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Magnetism

Background

From your everyday experience, you may have encountered the term magnet and learnt that magnets attract other materials such as razor blades, knives, and bottle tops. In previous classes, you may also have learnt about two groups of materials: those that are attracted by a magnet (magnetic materials) and those which are not, otherwise known as non-magnetic materials. In this topic, we shall learn more about magnetism.

Background

From your everyday experience, you may have encountered the term magnet and learnt that magnets attract other materials such as razor blades, knives, and bottle tops. In previous classes, you may also have learnt about two groups of materials: those that are attracted by a magnet (magnetic materials) and those which are not, otherwise known as non-magnetic materials. In this topic, we shall learn more about magnetism.

Topic objectives

By the end of this topic, you should be able to:

  • Describe the properties of magnets
  • Identify magnetic and non-magnetic materials
  • State the basic law of magnetism
  • Describe magnetic field patterns
  • Describe methods of magnetization and demagnetization
  • Describe uses of magnets

Topic objectives

By the end of this topic, you should be able to:

  • Describe the properties of magnets
  • Identify magnetic and non-magnetic materials
  • State the basic law of magnetism
  • Describe magnetic field patterns
  • Describe methods of magnetization and demagnetization
  • Describe uses of magnets

Magnetism

Introduction

Magnets are very important devices, either used on their own or as components of devices commonly used in everyday life. An example is the dynamo that lights the headlamp of a bicycle. In this topic, we are going to learn about some common properties of magnets and their applications.

 

Welcome

Welcome to Form 2 content of the digitized secondary physics series. In this volume, we are going to discuss some important concepts selected from the following topics:

    • Magnetism
    • Measurements II
    • Reflection at Curved Surfaces
    • Magnetic Effect of an Electric Current
    • Waves
    • Fluid Flow

     

     

     

     

Welcome to Physics for Form 2

In this series, we are going to learn some concepts selected from the following topics in Form 2 syllabus.

  • Magnetism
  • Measurements II
  • Reflection at Curved Surfaces
  • Magnetic Effect of an Electric Current
  • Waves
  • Fluid Flow

 

Magnetism

Properties of magnets
Magnets possess some common properties; and these include the pole and directional properties. We will investigate each property in turn.

(a) Pole property

What is the effect of putting a bar magnet into contact with iron filings and lifting it?

Observation:When a magnet is brought into contact with iron filings, the filings get attached all around the magnet but most of them cluster at the two ends of the magnet. This clustering is evidence that there is stronger magnetic attraction at the ends. These ends are called magnetic poles.

(b) Directional property
In the following demonstration, a bar magnet is suspended freely in a horizontal position by means of a piece of string, disturbed, then allowed to settle. Play the animation and note the direction in which the magnet finally comes to lie.

 

 

 

 

 

 

 

 


Observation

When a magnet is suspended freely in a horizontal position and given a slight displacement, it always rests pointing in a North-South direction. The pole pointing North is called a North-seeking pole while that pointing South is called a South-seeking pole.

 

 

 

 

 

 

 

 

Magnetic and non-magnetic materials

To investigate magnetic and non-magnetic materials

The following activity involves an investigation of magnetic and non-magnetic materials. Click on the button and observe the effect of bringing a bar magnet close to each of the following materials: match stick, steel needle, silver, rubber, piece of chalk, copper, steel pin, piece of iron, nickel and carbon rod.

 

 


 

Conclusion
It is observed that some materials get attracted to the magnet but others are not. Materials which are attracted by a magnet are called magnetic materials while those which are not attracted by a magnet are called non-magnetic materials. Magnetic materials include nickel, steel pin, steel needle and iron. This summary is given in Table 1.1.

Table 1.1:Magnetic and non-magnetic materials


Most of the common materials are non-magnetic.

 

 

 

 

 

 

 

Basic law of magnetism

What is the basic law of magnetism? The following video clip shows the behaviour of a magnet, A, when a second magnet, B, is brought close to it. The procedure is repeated with an unmagnetized iron bar, C, in place of B. Note the situations in which attraction occurs and those in which repulsion takes place.


 

 

 

 

Results

Table 1.2: Table of results


Conclusion

From the observations, two north poles repel each other. Repulsion also occurs between two south poles; but north pole attracts south pole. These results can be summarized in a general statement known as the basic law of magnetism. The basic law of magnetism states that:

Like poles REPEL each other while unlike poles ATTRACT each other.

 

 

 

 

 

 

Magnetic field patterns

When a magnetic material is placed near a magnet, it is attracted. This shows that there is magnetic effect in the area around a magnet. This area, where magnetic influence is felt is called a magnetic field. Magnetic field has a pattern which may be revealed by sprinkling iron filings around the magnet. Play the video and note the procedure.

The iron filings are observed to align themselves in a pattern which reveals the magnetic field pattern itself. It is represented in Fig.1.1.

Fig. 1.1: Magnetic field pattern
 

The lines along which iron filings align themselves are called magnetic field lines or magnetic lines of force.

Properties of magnetic field lines

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

2. They are closer together where the field is stronger.

3. They are at right angles to the surface of the magnet.

NB: A magnetic field must have directions indicated on the field lines.

The magnetic field associated with the pattern in Fig. 1.1 can therefore be represented as in Fig. 1.2.

Fig. 1.2: Magnetic field of a bar magnet

 

 

 

 

 

 

 

Domain theory of magnetism

In magnetic materials, each individual atom behaves as a tiny atomic magnet (magnetic dipole). A group of dipoles, with their magnetic axes aligned parallel to one another, form a magnetic domain. In different domains of the same magnetic material, the dipoles point in different and random directions.

 

 

Fig. 1.2: Magnetic material

 

 

 

Magnetization and demagnetization

Magnetization and demagnetization

When a material is magnetized, the dipoles align themselves in one direction. When all the dipoles are aligned in one direction, the material is said to be magnetically saturated.
 

 

Methods of magnetization

Magnetization process basically involves making the dipoles to be aligned in one direction as seen above. Suitable methods of magnetization are therefore those which enable us to achieve such alignment easily.

(a) Induction method

Activity 1.4: To demonstrate magnetic induction
Click on the play button to observe the effect of magnetization by induction.

 

 

 

 

 

Discussion

The pins are observed to attract and hold onto each other, showing that they have also acquired some magnetism. The process by which they have acquired magnetism is called induction. Induction is the method by which a magnetic material acquires magnetism when placed close to a magnet. What happens to the dipoles during the induction process? Click on the button to observe the effect on magnetic dipoles, of moving a magnet closer to a magnetic material such as iron bar.

 

 

 

 

The magnetic dipoles in the iron bar are observed to align themselves in the direction of the magnet and finally the iron bar moves towards the magnet. The alignment of magnetic dipoles causes the iron bar to become magnetized. This is what happens during the induction process.

 

 

 

 

(b) Electrical method

In this method, the material to be magnetized is placed in a solenoid connected to a direct current source (Fig.1.5). When a direct current (d.c) flows round a magnetic material, the material gets magnetized. Observe the behaviour of magnetic dipoles when the switch is closed and again when it is open.

On closing the switch, current flows and magnetic dipoles are urged to align themselves in one direction, causing the iron bar to become magnetized and attract the steel needle. The end Y becomes a North pole and X, a South pole. When the switch is opened, current stops flowing, the iron bar loses magnetism; so the falls off.

(c) Stroking method
Single stroke
In this method, the iron bar is stroked several times using a bar magnet as demonstrated in ANIMATION 1.6. Click on the button and take observations.

ANIM 1.6: Single stroke

During the process, the dipoles in the iron bar get more and more aligned with each successive stroking; so it gradually becomes magnetized. Finally the attraction becomes strong enough to move the pin.

 

 

 

Methods of demagnetization

Demagnetization process, being the opposite of magnetization, involves disrupting the alignment of magnetic dipoles. This can be achieved through any of the following methods.

  • Electrical method
  • Heating
  • Hammering a magnet


 

(a) Electrical method (using a.c. current)
Alternating current (a.c) is used instead of direct current (d.c). Alternating current keeps changing direction and hence disrupts the alignment of magnetic dipoles. Click on the button for ANIMATION 1.7 to observe the effect of alternating current on the alignment of magnetic dipoles.

Observation: With an alternating current in the solenoid, the alignment of dipoles becomes disrupted and face different directions. This leads to loss of magnetism.
 

(b) Heating
What happens to the particles of a substance when it is heated? When a magnet is heated, the dipoles vibrate faster and faster. This destroys the alignment of dipoles; so the magnet loses its magnetism. Click on the play button to observe this effect.

(c) Hammering

 

What happens to particles in a solid as the solid is hammered? Click on the button to observe the effect of hammering a magnet.Hammering a magnet while its poles are facing East-West direction also disrupts the alignment of magnetic dipoles and this explains how hammering causes demagnetization.

 

Applications of magnetism
Magnets have important applications in real life situations. These include:

  • Removal of magnetic objects that may accidentally get into the eye
  • Sorting of magnetic materials from non-magnetic materials
  • Lifting of containers at the sea port
  • Being a component of a loudspeaker
  • Navigation

Click on the button to observe some of these applications.

(a) A strong magnet being used to remove specs of a magnetic material from the eye

 

(b) A crane loading a container

 

 



 Magnetism 

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