8. MAGNETISM Salient features Ewing s molecular theory and its failures Domain theory Inverse square law of magnetism Magnetic induction Magnetic field due to a bar magnet Magnetic lines of force Magnetic properties of materials Magnetic classification of materials. EWING S MOLECULAR THEORY OF MAGNETISM AND ITS FAILURES According to this theory, every molecule of a magnetic substance behaves as a tiny magnet called molecular magnet. Postulates 1) In an unmagnetised state, the molecular magnets in an iron bar are randomly distributed with their N and S poles pointing in all possible directions, resulting in the net magnetic field zero. Non magnetic substance Magnetic substance 2) When the Iron bar is magnetized, with the help of a magnet the molecular magnets arrange themselves in an orderly way, pointing all N poles of individual magnets in one direction and all S poles in another direction. So it acquires magnetism. 3) Repetition of the process of magnetization, only allows the maximum alignment among the molecular magnets. This means the magnetization of the iron bar cannot be increased further. This limit of mangetisation of a substance is called magnetic saturation. 4) Magnetic poles always exists in pairs. Two poles have equal strength. Since any magnet contains two poles. A magnet is also called magnetic dipole. 5) Magnetism of a magnet can be destroyed by tapping, hammering, heating etc. Failures: Ewing s molecular theory failed to explain why 1) the individual molecules of a magnetic substance like iron behaves as tiny magnet 2) the individual molecules of a non-magnetic substances like brass or saw-dust are not magnets 3) the substances like bismuth or copper are repelled by a strong magnet. DOMAIN THEORY OF MAGNETISM 1) Domain in a ferromagnetic specimen is a small local region in which all the magnetic dipoles align parallel to each other giving rise to certain magnetization within the domain. 2) In a ferro magnetic substance, large number of domains occurs. The direction of magnetization with in a domain is different from that in the neighbouring domain, as a result the net magnetization of the specimen as a whole is almost zero. 3) When the substance is magnetized, the domains expand in space such that all the arrows align parallel to each other in a single direction. 4) For an ideal magnet, there would be a single domain with all molecular magnets pointing in one direction. INVERSE SQUARE LAW OF MAGNETISM
Pole strength (m) The ability of a pole of a magnet to attract or repel another magneticpole is called its pole strength, denoted by m Units of pole strength in MKS system: Weber (Wb) In S.I sysem: Amp - meter (A m) Inverse square law of magnetism The inverse square law of magnetism state that the force of attraction or repulsion between two magnetic poles ( F) is directly proportional to the product of their pole strengths (m 1 and m 2 ) and inversely proportional to the square of the distance (r 2 ) between them and acts along the line of joining the poles. Relative permeability µ 0 is the permeability of air. µ 0 = 4π x 10-7 Henry / meter F m 1.m 2 1 F 1/r 2 2 F m 1 m 2 /r 2 F = µ 0 /4π m 1 m 2 /r 2 The ratio of magnetic force in a medium to the magnetic force in free space is defined as Relative permeability, µ r. µ r = µ / µ 0 This relative permeability has no units. For air or vacuum µ r = 1 Magnetic flux: MAGNETIC INDUCTION The total number of magnetic lines of force in given area is called magnetic flux. Units of magnetic flux: Weber Magnetic Induction Magnetic field induction or magnetic flux density B is defined as the magnetic flux passing through a unit normal area. If A is the area through which a flux φ passes normally, B is given by B = φ/ A The magnetic field induction B due to a magnet at a point in a medium is defined as the force acting on a unit north pole placed at the point. For free space, B = µ 0 / 4π. m/r 2 If now a north pole of strength m is placed at a point where the magnetic induction is B, the force acting on it is given by F = mb or B = F/m The units of Magnetic Induction in M.K.S system: Weber / Meter 2 (or) Tesla
In S.I system : Newton / Ampere meter In C.G.S. system: Gauss. Intensity of magnetic field (H): The intensity of magnetic field (H) at a point is defined as the force acting on a unit north pole placed at the point independent of the medium. H = m/r 2 Units of H in C.G.S system: Gauss In S.I. System : Amp / meter Relation between B and H is B = µ 0 H Magnetic Moment: MAGNETIC FIELD DUE TO A BAR MAGNET Magnetic moment (M) of a bar magnet is measured by the product of its pole strength (m) and its magnetic length (2l). M = m x 2l S.I.Unit of magnetic moment is Amp meter 2 Magnetic field induction on the Axial line The imaginary line joining the north and south poles and passing through the centre of mass of a magnet is called the axial line. The magnetic field induction on Axial line is given by µ 0 2Mï ï ï ï ï B = N/ A m 4π d 3 Magnetic field induction on the Equatorial line: The imaginary line passing through the centre of mass and perpendicular to the line joining the poles of a bar magnet is called Equatorial line.
The magnetic field induction on Equatorial line is given by µ 0 Mï ï ï ï ï B = N/ A m 4π d 3 MAGNETIC LINES OF FORCE Magnetic line of force: Any line joining the two nearest points in a magnetic field is called as magnetic line of force. 1) These lines of force are parallel curves 2) These lines of force always pass from north to south. Neutral points: A point at which the magnetic field induction (B 0 ) due to a bar magnet is nullified by the horizontal component of earth s magnetic field induction is called a neutral point for the combined (resultant) magnetic field. Earth s magnetic field Location of neutral points a) N pole of a bar magnet facing N pole of the earth
ïïïïïïïï ï In this stage, the neutral points can be obtained on the equatorial line, such that they are equidistant from the centre of the magnet. b) S pole of a bar magnet facing N pole of the earth: ïïïïïïïïïïï ï In this stage, the neutral points can be obtained on the axial line, such that they are equidistant from the centre of the magnet. Magnetic properties of materials 1) Magnetic Permeability: Magnetic permeability of a medium (µ) is defined as the ability to allow the magnetic lines of force to pass through it or to allow itself to be influenced by magnetic field. 2) Intensity of magnetization: The magnetic moment acquired by a substance per unit volume is defined as intensity of magnetization (I) The units of Intensity of magnetization are A /m 3) Magnetic Susceptibility: Magnetic susceptibility ( χ ) of a substance is defined as the ratio of the intensity of magnetization (I) and intensity of applied magnetic field (H). χ = I /H 4) Retentivity
Retentivity of a magnetic material is a property by virtue of which it retains certain amount of intensity of magnetization even after the removal of applied magnetic field. MAGNETIC CLASSIFICATION OF SUBSTANCES Magnetic materials can be grouped into three classes based on their important magnetic properties. They are 1) Dia magnetic substances, 2) Para magnetic substances, 3) Ferro magnetic substances. 1) Dia magnetic substances Dia magnetic substances are those in which the resultant magnetic moment of individual atoms is zero. Examples are air, water, bismuth, gold, alcohol, mercury, hydrogen etc. When a specimen of diamagnetic substance is brought near a magnetic pole of a bar magnet, it develops a similar or like pole and therefore it is repelled. The relative permeability (µ r ) is less than or equal to one (µ r 1) The magnetic susceptibility is small and negative. 2) Para magnetic substances: Para magnetic substances are those in which the resultant magnetic moment of individual atoms is not zero. Examples are Oxygen, Solutions of salts of Nickel, and manganese, Aluminium, Platinum and chromium etc. When a specimen of paramagnetic substance is brought near a pole of a bar magnet, it develops an opposite or unlike pole and therefore it is attracted. The relative permeability (µ r ) is slightly greater than 1. (µ r >1) The magnetic susceptibility is small and positive. 3) Ferro magnetic substances: Ferro magnetic substances are those in which the resultant magnetic moments of individual atoms align themselves in parallel locally because of a special effect present in them giving rise to spontaneous magnetization. Examples are Fe, Co, Ni, Gd, Dy, etc. Ferromagnetic substances are strongly attracted by magnets. The relative permeability (µ r ) is greater than one. (µ r > > 1) The magnetic susceptibility is very high and positive. IMPORTANT QUESTIONS 4 Marks 1) What are the essential features of Ewing s molecular theory of magnetism? What are its failures? (March-2002) 2) State and explain the inverse square law of magnetism? (March- 06) 3) Explain the key concepts of Domain theory of magnetism? (March-08) 4) Explain the differences between Dia, Para and Ferro magnetic substances? (June-00,04,06,07) 2 Marks 1) Define magnetic permeability and magnetic susceptibility? (March-02) 2) Differentiate between magnetic and non magnetic substances? 3) What are the Dia magnetic substances? Give examples? 4) What are para magnetic substances? Give examples? 5) What are the Ferro magnetic substances? Give examples? (March-09)
1 Mark 1) Define magnetic moment? What are its units in S.I system? (June-05, 01) 2) State Inverse square law of magnetism? (March-04, June 00) 3) Define Intensity of magnetic field? What are its units S.I and C.G.S systems? BITS 1) Air, Water, Bismuth are magnetic substances (Apr 08, Mar-03) 2) Aluminium is an example of -------- magnetic substance. (March 10) 3) Magnetic field induction on Equatorial line is ----------- (March- 08) 4) Units of Intensity of magnetization (March-99) 5) S.I.Units of Magnetic Moment ---------------- (March 04) 6) Magnetic permeability of air is ------------------ (March 10, 08, 06, 00) 7) Magnetic susceptibility of Dia magnetic substances is -------- (March -05, June 01) 8) M.K.S. Units of Pole strength ------------- (June 06) 9) Magnetic moment of a bar magnet whose pole strength is 2 x 10-3 Amp-met, and length is 5 cm is --- -------------- (March- 08) 10) Relation between magnetic field induction (B) and magnetic field intensity (H) is---- (June-07) 11) The relative Permeability of Dia magnetic substances is --------- (March-02) 5 Marks 1) Draw the magnetic lines of force when N pole of bar magnet facing geographical N-pole and locate the null points? (June-05, March-03) 2) Draw the magnetic lines of force when N-pole of bar magnet facing geographical S-pole and locate the null points? (March-09, June-00, 01, Oct-99) Answers for bits 1) Dia 2) Para 3) B = µ 0 /2π x M/d 3 4) Amp/Met 5) Amp-met 2 6) 4π x 10-7 Henry / Meter. 7) Very small and negative 8) Weber 9) 1 x 10-4 Amp-met 2 10) B = µ 0 H 11) µ r 1 Reason for earth s magnetism