Overview on magnets | It’s 2 important types | Permanent and Electromagnet

Contents

  • History of Magnets
  • Types of Magnetic materials
  • Diamagnetic materials
  • Paramagnetic materials
  • Ferromagnetic materials
  • Types of Magnet
  • Hard magnets and Soft magnets
  • Permanent magnet and electromagnet
  • Applications of electromagnets

History of Magnets

From lodestones (or magnetite) first, people got an idea about the working of magnets, which are magnetized pieces of iron ore found in nature. The word magnet came from Greek, from the land named “Magnesia”, a part of ancient Greece where lodestones were found. By the end of the 12th century AD,magnets were used and magnetic compasses were built and used in navigation in different parts of the world like China, Europe, etc.

Overview on magnets | It's 2 important types | Permanent and Electromagnet
A naturally occurring permanent magnet: lodestone (black)
Image Credit : Teravolt (talk), Lodestone (black)CC BY 3.0

Basically, magnets are material that produces magnetic fields. Physicists Curie and Faraday observed that almost all materials have certain magnetic properties and according to their magnetic behavior divided them into three categories:

  • Diamagnetic materials
  • Paramagnetic materials
  • Ferromagnetic materials

Types of Magnet:

Hard Magnetic Materials: 

Hard magnets are generally ferromagnetic materials which have the ability to retain the magnetization for a quite a long period of time, i.e. the material should have high retentivity.

Hard magnets should also have a high degree of coercivity, i.e. only a large magnitude of the external magnetic field should be able to eliminate the residual magnetism retained by the material.

Some examples of hard magnetic materials are Alnico (an alloy formed by the combination of iron, cobalt, aluminum, nickel and copper) and lodestone (a naturally occurring metal).

Overview on magnets | It's 2 important types | Permanent and Electromagnet
Hysteresis loop for Hard Magnets

Soft magnetic Materials: 

Soft magnets are also ferromagnetic materials which can retain their magnetization as long as the external magnetic field exits, i.e. they have low retentivity. They also have a low degree of coercivity, i.e. their retained magnetization (although being very less) can be eliminated very easily.

Hence, they can be easily magnetized and demagnetized.

These kinds of materials (soft magnets) are being used to make electromagnet as an electromagnetic material should have a low retentivity and also a low coercivity. Soft iron is a suitable material as a soft ferromagnet.

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Overview on magnets | It's 2 important types | Permanent and Electromagnet
Hysteresis Loop for Soft Magnet

The two types of magnets: Permanent magnet and electromagnet

Permanent magnets:

magnets
Permanent magnets

The materials that can retain their ferromagnetic properties for long periods of time at normal room temperature can be classified as permanent magnets.

A high degree of retentivity (the magnet can retain its magnetism in the absence of external magnetic field) and also a high degree of coercivity (the magnetic property is not wiped out by external magnetic fields) is necessary to be a permanent magnet.

Permanent magnets should also be resistant to mechanical stress and temperature change. 

As stated before, a magnetic field is produced by a changing electric field. Hence it is theorized that the magnetic field of a permanent magnet is a consequence of the uniform spin of the electrons in a particular direction within the atoms of the material as the electric charge in motion produces a changing electric field. This kind of uniform spinning of the electrons in a material’s atoms is basically due to the atomic structure and electronic orientation of the material. Therefore, only a few types of substances have the ability to permanently sustain or retain a magnetic field.

Lodestone, Alnico, as mentioned in the hard magnets, can be an example of permanent magnet. From the discussions we had it can be inferred that steel is more suitable for the manufacture of permanent magnet than iron as steel has a much higher value of coercivity than iron although iron has a little higher retentivity than steel. A number of alloys with quite large values of retentivity and coercivity have been developed for the manufacturing of permanent magnet. Such an alloy with a very high coercivity value is named as vocally (an alloy made up of vanadium, iron and cobalt).

Electromagnets

Electromagnets are generally constructed by winding a material (usually ferromagnetic materials) by a wire in a coil and connecting the wires to a variable power supply (such that the current in the wires can be varied).

Overview on magnets | It's 2 important types | Permanent and Electromagnet
An Electromagnet

How does an electromagnet work?

When a current flows through the wires, the magnetic field produced by each of the individual coil loops is summed up with the magnetic field of the neighbouring loops, and altogether it works as a strong bar magnet with the distinct North Pole and the South Pole.

This resultant bar magnet with its distinct North and South Pole is much stronger than any permanent bar magnet which can be magnetized and demagnetized at will, i.e. it can behave as a magnet only when it is needed.

The material used as core should have high permeability, low retentivity and also low coercivity. In an electromagnet, the magnetic field and flux density can easily be varied according to the current in the windings. This property of an electromagnet is widely used in different applications, but unlike permanent magnet this one requires a power supply for it to work and also for electromagnet, there is some energy loss in magnetization and demagnetization of the core as studied previously in the form of the hysteresis loop.

The North Pole and South Pole formation when current flows through the windings depends on the direction of current flow in the loops. Where the North and the South Pole will be formed can be predicted by the diagram shown below.

Overview on magnets | It's 2 important types | Permanent and Electromagnet
North-South Pole according to the direction of current in the coil

Factors on which the strength of the electromagnet depends

The magnetic field strength or the magnetic flux density depends on the amount of current flowing through the windings and also to the number of turns in the coil. More specifically, the magnetic field strength is directly proportional to both of them, which is relevant from the expression of the magnetomotive force, which is as follows:

Magneto-motive force (MMF) = Overview on magnets | It's 2 important types | Permanent and ElectromagnetI X N 

where Overview on magnets | It's 2 important types | Permanent and Electromagnet is the current flowing through the winding and N is the number of turns.

Another condition on which the magnetic strength of an electromagnet depends is the material used as the core. Generally, the core is made up of ferromagnetic material with a high degree of permeability (the measure of the ease by which a magnetizing field can penetrate or permeate a given material). If we use any non-magnetic material like wood, plastic, etc., it can be assumed as if the core is made up of free space as the permeability of such material is very low and hence, the magnetic flux density will be negligible.

Overview on magnets | It's 2 important types | Permanent and Electromagnet
Application of Electromagnet
Image Credit: AntennamaxAGEM5520CC BY-SA 3.0

Applications of electromagnets

  • Electromagnets are extensively used in electrical devices such as electric bells, induction heaters, electric fans, telegraph, electric trains, electric motor-generator etc.
  • They are used for magnetic levitation as in maglev trains.
  • They are used in headphones, speakers, tape recorders and even in hard disks of our computers.
  • They are used as relays and in equipment such as mass spectrometers and even in particle accelerators.
  • They are even used in medical purposes such as for removing pieces of iron from wounds and also in MRI (magnetic resonance imaging) machines. 

For more Electronics related article click here

About Amrit Shaw

Overview on magnets | It's 2 important types | Permanent and ElectromagnetConnect to our ex-author: LinkedIn(https://www.linkedin.com/in/amrit-shaw/)

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