Physics Notes for NEET Chapter 22 Magnetism PDF

60 1242 Magnetism Chapter E3 22 Magnetism ID The molecular theory of magnetism was given by Weber and modified later by Ewing. According to this theory. U Every molecule of a substance is a complete magnet in itself. However, in an magnetic substance the molecular magnets are randomly oriented to gi...

60 1242 Magnetism Chapter E3 22 Magnetism ID The molecular theory of magnetism was given by Weber and modified later by Ewing. According to this theory. U Every molecule of a substance is a complete magnet in itself. However, in an magnetic substance the molecular magnets are randomly oriented to give net zero magnetic D YG moment. On magnetising, the molecular magnets are realigned (1) Directive properties : When a magnet suspended freely it stays in the earth’s N-S direction (in magnetic meridian). Magnetic axis in a specific direction leading to a net magnetic moment. (A) Unmagnetised (B) Magnetised U Fig. 22.1 N S Magnetic meridian Fig. 22.3 (2) Monopole concept : If a magnet is Broken into number of pieces, each piece becomes a magnet. This in turn implies that monopoles do not exist. (i.e., ultimate individual unit of Bar Magnet ST magnetism in any magnet is called dipole). A bar magnet consist of two equal and opposite magnetic pole separated by a small distance. Poles are not exactly at the ends. The shortest distance between two poles is called effective length (Le) and is less than its geometric length (Lg). for bar magnet Le = 2l and S Lg  R and LeL =2 2Rl e S N S N S N S N Fig. 22.4 2R Le = (5/6) Lg. for semi circular magnet N S N (A) and doesn’t attract in figure (B) then, Q is a magnetic and P Lg (A) Bar magnet (3) For two rods as shown, if both the rods attract in figure Fig. 22.2 (B) Semicircular magnet is simple iron rod. Repulsion is sure test of magnetism. Magnetism (A) P Q Q P (B) 1243 length as well as perpendicular to the length simultaneously as shown in the figure then b Fig. 22.5 b (4) Pole strength (m) : The strength of a magnetic pole to L Fig. 22.8 60 attract magnetic materials towards itself is known as pole L strength. Length of each part L'  (i) It is a scalar quantity. L , breadth of each part b '  b n m w , Mass of each part w '  , pole strength of each part m '  , n n m L M Magnetic moment of each part M '  m ' L'    n n n (ii) Pole strength of N and S pole of a magnet is conventionally represented by +m and –m respectively. (iii) It's SI unit is amp × m or N/Tesla and dimensions are [LA]. E3 n If initially moment of inertia of bar magnet about the axes ID passing from centre and perpendicular to it’s length is (iv) Pole strength of the magnet depends on the nature of material of magnet and area of cross section. It doesn't depends SS S S S S N NN N N N N A – more m – more S SS SS SS A – less m – less (B) (A) D YG Fig. 22.6 N NN NN N   then moment of inertia of each part I'  I  n2  (7) Cutting of a thin bar magnet : For thin magnet b = 0 so U upon length. S  L2  b 2 I  w   12 L'  L m I w , w'  , m '  , I'  3 n n n n Various Terms Related to Magnetism (1) Magnetic field and magnetic lines of force : Space around a magnetic pole or magnet or current carrying wire (5) Magnetic moment or magnetic dipole moment (M ) : It within which it's effect can be experienced is defined as represents the strength of magnet. Mathematically it is defined as magnetic field. Magnetic field can be represented with the help the product of the strength of either pole and effective length. i.e. of a set of lines or curves called magnetic lines of force. –m S N U M  m(2 l ) +m N S S N L = 2l ST Fig. 22.7 M (A) Isolated north pole (B) Isolated south pole (C) Magnetic dipole Fig. 22.9 (i) It is a vector quantity directed from south to north. (ii) It's S.I. unit amp×m2 or N-m / Tesla and dimensions [AL2] (6) Cutting of a rectangular bar magnet : Suppose we have a rectangular bar magnet having length, breadth and mass are L, b and w respectively if it is cut in n equal parts along the (2) Magnetic flux ( ) and flux density (B) 1244 Magnetism (i) The number of magnetic lines of force passing normally through a surface is defined as magnetic flux ( ). It's S.I. unit is weber (wb) and CGS unit is Maxwell. Relative permeability of the medium = flux density in material B . B0 flux density in vacuum (4) Intensity of magnetising field ( H ) (magnetising field) : Remeber 1 wb = 108 Maxwell. It is the degree or extent to which a magnetic field can external magnetic field the substance becomes magnetised. The number of magnetic lines of induction inside a magnetised substance crossing unit area normal to their direction is called magnetic induction or magnetic flux density (B). It is a vector It's SI unit is A / m.  N m 2  Tesla  B . N J J It's CGS unit is   wb m 3  Tesla m  wb Oersted. Also 1 Oersted = 80 A/m E3 quantity. magnetise a substance. Also H  60 (ii) When a piece of a magnetic substance is placed in an S r  and (5) Intensity of magnetisation (I) : It is the degree to which a N substance is magnetised when placed in a magnetic field. Fig. 22.10 ID It can also be defined as the pole strength per unit cross sectional area of the substance or the induced dipole moment per unit volume. It's m2  unit Tesla is which is N J volt  sec   amp  m amp  m 2 m2 equal to D YG wb SI U Hence I = m M . It is a vector quantity, it's S.I. unit is A V Amp/m. (6) Magnetic susceptibility (m) : It is the property of the and CGS unit is Gauss. Remember 1 Tesla = 104 Gauss. substance which shows how easily a substance (3) Magnetic permeability : It is the degree or extent to magnetised. It can also be defined as the ratio of intensity of which magnetic lines of force can enter a substance and is magnetisation (I) in a substance to the magnetic intensity ( H) denoted by . Or characteristic of a medium which allows applied to the substance, i.e.  m  magnetic flux to pass through it is called it's permeability. e.g. ST U permeability of soft iron is 1000 times greater than that of air. In air (A) can be I. It is a scalar quantity H with no units and dimensions. (7) Relation between permeability and susceptibility : Total magnetic flux density B in a material is the sum of magnetic flux In soft iron (B) Fig. 22.11 density in vacuum B 0 produced by magnetising force and magnetic flux density due to magnetisation of material B  B0  Bm  B   0 H   0 I   0 (H  I)   0 H (1  B m. i.e.  m ). Also  r  (1   m ) Force and Field Also  =  0  r ; where  0  absolute permeability of air or free space = 4  10 7 tesla m / amp. (1) Coulombs law in magnetism : The force between two magnetic poles of strength m1 and m2 lying at a distance r is Magnetism In CGS units r2 . In S.I. units k  0  10 7 wb / Amp  m , 4 k 1 (4) Gauss's law in magnetism : Net magnetic flux through any closed surface is always zero i.e. (i) Magnetic field due to an imaginary magnetic pole (Pole strength m) : Is given by B   m F also B  0. 2 m0 4 d As per the most established theory it is due to the rotation of the earth where by the various charged ions present in the molten state in the core of the earth rotate and constitute a (ii) Magnetic field due to a bar magnet : At a distance r from current. the centre of magnet Geographic Magnetic  B e g N S 2l  M a Axial line  Ba ID + D YG 0 M ; If l Ts.  If a rectangular bar magnet is cut in n equal parts then  When a magnetic dipole of moment M moves from unstable equilibrium to stable equilibrium position in a time period of each part will be 1 times that of complete n magnet (i.e. T '  T n ) while for short magnet T '  T. If n Magnetism 1253 nothing is said then bar magnet is treated as short magnet.  Suppose a magnetic needle is vibrating in earth’s magnetic field. With temperature rise M decreases hence time period (T) increases but at 770oC (Curie temperature) it stops vibrating.  An iron cored coil and a bulb are connected in series 60 with an ac generator. If an iron rod is introduced inside a coil, then the intensity of bulb will decrease, because some energy lost in magnetising the rod. loop = VAnt Joule; Where , V = Volume of ferromagnetic sample, A = Area of B – H loop P, n = Frequency of ST U D YG U ID alternating magnetic field and t = Time E3  Hysteresis energy loss = Area bound by the hysteresis