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Introduction to Magnetism

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Magnetism Learning Material
Tutorial ID	Title	Tutorial	Video Tutorial	Revision Notes	Revision Questions
16.1	Introduction to Magnetism

In this Physics tutorial, you will learn:

What are magnets?
How many types of magnets are there?
What is magnetic field? How do we represent the magnetic field?
What are magnetic poles? What do they have in common with Earth Poles and where do they differ?
What is magnetic force? What happens when we bring like and unlike poles close to each other?
How a magnet is produced?
How to remove magnetism from a magnet?
What magnetic tool do we use to orient ourselves when no direction is known?
What is magnetosphere? How does it protect us?
How magnetism is used by animals?
How do we use magnetism in electrical equipment? Medicine? Industry?

Introduction

What happens when you bring a magnet near a small iron object such as needle, bolt, etc.? Is it necessary to touch them to observe this phenomenon?

Try putting some iron filings on a paper and move the magnet slightly below the paper for a while. What do you observe? Do the iron filings spread irregularly throughout the paper or not? Explain.

This tutorial is an introduction to magnetism - the other sub-branch of electromagnetism, which is one of the building blocks of physics.

A Brief History of Magnetism. Natural and Artificial Magnets

Magnets are materials that are able to attract or repel metal objects made from iron, steel, cobalt, nickel and iron oxides.

The term "magnet" derives from Magnesia - a town in Asia Minor (present day Turkey) where this phenomenon was observed first. People living in that town have been aware of the existence of attractive or repulsive properties of some "strange" rocks since more than 3000 years ago. They considered such rocks as very precious given this rare property they have.

Such magnets that exist in natural form and are obtained through mining are known as natural magnets. Nowadays, natural magnets are not so common because of the exploitation of natural resources and the technology development that has made possible the production of artificial magnets in factories. Thus, the magnets we use are all artificial.

Materials attracted by magnets are known as magnetic materials. For example, iron is a magnetic material as we can attract iron objects using magnets while copper, paper, plastics, gold, aluminium etc., are non-magnetic materials as they are not attracted by magnets.

Magnetic Field of Magnets and Earth. Magnetic Poles.

By definition, magnetic field is the space around a magnet in which its attractive or repulsive effect is observed. As you see, the definition of magnetic field is very similar to those of other fields (gravitational and electric) we have discussed earlier.

A magnet behaves in opposite way when approaching a magnetic material in two opposite positions. For example, if we bring one end of a bar magnet near an iron nail the magnet attracts the nail while if we bring the other end of the magnet near the same iron nail, the magnet repels it. These two extremities are known as magnetic poles. To make distinction between poles, we call them North and South poles respectively. The North pole is coloured in red while the South pole in blue, as shown in the figure below.

Physics Tutorials: This image provides visual information for the physics tutorial Introduction to Magnetism

The general law governing the interaction of magnets is similar to that of electric charges, i.e.:

"Like poles repel each other while unlike poles attract each other."

The interaction between magnets is an indicator for the existence of the magnetic force - a force that causes attraction or repulsion between poles as in the law discussed above.

The reason why magnetic poles are named North and South is because when a bar magnet like the one shown below is suspended using a thread, it will align in the North-South direction of the Earth after it stops swinging (when the equilibrium is reached).

Therefore, the magnetic pole pointing towards the geographic North Pole is the South Pole of magnet. However, sometimes it is called the North seeking pole as it point towards the North Pole of Earth. The same for the North Pole of magnet as well. Therefore, one must be careful to avoid confusion.

The reason why this alignment occurs is the existence of a magnetic field produced by the Earth. Indeed, Earth is a giant magnet producing its natural magnetic field. As a result, one of the seven layers of atmosphere known as magnetosphere does exist. It is the outmost layer of atmosphere. It is very crucial in repelling the hazardous cosmic rays coming from all parts of the universe. Therefore, magnetosphere is irreplaceable in terms of the continuity of life on earth despite the fact it does not contain any air.

Magnetic Field Lines

The effect of magnetic field is understood better is we draw some imaginary lines (similar to those of electric field) to represent it. However, first we must do an experiment to determine the direction of magnetic field lines. This experiment is mentioned in the introduction part of this tutorial. When we place some iron filings on a paper and move slightly a bar magnet below the paper, the iron filings will align in the way shown in the figure:

This experiment is a demonstration that:

Magnetic field lines (unlike electric field ones) are closed lines that start from one pole (North) and end to the other pole (South).
The magnetic field lines are denser at the poles. This means that the magnetic field is the strongest at the poles.

We represent the magnetic field lines through arrows starting from the North Pole and ending to the South Pole of magnet as shown below.

Magnetic field lines are not always curved. In U-shaped magnets for example, magnetic field lines in the shortest path from N-pole to S-Pole are straight, as shown in the figure.

Like electric field lines, magnetic field lines have a very important property: they do not cross each other. This means the magnetic field changes gradually through a predictable process.

Remark! A magnet is a piece of metal that is coloured in the factory in which it is produced. The red and blue colour are just conventional signs; magnets are coloured to help the user identify which is the N-pole and which is the S-pole of magnet.

The following table summarizes the properties of magnetic field and comperes them to the properties of electric field discussed in Section 14.

From the above table we can deduce other properties of magnets such as:

All magnets have two poles; unipolar magnets do not exist.
When a magnet is cut in two pieces, two new magnets with two poles each are obtained.

Example 1

The magnet in the figure is cut in four equal pieces.

What is the polarity of the new magnets obtained?

Solution

Since the edges of the original magnet are not affected by the cutting process, they keep the original polarity and every new polarization is made by taking into account the fact that all magnets have two opposite poles. Therefore, the four small magnets will have the polarities as shown in the figure below.

How is a Magnet Produced?

A magnet can be divided in pieces (as discussed in the above example) to obtain smaller magnets. This process lasts until we obtain a single proton-electron pair. Such pairs known as dipoles are the smallest magnets possible. Dipoles are assumed as enclosed within small rooms called domains where each domain contains a single dipole. In normal conditions, all materials are neutral and their dipoles are randomly oriented as shown in the figure.

When we allow some electric current flow through material, the dipoles are oriented according the direction of electric current. In this way, a bipolar magnetic material is obtained. The total charge is still zero; this means the object is only magnetised, not electrically charged. This method of magnetisation is known as the electrical method. Look at the figure:

Other ways to obtain a magnet include:

Contact method consisting in touching a magnet with a metal that possesses magnetic properties. In this way, the metal is magnetised after a certain period due to the regular orientation of its dipoles;
Induction method, i.e. bringing a non-magnetized metal near a magnet. After a while, the metal is magnetised, i.e. its dipoles are oriented according the magnetic field lines;
Stroking method. If we strike gently a steel needle by a magnet, the needle is magnetised after a number of strokes.

Demagnetisation, i.e. the process of magnetism removal from a magnetised object, is carried out in one of the three following ways:

By striking the magnet with a hammer. Strong strokes distort the regular orientation of magnetic dipoles, turning the metal in the state is has been prior to becoming a magnet.
By dropping the magnet from a height. This method works in a very similar way to the previous one.
By heating the magnet. Heating an object results in a higher kinetic energy of its molecules. This brings a less regular orientation of dipoles and consequently, a decrease in the magnetic properties of the object.

Permanent and Temporary Magnets

In the first paragraph, we discussed about natural and artificial magnets. This is a kind of magnets classification but it is not very useful because the overwhelming part of magnets used today are artificial. Therefore, a new classification of magnets based on the ability to preserve the magnetic properties is used. Thus, we classify magnets in permanent and temporary based on their ability to keep the magnetism. Let's take a closer look at these categories of magnets.

Permanent magnets are magnets able to keep their magnetism for a long time. Cast-iron is an alloy that is used to produce permanents magnets. Most of magnets we use today are made from cast-iron. This is why we use these magnets for long periods without noticing any considerable decrease in the magnetic properties. Other materials used to obtain permanent magnets include steel, brass, iron oxide etc.
Temporary magnets are magnets that lose their magnetism very quickly; almost immediately after the electricity supply is removed from them. Iron is an example of a temporary magnetic material. As long as an iron piece is placed inside an electric field, it is magnetised. When we stop the current flow, i.e. when we remove the electric field, the iron piece loses immediately its magnetism. Such a property is used by engineers to produce electromagnets, the magnetism of which is controllable, i.e. we can use them as magnets when needed and remove their magnetism in other moments.

Temporary magnets are also known as soft magnetic materials.

The Earth as a Giant Magnet. Two North Poles.

We explained at the beginning of this tutorial that a bar magnet suspended in a thread points towards the two geographic poles. This is the reason why we use the terms North and South to describe the poles of a magnet. In addition, we explained that such a behaviour of bar magnets indicates the existence of Earth magnetic field, which determines the orientation of bar magnets in space.

Indeed, Earth is considered as a giant magnet containing its own magnetic field, whose lines as shown in the figure below.

In fact, magnetic and geographic poles do not lie exactly in the same direction; they diverge from each other by an angle of about 220. This means when a traveller starts moving from equator in the direction of North Pole indicated by the magnet, he will not reach the geographic North Pole but he will instead end his motion about 2000 km away from it. In other words, if a person starts travelling from Africa towards the North Pole indicated by the suspended bar magnet, he will end his travel at north of Canada, not at the North Pole of the Earth. That's why there exist two distinct North Poles: one geographic and the other magnetic.

The device used from travellers to orient themselves - especially when sailing in oceans, during which it is very difficult to know the direction as no land is visible - is called compass. It consists on a small magnetic needle placed on a thin pin, parallel to a circular case in which the main directions are written.

The Earth's magnetic field is similar to that of a bar magnet extending from the North to the South magnetic pole. This means the magnetic field lines near the poles are vertical while in other positions they are parallel to the Earth surface, i.e. they are horizontal.

Magnetosphere

The magnetosphere is the region of space surrounding Earth where the dominant magnetic field is the magnetic field of Earth, rather than the magnetic field of interplanetary space. The magnetosphere is formed by the interaction of the solar wind with Earth's magnetic field.

The magnetosphere shields the surface of the Earth from the charged particles of the solar wind and is generated by electric currents located in many different parts of the Earth. It is compressed on the day (Sun) side due to the force of the arriving particles and extended on the night side.

Magnetosphere acts like an umbrella, protecting Earth from hazardous cosmic rays coming from the sun. In the absence of the magnetosphere, these cosmic rays may cause cancer in humans and animals. Under these circumstances, life on Earth would end very soon.

The average distance of magnetosphere from the Earth is about 65000 km. It extends well beyond the other layers of atmosphere, which extend up to 1000 km above the Earth surface.

Magnetism in Animals

Some animals such as pigeons and magnetostatic bacteria use the Earth magnetic field to find the direction of motion. Thus, pigeons have some small particles known as "magnetites" on the back of their head. When pigeons fly, magnetites interact with Earth magnetic field and help them find the right direction.

On the other hand, magnetostatic bacteria - which live in Polar regions - move up and down in water according the Earth magnetic field lines in these regions, to find their food. In this way, they can move down to nutrient-rich sediments or stay in optimal depths in water.

Other animals that are found to contain magnetites include monarch butterflies, tuna fish, whales, dolphins, etc.

Magnetism in Use

Magnets have a wide range of use in today's world. In this paragraph, we will briefly discuss only a few of them.

Magnetism in home appliances
Magnets are used in many home appliances such as in refrigerators, computers, sound systems, etc.
Flexible magnetic strips are enclosed within plastic covers attached around a fridge's door. These magnets keep the fridge's door firmly closed so that no heat is exchanged with the surroundings.
Hard discs of computers are covered by a layer of magnetic material. Magnetism is used in computer storage because it goes on storing information even when the power is switched off.
Sound systems use magnetic materials to transmit the sound through loudspeakers. Inside almost every speaker, there is a magnet, a coil of wire, and a thin material to convey the sound into the air. The invention of strong rare-earth magnets allows speakers to create more sound using less electric current.
Magnetism in medicine
Magnetic resonance is one of the most marvelous applications of magnetism in medicine. Most imaging techniques make use of X-rays to see inside the human body. Magnetic resonance is one of the revolutionary and techniques besides echo that is not hazardous for the health. Nuclear Magnetic Resonance NMR technique is used in medicine to obtain three-dimensional images of the inner parts of the human body. NdFeB (neodymium) is a high-end rare earth magnet. In terms of medical treatment, NdFeB has been widely used in physical magnetic therapy. Due to its excellent magnetic properties, NdFeB magnets have been widely used in the field of medical treatment. The use of this magnet can increase the blood supply and supply of the brain, reduce the excitability of the nerve of the cerebral cortex, and increase the blood circulation to the organs of the lungs, spleen, liver and perianal, and boost the local blood supply by enhancing the biological electromagnetic energy of the body and collaterals. NdFeB is often used for the physical treatment of a variety of diseases.
Magnetism in industry
Magnetism in industry is mainly used in metal processing technology. Permanent magnets are used to separate metals from non-metals applying the procedure shown in the figure.

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Continuing learning magnetism - read our next physics tutorial: Magnetic Field Produced by Electric Currents