The world around us is filled with interesting objects. Some are very heavy and some are very light. Some objects swiftly let the electricity and heat pass through them, and some resist. Some materials attract objects of iron. These are called magnetic materials. We all have played with a piece of magnet in childhood. Refrigerator magnets were very popular among kids. An object which can create a magnetic field is known as a magnet.
Initially, the natural magnets were known as lodestones. They can be compared to the electric charge. As the electric charge creates the electric field and has field lines around the charge. The same can be said about the magnet also. They also create magnetic field lines. However, there are a few differences too. Magnets are almost everywhere in the era of technology. From the small motor to the space programs, they are prominent everywhere.
The history of magnets can be traced back to the early 600BC. There was an island called “Magnesia”. Shepherd used to go there with their cattle. They had reported that their shoes which had nails, used to get stuck on the ground. Their rods which had iron tips also got affected similarly. The reason was the presence of magnetic ore in that area. The name magnet was derived from the island “Magnesia”.
Greeks also found the piece of a stone attracted the magnet. It was magnetite. These natural pieces of magnetite are called natural magnets. We also get a hint from the ancient Chinese texts that they were aware of the directional properties of the magnet. They used it as a navigation system in ships. Interestingly the Chinese texts also mention a chariot on which they place a magnetic statue. They used this chariot to attack enemies in fog.
Ancient Egyptian were also aware of the magnet and used it for many recreational things. But these were natural magnets. The modern contribution in this field started in the 17th century when William Gilbert published his book “De Magnete”.
He suggested three techniques in his book to convert the steel to a permanent magnet. In 1820, Orsted demonstrated through his seminal experiment that electric current can also produce a magnetic field. From here due to the contribution of Faraday, Maxwell, etc. the field of Electromagnetism emerged rapidly.
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Fig:1 Earth magnetism
Generally, the magnet can be categorized into three types - Permanent Magnet, Temporary magnet, and Electromagnet.
Permanent Magnets - These are the types of magnets that don’t lose their magnet property once they are magnetized. They don’t need any external source for magnetism. These days permanent magnets are made of alloys. Some common examples are - Samarium-cobalt-magnet, Ceramics, aluminum-nickel-cobalt or alnicos, neodymium-iron-boron etc. Rare earth materials are also used for making permanent magnets, these are called rare earth magnets. A neodymium-iron-boron magnet is one such example.
Temporary Magnets- Some substances can be easily magnetized. Even if we expose them to the weak field they get magnetized, but as soon as we remove the field they lose their magnetic property. These materials are known as temporary magnets. Soft Iron alloys and a mixture of iron-nickel alloys are examples of such magnets. These kinds of magnets are often used in electric motors and electromagnets.
Electromagnets - In this kind of magnet, an electric current is used. A wire is wrapped around a metal core, an electric current is passed through the wire. As Orsted demonstrated through his experiments the electric current produces a magnetic field. The strength of a magnetic field can be controlled by changing the strength of the current and the number of coils in the wire. Electromagnets produce a very strong magnetic field. But the magnetic field remains as long as there is some current in the electromagnet. Electromagnets are used in computers, TV sets, electric motors, etc.
When we discuss a magnet, we use many important terms. Here we shall learn briefly about a few key terms.
Polarity - The endpoints of magnets are known as poles of magnets. There are two poles, North and South. The pole which points towards the geographical north is called the north pole. And the pole which points towards the geographical south is called the south pole. The north pole and south pole always exist in pairs which means there can’t be any magnet that has a monopole.
Attraction and Repulsion - The Magnets have properties of attraction and repulsion. Like poles repulse each other and unlike end attracts each other. This means North and north will repel each other but north and south will attract each other.
Bar magnet - A bar magnet is a rectangular piece of ferromagnetic material. It has permanent magnetic properties. An example is shown in the figure below-
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Fig:2 Bar magnet
Field lines - Magnets create a magnetic field around them. Magnetic field lines are a visual depiction of the field. These create a continuous loop, starting from the north pole and ending at the south pole. A tangent at these lines gives us the direction of the magnetic field
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Fig: 3 Magnetic lines of force
Compass - It is used for navigation purposes. A magnetic needle is used in it. It points toward the geographical north.
Magnetic moment - It tells about the magnetic strength of the magnet. It is a vector. The magnetic moment is generated either by the electric charge or spin angular momentum.
Magnets are useful in almost every area of science and technology. Some applications are summarised below -
Used in computers - Hard disk uses magnets inside them to store the data.
Used inside TV, speakers, and radios.
Magnets are used inside the motors and generators.
Used in the medical field - MRI scans, ventilators, blood separators, etc use a magnet.
Various strip card machines like ATM cards use magnetism to store data.
Magnets are used since ancient times. Ancient Greece,China, and Egypt used magnetic stones for various purposes. Magnets can have many types, Main types of magnets are - permanent magnets, electromagnets, and temporary magnets. In today's world magnet is used in various devices such as computers, motors, etc.
Q1. Why can’t a magnet have a monopole?
Ans: Maxwell’s equation predicts that magnetic monopole doesn’t exist. The right-hand side of gauss’s law of magnetism is zero. This implies there is no magnetic monopole.
Q2. Write two differences between electric and magnetic fields.
Ans:
The electric field is created by electric charge, the magnetic field is created by moving electric charge and magnets.
Magnetic field line always creates a closed loop, it is not necessary for electric field lines.
Q3. Which civilization is credited with the discovery of the compass?
Ans: Chinese are credited with the discovery of the compass.
Q4. What is the range of magnetic fields produced by the earth?
Ans: Magnetic field of the earth is between 0.25 Gauss to 0.6 Guass.
Q5. What is the unit of the magnetic moment?
Ans: The magnetic moment is measured in Ampere -meter square or Joules per tesla.
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