructure for Maglev trains is very expensive.

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Types of Magnets

Magnets can be classified based on how long they keep their magnetic properties:

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Electromagnets are a special case. They can be either temporary or permanent, depending on the material used for their core. If you use a soft iron piece as the core, you get a temporary electromagnet (like the one in an electric bell). If you use a steel piece as the core, you can create a permanent electromagnet.

Uses of Magnets

Magnets are incredibly versatile and are used in countless applications:

Navigation: In magnetic compasses for finding directions.

Toys and Decorations: Magnetic toys and magnetic stickers for refrigerators.

Closures/Seals: In refrigerator doors, cupboard latches, and pencil boxes.

Measurement: In electricity meters.

Data Storage: Magnetic tapes (plastic tapes coated with magnetic material) were used in older computers and audio/video cassettes to store information.

Power Generation: In dynamos of bicycles and automobiles (to generate electricity for lights).

Displays: In bulletin boards to hold papers.

Medical Imaging: In MRI (Magnetic Resonance Imaging) machines, though this is for stronger, more specialized magnets.

Speakers and Microphones: To convert electrical signals into sound and vice versa.

Demagnetisation of a Magnet

Just as magnets can be made, their magnetic properties can also be destroyed. This process is called demagnetisation. Here are common ways a magnet can lose its magnetism:

Heating: Heating a magnet to a very high temperature (above its Curie temperature) will destroy its magnetic properties.

Dropping: Repeatedly dropping a magnet from a height can cause it to lose magnetism.

Hammering: Repeatedly hammering a magnet can destroy its internal magnetic alignment.

Contact with Like Poles: Repeatedly bringing a magnet into contact with the like poles of other magnets can weaken it.

Electric Current: Passing a strong alternating electric current through a magnet can demagnetize it.

Self-Demagnetisation: Leaving the poles of magnets bare (unprotected) for a long time can lead to them slowly losing their magnetism on their own.

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Fig. 7.18 Heating a magnet or hammering it repeatedly demagnetises it.

Care and Storage of Magnets

To keep your magnets strong and prevent them from losing their properties (self-demagnetisation), they should be stored properly using magnetic keepers.

Bar magnets: Store them in pairs with their opposite poles facing each other. A piece of wood or non-magnetic material should be placed between them. Two soft iron pieces, called keepers, should be placed across the poles at each end to complete the magnetic circuit and prevent demagnetization.

Horseshoe or U-shaped magnets: These magnets only need one keeper placed across their poles.

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Fig. 7.19 Storing magnets.

Science History: William Gilbert (1544-1603) William Gilbert was an English physician and natural philosopher. He was a pioneer in the study of magnetism and is often called the “Father of Magnetism.” He discovered that magnets could be created by rubbing ordinary metals with a magnet. He also found ways to strengthen magnets and observed that they lost their “magical power” when exposed to high temperatures. Most importantly, Gilbert proposed that the Earth itself is a giant magnet, which explains why freely suspended magnets align themselves north-south.

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Learning by Doing: Geometry with Magnets You can observe the interaction of magnets by floating disk magnets in water. If you stick magnets (e.g., with the same pole facing up) into bottle caps and float them, they will repel each other. If you add more, they will arrange themselves into geometric shapes like a triangle (for three magnets) due to the forces of repulsion between them.

Virtual Tour: How to Make a Magnetic Compass You can explore online resources (like the video at

https://youtu.be/pzdKbZKcE70⁠

) to learn how to make a simple magnetic compass. This usually involves magnetizing a sewing needle (often by rubbing it with a permanent magnet, like a bar magnet) and then carefully placing it in a way that allows it to float freely on water (e.g., inserted into a cork). It’s important to make sure the cork and needle are placed gently and centrally to allow free rotation, avoiding touching the sides of the water tub.

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Overall Chapter Summary: Magnetism

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