Unit 4


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Unit 4

  2. 2. UNIT INDEX UNIT-IS.No. Module Lecture PPT Slide No. No. 1 Introduction L1 4-10 2 Dielectric constant L2 11 3 Electronic ,Ionic,& L3 12-26 orientational Polarizations 4 Internal fields in solids. L4-5 27-28 Clausuis Mossoti relation 2
  3. 3. 5 Dielectrics in alternating L6-7 29-31 fields. Frequency dependence6 Ferro and piezo L8 32-33 electricity. 3
  4. 4. INTRODUCTION LECTURE-1 Dielectrics represent a class of materials which, although insulators, exhibit a number of effects when placed in an electric field. A good example is their effect on capacitors. A capacitor has capacitance C0 when the space between its two conductors is a vacuum, filling this space with a dielectric increases the capacitance to a new value Cm. The ratio Cm/C0=εr is known as the relative permittivity of the dielectric. 4
  5. 5.  When the atoms or molecules of a dielectric are placed in an external electric field, the nuclei are pushed with the field resulting in an increased positive charge on one side while the electron clouds are pulled against it resulting in an increased negative charge on the other side. 5
  6. 6.  This process is known as polarization and a dielectric material in such a state is said to be polarized. There are two principal methods by which a dielectric can be polarized: stretching and rotation. Stretching an atom or molecule results in an induced dipole moment added to every atom or molecule. 6
  7. 7. Polarizability It can be defined as induced dipole moment per unit electric field.  i.e. µ= αE  Where α is the proportionality constant called Polarizability. 7
  8. 8. Polarization vector The dipole moment per unit volume of the dielectric material is called polarization vector. If µ is the average dipole moment per molecule and N is the number of molecules per unit volume then the Polarization vector P=N µ 8
  9. 9. Electric flux density (D) The flux density or electric displacement D at a point in a material is given by D=єr є0E. Where E is the electric field strength, є0 is the dielectric constant and єr is relative permitivity of the material. The 3 vectors D,E and P are related by the equation D= є0 E+P 9
  10. 10. Electric susceptibility(‫אּ‬e) The polarization vector can be written as P= ‫ אּ‬є0 ‫ אּ‬eE Where the constant ‫ אּ‬e is the electric susceptibility. ‫(= אּ‬є -1). e r 10
  11. 11. Dielectric constant (єr ) Lecture- 2 Dielectric constant (є) is the ratio r between the permitivity of the medium and the permitivity of free space. i.e є = є/ є . r 0 Є has no units. r 1 11
  12. 12. Electric Polarization Lecture- 3 If a material contains polar molecules, they will generally be in random orientations when no electric field is applied. An applied electric field will polarize the material by orienting the dipole moments of polar molecules. This decreases the effective electric field between the plates and will increase the capacitance of the parallel plate structure. 12
  13. 13.  The process of producing electric dipoles which are oriented along the field direction is called Polarization in dielectrics. P=NαE. 13
  14. 14. Polarization in dielectrics Electronic polarization. Ionic Polarization. Orientational Polarization. 14
  15. 15. Electronic polarization Electronic polarization represents the distortion of the electron distribution or motion about the nuclei in an electric field. The positive charge in the nucleus and the center of the negative charges from the electron "cloud" will thus experience forces in different direction and will become separated. We have the idealized situation shown in the image below. 15
  16. 16. Electronic polarization 16
  17. 17. Electronic polarization The separation distance d will have a finite value because the separating force of the external field is exactly balanced by the attractive force between the centers of charge at the distance d. 17
  18. 18. Ionic Polarization In the absence of electric field, The polarization of a given volume, however, is exactly zero because for every dipole moment there is a neighboring one with exactly the same magnitude, but opposite sign. 18
  19. 19. The dipoles can not rotate; their direction is fixed. 19
  20. 20. When field is applied In an electric field, the ions feel forces in opposite directions. For a field acting as shown, the lattice distorts a little bit The Na+ ions moved a bit to the right, the Cl– ions to the left. The dipole moments between adjacent NaCl - pairs in field direction are now different and there is a net dipole moment in a finite volume now. 20
  21. 21. The Na+ ions moved a bit to the right, the Cl– ions to the left 21
  22. 22. The distance between the ions increases by d 22
  23. 23. Orientational polarization. The polarization arising due to the allignment of already existing but randomly oriented dipoles in the polar substance is called the Orientational or dipolar polarization. It is denoted by α . o 23
  24. 24.  It depends on temperature T It decreases with T. α (T)=µ 2/3K T. o m B 24
  27. 27. Internal fields in solids. Lecture- 4 The total electric field at the site of the atom within the dielectric is called the local field or the internal field. It is also called the Lorentz field. We have P=NαE . i E =[ є (є -1)E]/N α. i 0 r E =E+(ГP/ є ). i 0 27
  28. 28. Claussius-Mosotti relation Lecture-5 It gives the relation between the microscopic polarizability and the macroscopic dielectric constant. Clasius Mossotti equation is given by (єr -1)/( єr +2)= N α/3 є0 . 28
  29. 29. Dielectrics in alternating fields Lecture-6 According to Maxwell’s theory of wave propagation V=√1/ єµ. C= √1/ є µ 0 0. Hence C/V=n=√ єr µr. If the materials are non magnetic, µr=1 29
  30. 30.  n=√ єr( or) єr =n2. Then the Clasius Mossotti relation becomes (n2-1)/( n2 +2)= N α/3 є0 . This is known as Lorentz-Lorentz relation. 30
  31. 31. Lecture-7 In case of the alternating fields, we write E=E (t) and P=P (t) to indicate that both E and P vary with time t. There will be some time lag between the response P (t) and the cause E (t). If the applied field E (t) is oscillatory, then P (t) is also oscillatory. If E (t) is given by E (t)=E0coswt, then P (t)=P0cos(wt+δ). 31
  32. 32. The ferroelectricity Lecture- 8 Some dielectrics become spontaneously polarized when their temperature is equal to critical temperature. This phenomena is called the ferroelectricity.It is not because of it is possessed by the ferrous materials but because its origin and characteristics are same as those of ferro magnetism. The critical temperature of the polar dielectrics is called the ferroelectric curie temperature. 32
  33. 33. PIEZOELECTRICITY When crystals are subjected to electric field, their geometrical dimensions are altered. This phenomenon is called electrostriction. If crystals are subjected to mechanical stress, electrical charges will be induced on the surfaces of the crystals. This phenomenon is called piezoelectricity. When an electric stress (voltage) is applied, the material becomes strained. This phenomenon is known as inverse piezoelectric effect. 33
  34. 34. UNIT INDEX UNIT-IS.No. Module Lecture PPT Slide No. No. 1 Introduction L9 4-9 2 Magnetic permeability, L10 10-11 Magnetization 3 Origin of magnetic L11 12-14 moment. 4 Classification of L12 15-22 magnetic materials. 34
  35. 35. 5 Hysteresis curve, L13 236 Soft & Hard Magnetic L14 24 Materials 35
  36. 36. Lecture-9 Introduction Magnetic materials play a prominent role in modern technology. They are widely used in industrial electronics and computer industry. The traditional methods of information storage and retrieval are rapidly replaced by magnetic storage. 36
  37. 37.  The magnetism of materials is mainly a consequence of interactions of uncompensated magnetic moments of constituent atoms and molecules. Basing on the response of materials in external magnetic field, and on the alignment of magnetic moments in the materials, they are classified into five types. 37
  38. 38. Magnetic Polestrength Magnetic poles always occurs in pairs. Magnetic Polestrength (m) : It is scalar quantity .It is independent of the shape of the magnet. .It depends on the state of magnetisation. SI unit is – Am. 38
  39. 39. Magnetic field strength(B) Magnetic field : The space around a magnet where its influence is felt is called magnetic field. Magnetic induction field strength (B): Magnetic induction at a point is the force experienced by a unit north pole at that point. B is a vector. 39
  40. 40. Intensity of magnetic field (H) It is defined as the field that induces magnetism in a magnetic material. H is measured in Ampere/metre When a medium is exposed to magnetic field of intensity H it causes an induction B in the medium. 40
  41. 41. Magnetic flux(Φ).Magnetic flux(Φ): It is the total number of lines of induction passing normal to the cross section. S I unit : weber.  Magnetic flux (Φ) µo.m  : Φ is a scalar. 41
  42. 42. Magnetic permeability. Lecture-10 Magnetic permeability: It is defined as the ability of a medium to allow the magnetic lines of force to pass through it. B = μo (H+M) = μo (H + χ m H) B =μo μr H. Where μ =1+χm. Which is called r relative permeability. 42
  43. 43. Intensity of magnetisation. Intensity of magnetization : It is the magnetic moment per unit volume or pole strength per unit area. I=M/V = (2l.m)/(2l.a) a= area of crossection.It is measured in ampere/metre. 43
  44. 44. Magnetic moment Lecture-11It is a product of Magnetic length and pole strength of a magnet .Magnetic moment M=2l.m S.I unit of Magnetic moment is =Am2. (or) N-m3/wb. 44
  45. 45. Magnetic susceptibility. Magnetic susceptibility is defined as the ratio of intensity of magnetization (I) to intensity of magnetizing field. Magnetic susceptibility(χ): χ = I/H. χ has no units. 45
  46. 46. Relative permeability. Relative permeability of material is expressed as the ratio of permeability of the material to the permeability of free space. Thus μr =μ/μo. (or) μ=μrμo. 46
  47. 47. Magnetic materials Lecture-12 These are the substances, which upon which being introduced into the external magnetic field, change so that they themselves become sources of an additional magnetic field. And they are classified into 5 groups.1Diamagnetic. 4.Antiferromagnetic2.Paramagnetic. 5.Ferrimagnetic 47
  48. 48. Diamagnetic materials The materials which when placed in magnetic field acquire feeble magnetism in the direction opposite to that of field are known as Diamagnetic substances. Diamagnetic materials exhibit negative magnetic susceptibility. The magnetization in diamagnetic materials is directed in opposite direction of the field applied. 48
  49. 49.  The relative permeability of a diamagnetic substance is slightly less than unity. μr< 1; which implies that substances are repelled by a magnetic field. The magnetic susceptibility of diamagnetic materials is practically independent of temperature. Examples: Hydrogen, air, water, gold silver. 49
  50. 50. Paramagnetic materials These are the substances which when placed in magnetic field acquire feeble magnetism in the direction of magnetic field. Examples: copper chloride, chromium, platinum. The magnetic susceptibility of paramagnetic substances is positive as the magnetization coincides the magnetic field. 50
  51. 51. Ferromagnetic materials Large magnetization occurs in thedirection of the field. The relative permeability is very high (several thousands). When placed in magnetic field, it attracts the magnetic lines of force very strongly. Permanent and electromagnets are made using ferromagnetic materials. Examples:ZnFe2O4, CuFe2O4, Zn-CuFeO4 & 51
  52. 52. Antiferromagnetic materials They show very little external magnetism. Magnetic susceptebility is positive and small. The magnetic dipole moments of adjacent atoms are antiparallel. 52
  53. 53.  Due to antiparallel magnetic dipole moments, the magnetic effect of antiferro magnetic material is zero, but possess magnetism due to temperature dependent disruption of the magnetic moment alignment. The susceptibility increases with temperature upto TN (Neil temperature). Above Neil temperature, susceptibility decreases with increasing temperature. 53
  54. 54. Ferrimagnetic materials Magnetic dipole moments of adjacent moloecules or atoms are antiparallel and unequal in magnitude. It results in a net magnetisation in the material. Magnetic susceptibility is large and positive. Above Curie temperature, thermal dnergy randimizes the individual magnetic moments and the material becomes paramagnetic. Examples: copper, zinc, cadmium, iron, cobalt, nickel, etc. 54
  55. 55. Hysteresis Lecture-13 When a magnetic field is applied on a ferromagnetic material then magnetization takes place. This magnetizatio9n always lags behind the applied magnetic field. This phenomenon is known as hysteresis of a ferromagnetic material. 55
  56. 56. Magnetic materials are classified into soft materials and hard materials. Lecture-14 Soft magnetic  Hard magnetic materials are easily materials retain magnetised and magnetism on a permanent basis, and demagnetised, and are used in producing therefore used in ac permanent magnets . applications.  These materials play an important role in information storage devices. 56