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Tutorial ID | Title | Tutorial | Video Tutorial | Revision Notes | Revision Questions | |
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16.1 | Introduction to Magnetism |
In this Physics tutorial, you will learn:
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.
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.
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.
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.
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:
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:
The magnet in the figure is cut in four equal pieces.
What is the polarity of the new magnets obtained?
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.
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:
Demagnetisation, i.e. the process of magnetism removal from a magnetised object, is carried out in one of the three following ways:
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.
Temporary magnets are also known as soft magnetic materials.
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.
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.
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.
Magnets have a wide range of use in today's world. In this paragraph, we will briefly discuss only a few of them.
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