Magnet is a material which carries a static magnetic field around with it. Here we are going to give some clear idea about Magnet and Magnetism. Most commonly we find are the permanent magnets made by some metals especially iron, or mixture of iron with other stuff like ceramics. Other kinds of magnets are called electro magnets made by electrical coils, around a piece of iron or iron core, which creates a magnetic field temporarily when electric current passes through the coil. Both types of magnets have the capacity of attract the magnetic substance like Iron, nickel, etc. and the opposite pole of other magnets.
Magnets plays very important role in our modern life. Vast number electrical devices are works on the electromagnetic principle. Natural magnets are now little used because of their weak attractive properties and irregular shapes. With the help of another magnet or using electric current, the magnetic properties may be infused in a variety of ways into some metals. Such type of infusion is called magnetisation, and the magnets prepared by such process are called artificial magnets. Generally the fallowing types of magnets are made artificially:
Types of Magnets:
a) Bar magnet : These type of magnets are made from uniform bars of rectangular or circular cross section.
b) Horse-shoe magnet: this is also bar magnet but in the form of horse shoe or U shaped.
c) Needle magnet: Magnetised steel or any other suitable alloy in the shape of thin strip and ends of which are pointed and pivoted at the middle so that in can rotate freely in horizontal plane.
d) Ball ended magnet: It is also a bar magnet of circular cross section which ended in two balls.
e) There has also some other types of magnets made as per different purpose of use.
Terms related to Magnet and Magnetism: –
i) Pole of Magnets:- If a bar magnet is dipped into iron filings it is found that when the magnet is lifted, lumps of filings clings to it at or near the ends only. Little or no filings cling to the middle portion. So we can say that the attractive power of the magnet is most pronounced at the region near the ends and this regions are called the Pole of the magnet. It has also established that the poles are situated at points near the ends but not at the extreme ends. If a magnet suspended freely it always comes to the rest with its length lying nearly North and south direction and with the same end always pointing towards the same direction. The pole which points towards the north is called North Pole and the other pole is called South pole. The attractive power is differs from magnet to magnet. To measure the ability of attraction of the magnet the term Pole Strength is introduced. The pole strength of the North Pole and South pole are equal.
ii) Magnetic Axis :- The imaginary line which joins the two pole of a magnet is called its magnetic axis.
iii) Magnetic Meridian :- At ant place, the imaginary vertical plane passes through the magnetic axis of a freely suspended magnet is called the magnetic meridian at that place.
iv) Equivalent Length :- The distance between the poles of a magnet along its magnetic axis is known its equivalent length.
v) Neutral region :- There is no attracting power near the middle portion of the magnet and is known as the neutral region of the magnet.
vi) Magnetic and non magnetic substances: – Substances which are strongly attracted by magnets are called magnetic substances, e.g. iron, Cobalt, Nickel etc. Substances which are not at all acted on magnets are called non magnetic substance, e.g. wood, paper, brass, plastic etc.
vii) Permanent and Temporary Magnets: –There are two types of artificial magnets according to the duration of their retention of magnetic properties. On being magnetized some substance can retain magnetic properties for a long time, e.g. steel, tungsten steel, alnico, stalloy etc. Magnets made artificially from these substance are called Permanent Magnets. But magnets made from soft iron lose their magnetic properties within a short time after magnetization and these magnets are called Temporary Magnets.
Magnetic attraction and repulsion
When a magnet is brought near another magnet, mechanical forces are experienced by each of them. The Magnet and Magnetism has some general rule about such forces is that, like poles repel and unlike poles attract each other. If a bar magnet and a magnetic needle are taken and if north (N) pole of the bar magnet is brought near the N pole of the needle that ends of the needle moves away, which indicates the repulsion. But if the south (S) pole of the bar magnet placed near the N pole of the needle, the said end of the needle turns towards the bar, which indicates the attraction.
Artificial magnets are generally made by two methods i.e. Mechanical and Electrical. By mechanical method bar magnets are made from bars of magnetic substances by rubbing them mechanically with one or two magnets. On the other hand in electrical method an insulated copper wire is wound over the rod to be magnetized from its one end to other. When direct current is passed through the wire, the rod will be magnetized. The end of the rod where the current flows in counter clockwise direction when viewed from that end will acquire North Polarity and the other end will be South Polarity.It also to be noted that if the bar is made of steel, tungsten-steel, Stalloy, perm-alloy, alnico etc. it will be magnetized permanently. But if the rod be made of soft iron it will retain strong magnetism as long as current passes through the coil, and at the instant of current is stopped it will almost completely loss its magnetism. Such a magnet is called an Electromagnet.
On a particular piece of magnetic substance the degree of magnetization depends on the nature of the substance itself and on the magnetizing force. In the process of mechanical rubbing the magnetizing force increases with the increase in strength of the stroking pole and with duration of rubbing. In electrical method it increases with the number of turns per unit length of the coil and with the strength of the current. With the increase of magnetizing force, the degree of magnetization increases initially. But, there is a maximum limit to the degree of magnetization, as the magnetization induced in a piece of magnetic substance cannot be increase any more by increasing the magnetizing force to ant extant, then the piece of the magnetic substance may said to have reached the stage of Magnetic Saturation.
If a piece of magnetic substance be placed in the neighborhood of or in contact with a magnet, it is found to acquire some temporary magnetism.
This is known as magnetic induction. After removal of the magnet, the magnetic substance loses its magnetic property. In this case , the magnetic substance is said to be magnetized by induction , the magnetism developed in it is called induced magnetism and the magnet responsible for the induction is called the inducing magnet. When a magnetic substance is magnetize by induction, opposite polarities developed at the end of it which is nearest to the inducing pole and similar polarity at the end furthest from the inducing pole. The Degree of induced magnetism in a magnetic substance depends upon (i) the strength of the inducing pole, (ii) the distance between the inducing pole and the magnetic substance, and (iii) the nature of the magnetic substance.
When the pole of a magnet is brought near a magnetic substance, opposite polarities induced at the near end of the substance, while the rare most end is acquires the same polarity. Attraction then follows between the inducing pole and the induced opposite pole. There is also a force of repulsion between the inducing pole and the induced like pole at the furthest end. As the distance between the latter two beings much larger than that in the former case of attraction, the attractive force prevails over the repulsive one. So, the magnetic substance attached towards the magnet. It we can say that the induction always proceeds attraction.
The tendency for magnetic lines of force to pass more readily through a magnetic material, such as iron, than through a magnetic substance like air, is measured by the permeability of the material. Iron is said to be more permeable than air or to have greeter permeability than air. Permeability is defined by the relation: Permeability = Number of lines of induction per unit area / Number of lines of force per unit area.
Permeability of a magnetic substance depends upon its nature and condition. The case with which a substance can be magnetized is measured by a quantity, called its Susceptibility. It is defined as the ratio of the intensity of magnetization to intensity of the magnetizing field. The Susceptibility of vacuum or air is zero. For magnetic substances, the susceptibility is much larger than for non-magnetic substances. Susceptibility of soft iron is greater than steel, nickel or cobalt. Its value also varies with temperature.
When a magnetic substance is placed in a magnetic field, it gets magnetized. The Retentivity of a magnetic substance measures its ability to retain its magnetism when the magnetizing field is withdrawn. The Retentivity of soft iron is smaller than that of steel. Generally, the substances having small susceptibility have high Retentivity.
In order to remove the residual magnetism, a magnetic substance must be subjected to some demagnetizing influence. The substance may be demagnetized by placing strong magnetic field acting in a reverse direction to that by which the specimen was magnetized. The intensity of the reverse field just sufficient to completely destroy the residual magnetism in the substance is known as its Coercively. Steel has a greater Coercively than iron.
We know that a freely suspended magnets points towards north-south direction. What is the cause? The earth can be likened to a big magnet having to magnetic poles, i.e. North Pole of the earth’s magnet is located near the geographical south pole of the earth, and the south pole of the earth’s magnet is located near the geographical north pole of the earth. As opposite poles of the earth and those of freely suspended magnet will attract each other, the magnet will always point towards north- south direction.
Usually a magnet is supposed to attract a magnetic substance, which is the general thinking about Magnet and Magnetism but always not happened so. It is found that some substances are repelled slightly by a very strong magnetic field, where as some substances are attracted to a little extent. Indeed these are substances like iron which are strongly attracted by the magnetic field applied. Depending upon these three types of behaviour, the substances are classified under three heads, namely Dia-magnetic substances, Para -magnetic substances and Ferro-magnetic substances respectively. The substances like Zinc, Bismuth and Antimony are called dia-magnetic substances. The substances like Manganese, Platinum, and Palladium are termed as Paramagnetic substances. Substances like Iron, Steel, Nickel, Cobalt and some alloys shows very strong attraction to a magnet and are called Ferro-Magnetic substances.