# Basic Physics lesson -15: Magnetism

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## Introduction

We have learned so far in this series of basic Physics articles about the linear motion of the bodies as well as the circular or rotational motions. Thereafter, we learned about the force, work done, and energy also. Subsequently, we learned about the entities called voltage and current which are so often mentioned in the field of electricity.

Moving ahead with a quest to understand the other topics of basic Physics, we now take up the subject of magnetism which had intrigued scientists and scholars alike in past and still posing up challenges in the areas of research in magnetism.

## That invisible power

Everyone might have seen a magnet and how it attracts an iron nail or pin towards it which gets attracted and clings to it. Many magicians use magnets hidden in their sleeves to show the tricks where something vanishes to nowhere. Anyway, we will now try to understand what is there which brings a magnetic power or magnetic property to a material. Why a wooden piece cannot be a magnet while an iron piece can be made a magnet through some simple magnetising methods. It is obvious that every material is not a magnetic material.

## The magnetic property

Some elements like iron have this unique property of acquiring magnetism and then to retain it either for some short time or a longer one. Scientists have tried to explain it on the basis of domain theory where the material is supposed to be consisting of a large number of domains where the atoms are aligned in a particular direction within a domain. So normally these domains will be aligned in different different directions in a chaotic fashion but when due to some magnetising force if all the domains align in one direction then the material acquires a magnetic property and starts attracting the other materials having similar characteristics. I remember that during my school times some of my friends were keeping a big nail on a Railway track and once the train passes above it the nail gets magnetic properties. The enormous tangential force applied by the Train wheels over the nail does the trick of magnetising the nail. We had also done practicals in which we rubbed a nail with a magnet, a few times in the same direction and found that the nail itself became a magnet.

## Natural magnets

In nature, lodestone, a mineral compound of iron, also known as magnetite, shows magnetic properties. This was a great discovery as this material had some peculiar properties which the early navigators used for navigation while crossing the oceans. A piece of lodestone when hanged freely, tied with a string, rotates to align itself in an approximately North-South direction. Why did it do so? The reason was simple and was explained based on the Earth's magnetic field which itself is in the same direction and is powerful enough to align the lodestone piece in that direction.

Earth is a huge body inside which molten lava is there and that contains metallic ions. Earth is rotating around its axis and those metallic ions are also rotating alongwith Earth. All these motions give rise to a magnetic field in approximately North-South direction. So, Earth behaves like a magnet.

Any magnet will behave like that and we can perceive two distinct magnetic poles within a magnet. They are called North and South poles. If we have two magnets with us then we can feel this by bringing them near and if we bring their similar poles to each other there will be a repulsive force and if we bring opposite poles to each other there is an attraction. This is a very important property of the magnets.

## Artificial magnets

Electric current also has an associated magnetic property and if we make a coil of an electric wire and pass an electric current through it then the coil starts behaving like a magnet. If we insert a magnetic material rod or core inside this coil then as soon as the current flows in the coil the core or iron rod becomes a magnet due to the induced effect of the current. This assembly is then known as an electromagnet. These electromagnets have a large scale application in the industry in various ways.

So, the electric current flowing in a loop or coil produces a magnetic field. If we change the direction of the current in the coil then the magnetic field is also produced in reversed direction.

## Understanding Magnetic Field

The magnetic field created by a magnet is well known and the students know about this while studying and doing related experiments in schools. The effect of a magnet on the iron dust (very small pieces of iron) kept near it on a plane board where the board is softly vibrated along with the magnet is a common experiment to visualise the magnetic lines of forces as per the pattern generated on the board by the iron dust. These lines which curve around the magnet in a particular fashion are actually indicating the direction of the magnetic field at that point. In analogy just like the gravitational and electric forces, we have a magnetic force also and it is evident when we bring a small iron needle or pin near the magnet and it clings to it.

The magnetic field has a very close relationship with the electric current and if a current is flowing through a wire then actually magnetic field is created around it and the same magnetic lines of forces are created as that created by a magnet. Interestingly if the current is flowing through a helical coil of some metal wire then the magnetic field gets aligned in its axial direction and that property is used to make electromagnets. To enhance such a magnetic field, we generally wrap the wire coil over a magnetic material like iron or an alloy of iron, containing magnesium and nickel.

Magnetism is inherited in the electromagnetic waves which move from one place to another are used for communication in Radio and TV. They are named so because they possess both the qualities - electric field as well as the magnetic field. The Sunrays reaching Earth and X-rays used in medical diagnosis are also electromagnetic waves. They all have these peculiar properties.

## Magnets in Industry and in our lives

Magnets have many interesting applications in the industry as well as in our lives. These magnets can be manufactured in various shapes depending on their use either alone or in some equipment. Many magnets are manufactured using iron alloys and use various materials like neodymium, samarium cobalt, rare earth materials, and ceramics etc.

Some of the main uses are -

• Magnets are used in schools and colleges for doing practicals for understanding the behaviour of magnets and their characteristics.
• These are commonly used in speakers and loudspeakers for converting electrical signals to sound through the movement of a diaphragm connected to a coil through which the electrical signal is flowing. The earphones we use for hearing personalised music also contain tiny magnets.
• They are used for holding hazardous materials kept in iron containers and with strong magnets, these containers can be picked up remotely.
• In some industries, magnets are used to remove iron chips and iron pieces from a mixture of plastic parts and other non-magnetic material mixed with iron fillings and chips etc.
• Magnetic cleaners are used to clean and brush the runaways in the airports to remove any unwanted iron scrap or such material which can harm the plane tires.
• In the pharmaceutical industry also, magnets are used for cleaning the chemicals for any iron dust in them.
• Magnet strips are often used in the doors, windows, and cupboards for sealing them in closed conditions

## The Earth's Magnetic field

The earth's magnetic field is something that always intrigued humans and scholars alike and was a matter of great research from the beginning when scientific advancements started to take place.

The magnetic field of Earth is also thought to be produced by its rotational motion around its axis as the charged particles and ions moving in circular paths in the molten lava are nothing but some sort of electric current only and any such current flowing in so big loops is capable to generate the magnetic field. Hence, there could be many reasons for the generation of the magnetic field but it is a property that requires understanding in a detailed way.

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The measured magnetic field on the Earth surface varies from 0.25 to 0.65 Gauss where Gauss is the CGS unit of the magnetic field. The international SI unit of the magnetic field is called Tesla and being a big unit it is generally represented in nano Tesla and is shown as nT. In this unit the Earth magnetic field is mentioned as 25000 nT to 65000 nT. The conversion from Gauss to Tesla is given by 1 Tesla = 10000 Gauss.

If we compare this field with that of a bar magnet which most of us had used in science practicals during our school days then in comparison the magnetic field of those bar magnets is in the range 80 to 120 Gauss. It is evident that Earth magnetic field (which is quite less than 1 Gauss) is a weaker field.

## Measuring magnetic field

For measuring the magnetic field there are some instruments known as magnetometers. The basic magnetometers are based on the interrelationships between a magnetic field and electric current. For example, if we move a coil ( few rounds of a metallic wire) 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. There are other ways of measuring a magnetic field strength based on hall effect, induction, magnetic resonance, flux measurements etc.

Hall effect is one of the scientifically observed things that are very commonly used for magnetic field measurement in many types of equipment. Let us try to understand the basics of it. In the year 1879, Dr Edwin Hall discovered that when a current is passing through a thin conducting sheet of a material and a magnetic field is applied perpendicular to the sheet, then a voltage (called the Hall voltage) is generated across the direction perpendicular to both of these. The amount of Hall voltage generated is proportional to magnetic field strength. This helps us in measuring the unknown magnetic fields.

## Units of Magnetic Field

The magnetic field is having two types of main units. One is in the international unit (system SI) and is called Tesla. Another common unit used in the CGS system is Gauss. These two units are connected to each other through the conversion 1 Tesla = 10000 Gauss.

## Conclusion

Magnetism is a unique property and magnets are used for a variety of purpose in our lives. They are used in industry as well as educational matters and there is great potential in the area of magnets and their applications. Understanding the working of magnets and their close relation with electric current gives us insight into this topic.

## Other lessons in this series of basic Physics

Lesson-1: Distance and Displacement.

Lesson-2: Speed and Velocity.

Lesson-3: Acceleration.

Lesson-4: Mass and Weight.

Lesson-5: Gravity.

Lesson-6: Volume and Density.

Lesson-7: Momentum.

Lesson-8: Force Work Done and Energy.

Lesson-9: Heat and Temperature.

Lesson-10: Circular Motion.

Lesson-11: Friction.

Lesson-12: Rotational Motion.

Lesson-13: Simple Harmonic Motion.

Lesson-14: Voltage and Current.

Lesson-16: Light.

Lesson-17: Sound.

Lesson-18: Electrical Resistance.

Lesson-19: Capacitance.

Lesson-20: Atomic Structure.

Lesson-21: Kinetic Energy.

## References

1. https://en.wikipedia.org/wiki/Lodestone

2. https://web.ua.es/docivis/magnet/earths_magnetic_field2.html

3. https://www.sciencebuddies.org/science-fair-projects/project-ides

4. https://www.dkfindout.com/us/science/magnets/using-magnets/

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.