Magnet:
Introduction
Science of magnetism dates back to 600 B.C., when Thales of Miletus had a knowledge that an ore of iron called, magnetite, possessed the properties of attracting small pieces of iron towards it. It had an additional characteristic that, when suspended freely it pointed in a particular direction. That is why it was given a name lodestone or the Leading stone i.e., a stone which shows the direction. Later on this stone was used for navigation purposes. Work of William Gilbert showed that the earth itself behaves like a huge magnet and the directional property of lodestone was due to this reason.
Magnet
A piece of substance which possesses the property of attracting small pieces of iron towards it is called a magnet.
If the property of magnetism occurs naturally the magnet is known as a natural magnet. It is possible, artificially, to induce magnetism by rubbing the given piece of substance with magnets. The magnet, thus produced, is called an artificial magnet.
Following terms are often, used in connection with a magnet:
1) Pole : It is a region of the magnet where it shows maximum magnetic characteristic. It is situated near the end of magnet. We shall denote it by m. Its unit, in S.I., is 'Am'.
2) Magnetic axis : A line joining the two poles of the magnet is called magnetic axis.
3) Magnetic meridian : A vertical plane passing through the axis of the magnet is called magnetic meridian.
4) Magnetic length : Distance between the two magnetic poles is called magnetic length. It is denoted by '2l'.
5) Magnetic moment : Product of pole strength of a magnet and its magnetic length is called magnetic moment. It is denoted by M (Rightward direction).
M = 2ml
It is a vector quantity. Its direction is from south pole to North Pole inside the magnet. In S.I., it is measured in 'Am²'.
Properties of a Magnet
A magnet possesses following properties :
1) Two poles of a magnet. A magnet has two poles. One is 'North seeking pole' or simply north pole (N) while the second is 'South seeking pole' or the south pole (S). These poles are situated a little distance inside the faces of the magnet (Fig. 1).
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| Fig. 1. Magnetic poles. |
Face to face length of the magnet is called geometric length while pole to pole length is called the magnetic length of the magnet. It is quite evident that the magnetic length is slightly less than geometric length.
Magnetic length, 2l = 7/8 × geometric length
2) Attracting property of a magnet. A magnet is capable of attracting small pieces of iron towards it. These pieces (iron filings) are attracted towards both the poles, north as well as south. This attraction is greatest at the poles and decreases as move towards the centre of magnet. It will be observed that the iron filings sticking to the magnet are largest in number near the poles (Fig. 2) and are smaller on the portion of magnet in between.
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| Fig. 2. Magnetism strong at poles. |
3) Directional property of a magnet. When freely suspended a magnet always points in a particular direction. North Pole of the magnet points towards geographic north while south pole points towards the geographic south (Fig. 3).
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| Fig. 3. Suspended magnet points N-S. |
That is the reason these poles are called north seeking pole (or simply north pole) and south seeking pole (or simply south pole) respectively. The cause of this property is that earth behaves as a huge magnet which pulls the given magnet in a particular direction. This property led to the development of mariner's compass which is extensively used for navigation purposes.
If the suspended magnet is deflected a bit from equilibrium position and released, it will execute torsional vibrations with N-S line as the mean position. Amplitude of vibrations will gradually decrease due to air resistance and the magnet will, ultimately, come to rest along N-S line.
4) No existence of isolated magnetic poles. The magnetic poles exist only in pairs of opposite nature. It is not possible to obtain an isolated magnetic pole. If we break a magnet into two parts we get two independent complete magnets each with a pair of opposite pole. On further sub-division into four pieces, we shall obtain four complete magnets again with a pair of opposite poles (Fig. 4). This process continues till we reach the smallest particle i.e., an atom. The atom also behaves like a magnet. That is the reason we say that it is not possible to obtain an isolated pole. Sometimes, however, we need an isolated magnetic pole for theoretical consideration. In that case we can assume the pole of a very long magnet to be an isolated one, since the second pole will be situated at such a large distance away so as to be ineffective.
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| Fig. 4. Breaking a magnet. |
5) Nature of force between two poles. Magnetic poles exert forces upon each other. The nature of force between similar poles is repulsive while that between opposite poles is attractive.
This can be observed from following simple experiment.Bring the north Pole of magnet near the north Pole of a freely suspended magnet. The north Pole of suspended magnet also rotates away [Fig. 5(i)]. Similarly, it can be observed that south pole of suspended magnet also rotates away [Fig. 5(ii)] from the south pole, while the north pole of suspended magnet rotates towards [Fig. 5(iii)] the south pole of the magnet. This indicates that, "like poles repel each other while un like poles attract each other". This is called basic law of magnetistatics.
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| Fig. 5. Magnetic attraction and repulsion. |
Coulomb's law of magnetistatics. It states that the magnitude of the force between two magnetic poles (supposed isolated) varies directly as the product of their pole strengths and inversely as the square of the distance between them.
Consider two magnetic poles of similar nature (n) of strength m₁ and m₂ separated a distance r from each other (Fig. 6). The force (of repulsion) between them.
F ∝ m₁m₂
F ∝ 1/r² or F ∝ m₁m₂/r²
or F = k × m₁m₂/r²
where 'k' is the constant of proportionality.
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| Fig. 6. Force between two magnetic poles. |
In S.I., k = μ₀/4π
∴ F = μ₀/4π × m₁m₂/r² ... (1)
where μ₀ = 4π × 10-7 Wb A-1 m-1
μ₀ is called the "absolute magnetic permeability" of free space.
In C.G.S. system, k = 1
∴ F = m₁m₂/r² ... (2)
Equation (1) and (2) represent mathematical forms of Coulomb's law of magnetostatics in S.I. and C.G.S. system respectively.
6) Unit pole. Coulomb's law in magnetism, can lead to the definition of a unit pole.
F = μ₀/4π × m₁m₂/r²
If m₁ = m₂, r = 1 m and F = 10-7 N and (we know) μ₀ = 4π × 10-7 Wb A-1 m-1
Substituting these values, we get
10-7 = 4π × 10-7/4π × m²/1
or m² = 1 or m = ±1
So, a unit pole is that pole which when placed, in vaccum, at a distance of one metre from a similar pole repels it with a force 10-7 N.
Repulsion is the sure test of magnetism
Supposing you are in possession of an iron bar and you want to know whether it is magnetised Or not. If it is magnetised it can show attraction towards a dissimilar pole of suspended magnet. If it is not magnetised it will again again show attraction with either of the magnetic poles. To be sure whether it is magnetised or not, we have to rely upon the repulsive characteristics of magnetic pole.
Bring the same end of the given bar near the two poles of the suspended magnet. It indicates attraction at one end repulsion at the other, it is magnetised. If it shows attraction at both ends it may be an unmagnetised iron bar. It will never show repulsion at both the ends.
Hence, "repulsion is the sure test of magnetism".
Important Notes
- There is no existence of isolated magnetic poles.
- Magnetic poles are situated slightly inside the edges of magnet. Therefore, magnetic length is slightly less than geometric length.
- Strength of the two poles are always equal.
- Like poles are repel each other while unlike poles are attract each other.
- When freely suspended a magnet always points along N-S direction.
- Force of attraction/repulsion between two poles obeys inverse square law.
- Magnetic force between two poles is a central force.
Atomic/Molecular Theory Of Magnetisation
Every atom of a magnetic substance is a magnetic dipole. In an unmagnetised piece, these magnetic dipoles form closed chains [Fig. 7(i)], thus neutralising each other's effects. The process of magnetisation consists in arranging these dipoles in regular manner, so that their magnetic moments are directed in one direction as shown in [Fig. 7(ii)].
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| Fig. 7. Effect of magnetic field on matter. |
As a result of this, one face of the specimen acquires a north polarity and the other one acquires a south polarity. The specimen is said to be magnetised and is called a magnet. The resultant magnetic moment 'M' of the magnet is given by
M = 2 ml
where m = pole strength of the magnet
2l = distance between the poles
Magnetic moment is a vector quantity. Its direction is from south to the north pole.
Evidence Favouring Molecular Theory
Following experimentally observ
ed facts established the truth of molecular theory.
(i) Evidence of the two poles. During magnetisation the strength of the two poles always remains equal. This is due to the reason that alignment of one dipole results in liberation of one free north and one south pole. Greater the number of dipoles arranging themselves in regular manner, greater is the strength of magnet.
(ii) Magnetic saturations. When the manetising field is so large that all the dipoles have aligned themselves along the field, further efforts for magnetisation cannot increase the strength of magnet beyond that value. The magnet is said to have achieved saturation stage.
(iii) Magnetic induction. When a piece of soft iron is placed in the neighbourhood of a magnet, a pole of opposite nature is induced on its nearer end while a pole of similar nature is induced on further end. The north pole of the magnet attracts the south poles of the dipoles of soft iron towards it, thus making the nearer end south end farther end north.
(iv) Demagnetisation. It has been observed that due to rough handling or heating, a magnet loses its magnetism. This is due to the reason that heating results in an increase in kinetic energy of atoms. As they vibrate more vigorously, they form closed chains again, thus losing the magnetism.
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