Keywords

• Atom  
• Compass.
• Electron
• Element magnet
• Magnetic declination
• Magnetic domain
• Magnetic field
• Magnetic field lines
• Magnetic force
• Magnetic pole neutron nucleus
• Permanent magnet
• Proton
• Temporary magnet

By the end of this chapter, you will be able to:

1. Know that a small number of materials are magnetic, but most are not.
2. Know how magnets can be made and destroyed.
3. Understand the behavior of magnets and magnetic fields
4. Know that the earth is a magnet and how a compass is used to determine direction

6.1 The Nature of Magnets

Introduction

People have been aware of magnets and magnetism for thousands of years. The earliest records date to well before the time of Christ, particularly in a region of Asia Minor called Magnesia (the name of this region is the sources of words like magnetic). Magnetic rocks found in Magnesia, which is now part of western Turkey, stimulated interest in ancient times. The use of magnets in compasses resulted not only in improved long – distance sailing , but also in the names of ” north ” and ” south ” being given to the two types of magnetic poles of the earth

In this chapter, you will investigate and understand the properties of magnets and explain how the earth behaves as a magnet.

Magnetic and non – magnetic materials

 Activity 6.1 Identifying / classifying magnetic and non-magnetic materials

What you need

• Copper coin, iron nails, plastic, wood, glass, aluminium, and a horseshoe magnet.

 What to do

 (a) Spread your materials on a table.

 (b) Classify the materials basing on their ability to be attracted to a magnet.

(c) Bring a horseshoe magnet with its poles moving towards the materials.

 (d) State what happens when the magnet comes closer to each end of the materials. How do you conclude?

(e) Why do you think some of the particles were attracted to the terminals of the U – shaped material?

 (i) What special feature does the U – shaped material possess that made it attract the particles?

(g) Suggest other materials and fill in the table below:

Research 6.1

 Investigating strength of magnetic materials

 In pairs, do as follows and come up with a report:

 (a) Investigate the strength of available magnetic materials like nails and pins from your laboratory using a chain as a measure.

(b) Communicate your report to your teacher and then, finally, to the whole class.

Earth as a magnet

 William Gilbert, a physicist, first proposed in 1600 that the earth itself is a magnet, and he predicted that the earth would be found to have magnetic poles. The earth’s south magnetic pole is located near the south geographic pole and the earth’s north magnetic pole is located near the north geographic pole.

Figure 6.1: Showing earth behaving as a magnet

6.2 Magnetic Poles

Activity 6.2 Identifying the poles of a bar magnet

 Figure 6.2: Poles of a magnet

What you need

• A bar magnet, iron filings and a piece of paper. .

 Key Question: How can you locate or identify poles of a magnet?

What to do

• Place a bar magnet horizontally on iron filings spread uniformly on a piece of paper.
• Lift the magnet and observe the distribution of filings on it.
• Which points on the bar magnet have attracted the filings the most?

 DID YOU KNOW that the parts of a magnet where there is a greater force of attraction are called the magnetic poles?

 Activity 6.3 Identifying the poles of a bar magnet

What you need

• A bar magnet, a retort stand, a piece of thread, and a sticker.

Key Question: How can you locate or identify poles of a magnet using earth’s magnetic field?

What to do

1. Suspend a bar magnet horizontally on a retort stand using a piece of thread as shown in Figure 6.3. Thread Magnet Paper
2. Slightly adjust the magnet once so that it swings horizontally before it finally comes to settle. Place a sticker on the end that faces the geographic north when the magnet finally settles. Stand
3. Repeat step (b) above without removing the sticker (the stickers should be left intact for the next experiment).
4. One end of a bar magnet usually has a mark put by the manufacturer. Which pole does this mark indicate?
5. Why was it necessary to repeat step (b) of the procedure?
6. Explain why the string used should be flexible.
7. Give a reason why the magnet was slightly adjusted in step (b).
8. In which direction does the magnet finally settle when freely suspended?
9. Suggest a reason why a magnet always settles in the same direction.

Figure 6.3: Determining the pole of a magnet

When a magnet is freely suspended so that it can swing in a horizontal plane, it oscillates to and fro for a short time and then comes to rest in an approximate N – S direction.

The pole which points towards North Pole the north is called north – seeking or simply the N – pole; the other is called the south – seeking or S – pole. By convention, we denominate as the North Pole of the needle the pole directing towards the North magnetic pole in the equilibrium position, while the South Pole of the needle is the one directing towards the south magnetic pole.

6.3 Shapes of Magnets

Exercise 6.1

The different shapes below are magnetic

Figure 6.4: A plotting compass

Figure 6.5. Different shapes of magnets

1. Identify the different shapes of magnets shown in Figure 6.5.
2. Discuss where each of the shapes of magnets is applied.

Uses of magnets

Magnetism is often used to do work, but it can also be used to create art. People use magnets to hold things closed and to link things. They do also use them in televisions and electric motors.

 A farmer may put a magnet in a cow’s stomach. The magnet attracts bits of metal that the cow may eat. This keeps the metal from hurting the cow.

LAW OF MAGNETIC POLES

Activity 6.4 Finding out interactions of poles of a magnet

What you need

• 2 bar magnets

Key Question: What happens when like poles and unlike poles are brought together?

 What to do

Perform this activity in groups:

(a) Bring close to each other two ends of bar magnets that pointed in the same direction (like poles). What happens?

b) Repeat the experiment with two ends of magnets that pointed in different directions (unlike poles). What happens?

 (c) Deduce the law of magnetism using your findings in 1 and (b) above.

Explanation

The poles which point in the same direction when they are brought close together repel each other. Those which point in opposite direction attract each other. This observation is summed up in the first law of magnetism which states that like poles repel and unlike poles attract.

If the N – pole of a magnet is brought near the N – pole of a suspended magnet, it is noticed that repulsion occurs. Similarly, repulsion is observed between two SS – poles. On the other hand, an N – pole and S – pole always attract each other.

Figure 6.6: Attraction and repulsion of magnets

6.4 Tests for Polarity of Magnets

The polarity of any magnet may be tested by bringing both its poles, in turn, near to the known poles of a suspended magnet. Repulsion will indicate similar polarity. If attraction occurs, no firm conclusion can be drawn, since attraction would be obtained between either:

 (a) Two unlike poles; or

 (b) a pole and a piece of unmagnetized material.

Research 6.2

(a)On the internet, surf what happens when a magnetic needle is freely suspended.

 (b) Write a report and present your findings to your friends and then finally to the whole class.

6.5 Magnetization and Demagnetization of Magnets

Magnetization: This is the process in which magnetic materials attain magnetism.

 The methods of magnetization are:

• By bringing magnetic materials near the magnet.
• By stroking. A piece of magnetic material can be turned into a magnet if it is stroked by a magnet. As the magnet moves along the magnetic material, it causes the magnetic dipoles in the magnetic material to become aligned in one direction and give rise to a magnetic field.

Note: When an electrical current is made to run through a coil, the energized coil creates a magnetic field, hence forming electromagnets.

Demagnetization: This is the process of removing magnetic property from the magnet. Magnets can be demagnetized by:

• rough handling like hammering.
• heating
• passing it through a solenoid which is electric itself.

DOMAIN THEORY.

In ferromagnetic materials, smaller groups of atoms band together into areas called domains, in which all the electrons have the same magnetic orientation. That is why you can magnetize them.

A domain is a region inside a material where groups of magnetic moments naturally align in the same direction.

 There can be numerous domains within an object. When there is no external magnetic field present, the domains are also oriented randomly so that there is no net magnetic field. However, when an external magnetic field is present, the domains will rotate and align with the external magnetic field. When all or most of the domains are aligned in the same direction, the whole object becomes magnetized in that direction and becomes a magnet.

Figure 6.7: Magnetic domain

Types of Magnets

 Assignment 6.1

Discussing types and applications of magnetic materials.

 In pairs, brainstorm

(i)  types of magnets.

(ii)  applications of magnetic materials

(ii) share your findings with the rest of the class members.

There are two basic types of magnets: permanent and temporary magnets.

 Temporary magnets

Temporary magnets become magnetized in the presence of a magnetic field. They lose their magnetism gradually, when the magnetic field is removed. Some irons and iron alloys, as well as paper clips and nails, function as temporary magnets.

 Figure 6.8: Screwdrivers can be temporarily magnetized

Permanent magnet

Permanent magnets do not easily lose their magnetism. These materials retain their magnetism even after the removal of the applied magnetic field. Hence, these materials are used for making permanent magnets. In permanent magnets, the movement of the domain wall is prevented. These magnets may be naturally occurring (“rare – earth ” ) elements or chemical compounds.

 Permanent magnets are used to:

1. Lift heavy loads in industries. They handle loads with extreme easiness in the minimum area. This makes them efficient because they always operate from the top without compressing or deforming the load.
2. Remove iron pieces from the eyes of patients in hospitals.
3. Set Six’s maximum – and – minimum thermometer in weather stations.
4. Show direction, as in compass needles for navigation.
5. Magnetic tapes use permanent magnets in audio and video recorders.

Examples of permanent magnets include Alnico (an alloy of aluminum, nickel and cobalt) and ferrites (ceramic – like material made from a mix of iron oxides with nickel, strontium or cobalt).

MAGNETIC FIELD

The compass or magnetic needle furnishes a clear example of the action of the terrestrial magnetic field on a magnetic needle. It is a device for finding directions, usually with a magnetized needle that automatically swings to the magnetic north.

Activity 6.5 Plotting magnetic field using iron filings

 What you need

• Bar magnet, U – shaped magnet, iron filings, and piece of paper.

Key Question: How can you plot the magnetic field of a given magnet?

What to do

• Sprinkle the piece of paper with iron filings when it is placed far from magnets. Do you observe lines in iron filings? Put the piece of paper above the bar magnet. Do you. Observe lines in iron filings?
• Gently tap the piece of paper. What happens to the lines?
• Sketch the lines that you have observed.
• Repeat the experiment with a U – shaped magnet.
• Discuss your findings to the whole class.

Did you know that the region around a magnet in which the magnet exerts a magnetic force is called a magnetic field? When the piece of paper is placed in magnetic fields of the magnet, iron filings form a pattern of lines. These lines represent the magnetic pattern. When iron filings are spread over a bar magnet, they are oriented along magnetic field lines.

 Figure 6.9 : Magnetic field around : ( a ) a bar magnet , ( b ) U – shape magnet , ( c ) 2 unlike poles of 2 bar magnets , ( d ) like poles of 2 bar magnets and ( e ) unlike poles of 2 U – shape magnet

In (d) of Figure 6.9, x is the neutral point, a point where there is no resultant magnetic force.

Chapter Summary

 In this chapter, you have learnt that:

• A magnet is an object that exhibits magnetic properties such as:
• Exerting an attractive force on iron or other ferromagnetic materials.
• Exerting both attractive and repulsive forces on other magnets. There are two types of magnets, namely permanent magnets and temporary magnets.
• Magnetic poles are regions on a magnet where the magnetic forces of attraction or repulsion are the strongest.
• Magnets come in two types – North Pole and South Pole, often shortened to north and south or abbreviated N and S.
• The law of magnetism states that “like poles repel and opposite poles attract
• A compass is any dipole magnet that is free to rotate.
• A magnetic field is the region around a magnet in which the magnet exerts a magnetic force.
• The direction of the magnetic field is determined by following the north pole of a compass.
• Magnetic field lines diverge from the north pole of a magnet and converge on its south pole.
• The strength of the magnetic field at any location is proportional to the density of the lines drawn.
• Magnetic field lines never intersect (since the field can only point in one direction at any location).
• Electromagnets are created by running an electrical current through a coil with a metal core.