ICSE Class 8 Physics Chapter 07 Magnetism and Electricity

Magnetism and Electricity

Learning Outcomes

Properties of magnets

Magnetic field

Earth's magnetism

Electromagnetism

Solenoid

Uses of electromagnets: electric bell

Electromagnetic induction

Introduction

Read the following questions that Sheena and her friends have in mind. See if you can answer them.

Sheena: How do such huge pieces of iron stick to the block on the crane?

Rohit: How do the pins stick to the middle rod in the pin holder?

Sharmila: Hey! If I leave the door of the refrigerator open, it closes by itself. How?

What particular thing are Sheena and her friends talking about? Yes! They are talking about magnets. You might have seen magnets and enjoyed playing with them. You might have seen that some pencil boxes close by themselves if you leave their lid free? Try to locate the magnet in them.

In this chapter, you will study about magnets, their properties, how magnetism is related to electricity, and its everyday applications.

According to a legend, the first magnet was discovered by a Greek shepherd named Magnes. The Greek named this strange type of rock 'magnetite'.

Ancient people believed that magnets had magical properties, like the ability to scare away ghosts. Ancient navigators also used magnets for navigation.

Properties of Magnets

More than 2,500 years ago, it was discovered that certain rocks had the property of attracting iron. It was also found out that small pieces of these rocks also have directive property. These naturally occurring materials are called magnets. Later on artificial magnets were made from steel, an alloy of iron.

Do you know that not all materials are attracted by magnets? Materials like iron, nickel, and cobalt that are attracted by magnets are called magnetic materials. On the other hand, materials like aluminium, copper, and silver that are not attracted by magnets are called non-magnetic materials.

Now-a-days magnets are made in different shapes and sizes, depending on their usage. Can you name the different types of magnets that you see alongside?

You might have studied about some basic properties of magnets in your earlier classes. Do you remember some of them? Let us briefly revise them before going any further in this chapter.

Property of Attraction

You might have experienced that when you dip a magnet in a bowl of steel ball pins, maximum number of pins are attracted at the two ends of the magnet, which are called the poles of a magnet. This means that the magnetic force is stronger at the poles and weaker at the centre of a magnet. When a magnetic material (soft iron) is brought close to, or touches, the pole of a magnet, the magnetic material becomes a magnet itself and gets attracted. Here, we say that magnetism is induced in it. The end of the magnetic material nearer to the north pole of the magnet gets south pole and the farther end gets north pole induced in it. This magnetic induction depends on the strength of the magnet, the type of the magnetic material, and the distance between the two.

Property of Direction

If you tie a magnet to a string and suspend it freely, or place a magnet on a thermocole in a bowl of water and allow it to float, you will find that the magnet comes to rest in a particular direction, i.e., in the north-south direction. If you further disturb the magnet it will again come to rest in the same direction. This shows that a freely-suspended magnet always comes to rest in the north-south direction. The pole/end of the magnet facing north is called the north pole (N) of the magnet and the other facing south is called the south pole (S). These two poles cannot exist independently and always remain in pair. If a bar magnet is broken in the middle, the two pieces that we get have both north and south poles. If we continue breaking the magnet into still smaller pieces, every piece of magnet that we get will have both north and south poles.

Just like in electric charges, like charges repel and unlike charges attract; similarly, in magnets like poles repel and unlike poles attract. This can be verified by bringing two magnets close together. Since repulsion happens only between like poles of two magnets, repulsion is considered to be a sure test for magnetism. After having studied about the properties of magnets, can you answer a simple question? Can the attractive property of a magnet be felt at any distance from the magnet? Place some iron nails near a magnet so that they experience a force of attraction. Now gradually increase the distance between the magnet and the iron nails. At one point you will find that the magnet no longer attracts the iron nails. What does this show?

This shows that there is a specific region around a magnet where its magnetic effect is felt. This region is called the magnetic field of the magnet.

Magnetic Field

The space around a magnet where its influence (effect) is felt is called the magnetic field. Any magnetic material placed inside the magnetic field of a magnet experiences a force, and the direction of this force is given by closed continuous curves called magnetic lines of force. Let us look at Figure 7.3 to understand the magnetic lines of force.

If you scatter iron filings on a piece of paper, where a bar magnet is placed, you will see that the iron filings arrange themselves in specific curves around the bar magnet. These curves are called magnetic lines of force. The direction of these lines is always from the north pole to the south pole outside the magnet and from the south pole to the north pole inside the magnet.

An important property of these lines of force is that they never intersect each other. This is because at each point in the magnetic field of a magnet, the force of the magnet acts in only one direction. Also, you can see that the lines of force are highly concentrated near the poles, where the magnetic field is strong.

Our earth also behaves like a huge bar magnet and has its own magnetic field. Let us study about it.

Magnetic Field of the Earth

We have studied that a freely suspended magnet always comes to rest in a particular direction. This happens because this magnet is under the influence of another magnet, which is our earth. The earth acts like a giant bar magnet. It influences all the magnets so that they align themselves along the north-south direction of the earth.

Look at Figure 7.4. The bar magnet shows the magnetic effect of the earth. One interesting thing to note is that the bar magnet is aligned in the north-south direction, but its north pole points towards the earth's geographic south pole, and its south pole points towards the earth's geographic north pole. Hence, when a magnet is freely suspended, the north pole of that magnet always points towards the earth's magnetic south pole (which is the geographic north pole) and vice versa. The magnetic compass is based on this principle. Do you notice in the figure that the magnetic and geographic poles do not coincide? There is a difference of some degrees, and navigators need to calculate this difference to reach the correct destination during navigation.

Activity: Plotting Magnetic Lines of Force

Aim: To plot magnetic lines of force using a compass needle.

Materials required: A bar magnet, a compass needle, a sheet of paper, and pencil.

Procedure:

1. Place the bar magnet on a piece of paper and the compass needle near one pole of this magnet. Mark the positions taken up by the poles N and S of the compass as dots 0 and 1. Move the compass needle so that pole S is exactly over 1, and mark the new position of N as dot 2.

2. Continue the process until the S pole of the bar magnet is reached. Join the dots together to get a single magnetic line of force.

3. Other magnetic lines of force can be plotted by starting at different points around the magnet. Like this you will get a typical field pattern as shown in the figure.

Now you know that every magnet has its own magnetic field. You have studied about electricity in class 7. Do you know that magnetism and electricity are inter-related?

Electromagnetism

When an electric current flows through a conductor, it creates a magnetic field around the conductor making it behave like a magnet. This forms the basis of electromagnetism. Let us study this in detail.

The term electromagnetism comes from the fact that electric and magnetic forces are linked together. A Danish scientist, Hans Oersted, in 1820 noticed that when a compass needle is kept near a wire carrying electric current, the needle gets deflected. This means that a magnetic field can be produced by an electric current.

The branch of physics that deals with the magnetic effect of electric current is called electromagnetism.

When a wire is wound around a soft iron bar and electric current is passed through the wire, the soft iron bar behaves like a magnet, whose strength is much more than the magnetic effect produced by the wire alone. But is the magnetism of the soft iron permanent? No! the magnetism is soon lost when the current is switched off. This shows that the soft iron behaves like a magnet only when the wire around it carries current. A magnet of this kind is called an electromagnet.

After studying that a wire carrying current produces a magnetic field around it, let us see how this magnetic field is produced in a straight wire and in a circular coil, carrying current.

Magnetic Field Due to a Straight Wire Carrying Current

Figure 7.5 shows what happens when current is passed through a straight wire. A straight wire is passed through the centre of a cardboard, which has iron filings scattered on it. When a current of few amperes is passed through the wire and the cardboard is tapped, the iron filings are seen to arrange themselves in concentric magnetic lines of force with their centre lying on the wire.

The direction of the magnetic field produced like this can be found by Maxwell's right-hand thumb rule, if the direction of the current is known.

Maxwell's right-hand thumb rule states that if you hold the current-carrying wire in your right hand so that your thumb points in the direction of the current, then the direction in which your fingers encircle the wire gives you the direction of the magnetic lines of force around the wire. On reversing the direction of current in the wire, the direction of the magnetic lines of force is also reversed.

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