Free Tutorial in Physics - Understanding Magnetism

Magnetism is a very interesting and unique property of some metals like iron. We all have seen a magnet attracting iron nails and iron pieces. In this article, we will try to understand the science behind this hidden magnetic power.

Introduction

Magnetism is a very interesting property of some metals and materials like iron and iron compounds. Most of us have seen magnets pulling iron nails or pins and might get amused by it. We will now try to understand the science behind this hidden magnetic power.

Discovery of magnetism

Lodestone (also known as magnetite), a mineral of iron, was known to have magnetic properties, for quite a long, and another important observation that was found was that when a piece of lodestone was hung freely using a string then the piece rotated itself in North-South direction. this was a surprising behavior but scientists explained it based on the natural magnetic field of earth. scientists have found that the earth behaves like a big magnet and the axis of this magnet is in almost North-South direction. The property of a magnet, when hung freely, aligning itself in a North-South direction was utilized by early navigators to find the direction while undertaking long sea voyages.

Understanding magnetism

A magnetic material like iron can be considered to have small domains, containing iron atoms, the magnetic properties of which are aligned in different directions. Once the magnetic strength of all these small domains are aligned in one particular direction then this material starts exhibiting an overall combined magnetic property. For example, if a nail of iron is kept on a railway track then continuous rubbing of rail wheels and acting of a tangential force on it, once a train passes over it, makes it a magnet as the magnetic properties of those small domains where the atoms are now aligned in a particular direction. If this alignment inside the nail remains like that for a long time then the nail will become a permanent magnet otherwise it will remain magnetic for some short time and then slowly return to its initial state and become a normal nail. Another example is that if we rub a magnet on a nail only in one direction repeatedly then also the small domains start aligning in that direction and slowly the nail starts to behave like a magnet. These observations suggest that the magnetic alignment of the domains is bringing the magnetic strength in one particular direction and the material starts behaving like a magnet.

It is interesting to note that most of the materials do not have this property and will not exhibit it. For example, wood, plastic, glass, paper, copper, gold, brass, aluminum, etc, and many other materials do not have magnetic properties.

Magnetic field of Earth

Now let us try to understand the behavior of the Earth as a big magnet. We all know that there is molten lava deep inside the Earth and it contains metallic ions. Earth is rotating about its axis (this axis is almost in a North-South direction), the ions in molten lava are also moving in large circular paths and whenever electric charges move in that fashion they create electric currents and with each electric current a magnetic field is associated. The direction of this magnetic field is perpendicular to the plane in which the electric current is flowing. So, Earth behaves like a big magnet and its magnetic field is approximately in a North-South direction.

Magnetic poles

We all have seen small magnets called bar magnets in the school laboratories for doing experiments with magnets. Every bar magnet has two poles in it located very near to its ends. These poles are named the North Pole and South Pole. If we bring two bar magnets near each other then we will observe that the same poles repel each other while different poles attract each other. This is the reason that when we hang a small needle magnet with a string freely then it moves and aligns itself in a North-South direction and the South pole of this needle magnet indicates towards North of Earth where the North magnetic pole of Earth is situated and North pole of this small needle magnet indicates towards South of Earth where the South magnetic pole of Earth is situated.

Artificial magnets

As we have learned that the circular movement of charged ions in molten lava inside Earth gave rise to an electric current and that caused Earth to behave like a magnet, similarly if we pass an electric current through a circular metal wire coil than we get a magnetic field associated with it in the perpendicular direction to the plane of the coil. By placing an iron rod or bar in this direction the strength of the magnetic field is increased and this design is called an electromagnet. These artificial magnets have a large-scale application in the industry.

Electromagnetic Waves

Some students ask why the word 'magnetic' is there in the name 'Electromagnetic Waves'. The answer is that these waves have an inherent electric and magnetic field and they travel ahead with that energy. These waves have a broad spectrum. The Sun rays coming to Earth, the light and other invisible waves coming from distant stars, radio waves, TV signals, X-rays, etc all are electromagnetic waves.

Magnets in Industry and Our Lives

Magnets have many useful applications in the industry as well as in our lives. These magnets can be manufactured in various shapes and sizes depending on their use. Many magnets are manufactured using iron alloys and various materials like neodymium, samarium cobalt, rare earth materials, and ceramics, etc are added for better performance.

Some of the uses of magnets are -

Used in laboratories in schools and colleges for understanding their behavior and characteristics.
Commonly used in loudspeakers, earphones, and other such audio equipment for converting electrical signals to sound enabling us to hear it.
Strong magnets are remotely used to handle hazardous materials kept in iron containers.
Used for segregation of iron chips or dust from a mixture.
Magnetic cleaners are used for removing any unwanted iron dust or small pieces of nails or screws.

Measurement of magnetic field and its units

For measuring the magnetic field we use some instruments known as magnetometers. They work based on the inter-relationships between a magnetic field and an electric current. For example, if we take a metal wire coil and move it in a magnetic field, some little current would be generated in this coil, and by measuring that we can find out the strength of the magnetic field at that particular place. In the CGS system, the unit of magnetic field is Gauss while in the SI system, the unit is Tesla. As Tesla is a big unit so nano Tesla (nT) is used in practice.

The measured magnetic field strength of Earth on the Earth's surface varies from 0.25 to 0.65 Gauss (25000 nT to 65000 nT). The conversion relation between Gauss and Tesla is -
1 Tesla = 10000 Gauss.

Earth's magnetic field is a weaker force. We can understand it by comparing it with the magnetic field very near to a school laboratory bar magnet which is of the order of 100 Gauss.

Conclusion

Magnetism is a very interesting but unique property and magnets are used for a variety of purposes in the industry today. Learning about magnets and magnetism helps the students understand various related scientific and technological processes.

Frequently Asked Questions

What will happen if we cut a bar magnet in two pieces?
Both the pieces will become a bar magnet.

What is the use of electromagnets?
They are used for a variety of purposes like in relays for closing or opening of electrical contacts, in buzzers for making sound, for lifting metal weights, etc.

Does the Moon have a magnetic field?
No. The moon does not have sufficient convective currents in its core to create a magnetic field.