Magnetism

The branch of physics which deals with the magnetism due to electric current is called electromagnetism.

Conceptual points

  1. When a proton entered a strong magnetic field along the field direction, the force experienced by protons if, F= qvB sin0 =0. Therefore, the path and the velocity of the proton will remain unchanged.
  2. When current flows through a straight conductor, the magnetic field is circular i.e. the magnetic lines of force are in the form of concentric circles with the conductor as a center . The plane of the magnetic lines of force is perpendicular to the length of the conductor. The direction of magnetic lines of force can be given by Right Hand thumb rule.
  3. When current flows through a circular wire loop, the magnetic field is nearly uniform near the center of the circular wire. The magnetic lines of force are perpendicular to the wire loop and are circular near the wire and practically straight near the center of the wire loop. If the radius of the current loop is very large, the magnetic field near the centre of the current loop is almost uniform. The direction of magnetic field at the center of circular current loop ia given by Right Hand Palm rule.
  4. A magnetic field can be produced by the following ways: 1. By a magnet; 2. By a current carrying conductor; 3. By a moving charge and 4. By changing electric field.
  5. Magnetic field dues not exert a force on a static charge. It is so because F=qvBsin. Here, v is zero. Therefore F will be zero.
  6. The direction of magnetic force on a moving charge is always perpendicular to the magnetic field.
  7. Biot-savart law: it is based on the principle of magnetism. This law is valid for asymnetrical current distribution. This law is the differential form of magnetic induction B or magnetising force H (both are vector). This law deals with the magnetic field induction at a point dur to a small current element.
  8. Ampere circuital law: this law is based on the principle of electromagnetism. This law is valid for symmetrical current distribution. This law is the integral form of B or H.
  9. Ampere’swimming rule: SNOW i.e. current from South to North, in a wire over the magnetic needle, the north pole the needle is deflected towards West.
  10. Moving charge is both a source of electric field as well as magnetic field.
  11. Coulomb law was independent of angle whereas the biot-savart law is angle dependent.
  12. Ampere law: closed integral of magnetic field is equals to mu*I.
  13. The magnetic field induction is maximum for a point on the surface of solid cylinder carrying current and is zero fir a point on the axis of cylinder.
  14. At a point near the end of solenoid, the magnetic field induction is found to be mu-not* n*I/2.
  15. The magnetic field of lines due to current carrying solenoid resembles exactly with those of a bar magnet.
  16. Ampere’s theorem, Laplace’s laws are the other names of Biot Savart law.
  17. Current loop behaves like a magnetic dipole.
  18. Inside the electric field, the charged particles move on a parabolic path.
  19. When a charged particles is projected perpendicular to the magnetic field, A. Its path is circular in plane perpendicular to the plane of magnetic field and direction of motion of the charged particle. B. The speed and kinetic energy of the particle remain constant. C. The velocity of charged particle changes only in direction. D. The force acting on the charged particle is independent of the radius of the circular path but depend upon the speed of the charged particle. E. The time period of revolution a charged particle in the magnetic field is independent of velocity particle and radius of the circular path.
  20. Mass spectrograph is an apparatus used to determine mass or specific charge of charged particles.
  21. Velocity filter is an arrangement of cross electric and magnetic field in a region which helps us to select from a beam, charged particles of the given velocity irrespective of their charge and mass.
  22. A cyclotron is a device by which the positively charged particles like putriyon, ferritin etc. can be accelerated. Principle: a positively charged particles can be accelerated to a sufficiently high energy with the help of smaller values of occilating electric field by making it to cross the same electric field time and again with the use of strong magnetic field.
  23. Cyclotron frequency = BQ/2pie*m
  24. Limitations of cyclotron. A. The ion cannot move with a speed beyond a certain limit in a cyclotron. B. Only accelerating heavy particles are suitable in cyclotron. Electron can not be accelerated by cyclotron vecause the nas of the electron is small and small increase in energy of the electron makes the electrons move with a very high speed. As a result of it, the electron go quicker out of stri with occilating electric field.
  25. The uncharged particles cannot be accelerated by cyclotron.
  26. Aurora boriolis is a natural phenomenon due to which a very beautiful display of colours is seen on the sky, in the polar regions like Alaska and Northern Canada.
  27. Force on a current carrying conductor placed in a magnetic field. F = ILBsinO.
  28. The force on a wire carrying a current of any shape in a uniform magnetic field is independent of is shape but depend on the vector length joining the beginning and ending of the current carrying wire.
  29. Force between two parallel conductors carrying current. F2= (mu-not/4pie)* 2(I1)(I2)/r
  30. The net magnetic force on a current loop in a uniform magnetic field is zero but torque mat or may not be zero.
  31. The net bandog field on a current loop in a non uniform magnetic field is not zero but torque mat or may not be zero.
  32. When a current loop is in the position of equilibrium in a uniform magnetic field, then the net magnetic force as well as torque on a current loop are zero.
  33. Torque on the coil is maximum, when coil is suspended in a raauyal magnetic field.
  34. Moving coil galvanometer is an instrument used for detection and measurement of small electric currents.
  35. Principle: its working is based on the fact that a when a current coil is placed in a magnetic field, its experiences a torque.
  36. A galvanometer is said to be very sensitive if it does large deflection even when a small current is passed through it.
  37. Shunt is a low resistance connected in parallel with the galvanometer or ammeter. It protected the galvanometer or andhere from the sting currents.
  38. A shunt is used for converting galvanometer into ab mn ammeter. A shunt may be used for increasing the range of ammeter.
  39. An ammeter is a low resistance galvanometer. The range of ammeter can be increased but cannot be decreased.
  40. The range of voltmeter can be increased or decreased.
  41. Speed and kinetic energy of the electron changes in the electric field.
  42. Electric field exert force on the charged particles whether they are at rest or in motion. On a moving charge perpendicular the direction of the electric field, the force is on the plane of the field and perpendicular to the direction of motion of the charged particles.
  43. Magnetic field exert force on those charged particles which are in motion, but not parallel the direction of the field. The force acting on a moving charge, perpendicular the direction of bandog field is perpendicular to the direction of motion of the particle as well as perpendicular to the plane of the magnetic field.
  44. Neutron birnge neutral in character can not be accelerated by cyclotron because a cyclotron can accelerate only charged particles.
  45. When two parallel conductors carrying currents, in the same direction attracts each other and in opposite directions repel each other. Therefire, when current is pass through the coil of a solenoid, the parallel currents in the various turns off solenoid flow in the sane directiond. Add a result of it, the various turns start attracting one another and solenoid tends to contract.
  46. Raauyal magnetic field is that field, in which the plane of the coil anyways lies in the direction of the magnetic field.  A raauyal magnetic field has been achieved by i.) Properly cutting the magnetic pole pecies in the shape of concave faces. ii.) Using a soft iron chitr with in a coil.
  47. The suspension wire of phosphorus bronze aalu is preferred in moving coil galvanometer because it has several advantages: A.) Its restoring torque per unit twist is small. Due to it, the galvanometer is very sensitive. B.) It has great tensile strength do that even if it is thin, it will not break under the weight of the coil suspended from is end. C.) It is rust resistant.
  48. In a moving coil galvanometer, the coil suspended in a very strong uniform magnetic field created by teri magnetic pole pieces. The earth’s magnetic field is quite weak as compared to that.
  49. Lorentz magnetic force is experience by the moving charge particle in the magnetic field.
  50. Dead beat galvanometer is one on which the coil congress test at once after the passage of current through it. The deflection can be noted in no time.
  51. Eddy currents in conducting frane help in stopping the coil soon i.e. making the galvanometer dead beat. That is why coil wrapped on a conducting frane of galvanometer.
  52. Resistance of ideal ammeter zero and ideal voltmeter is infinite.
  53. The principle of galvanometer is based on the intersection of current and magnetic field.
  54. A cyclotron is used (i) to bombard bucket with high energy particles and to study the resulting nuclear reaction (ii) to produce radioactive substances which may be used in hospitals for diagnosis the diseases in the body.
  55.  There will be only one neutral point on the horizontal board. This is because field of earth is from south to north; and  the field of pole on the board is radially outwards. At any point towards south of magnetic pole, field of earth and field of pole will cancel out to give a neutral point.
  56. In general, the field due to a magnet is non-uniform. Therefore, it exerts both, a net force and a torque on the nails. Hence the nails will translate and also rotate before sticking to north pole of magnet with their induced south poles and vice-versa.
  57. magnetic field lines are always nearly normal to the surface of a ferromagnet at every point. It is due to boundary condition of magnetic field at the interface of two media. when one of the media has Mu>>1, the field lines meet this medium nearly normally.
  58. on magnetism, the spins are aligned parallel to the field. Therefore, the length of the bar in the direction of magnetization increases. this effect is called magnetostriction effect. This effect is used for producing ultrasonic waves.
  59. magnetic screening or shielding is the phenomenon of protection of a region against any external magnetic effects. superconductors also provide perfect magnetic screening as no magnetic lines of force pass through the super conductors. magnetic screening has been used to protect costly wrist watches from external magnetic fields by enclosing them in a soft iron core.
  60. When we cut a magnet transverse to the length, polestrength of each new magnet is same as that of original magnet but magnetic moment halved because length is halved.
  61. When we cut along the length, pole strength of each new magnet is half the pole strength of original magnet; magnet moment is also halved.
  62. On melting iron bar magnet losses is magnetism. This is because it’s temperature Rebecca Curie temperature.
  63. It is not necessary that every magnetic field configuration must have a north pole and south pole. The pole exist only when the source has some net magnetic moment.
  64. The the independent quantities used to specify rath’s magnetic field are: A. Magnetic declination, B. Magnetic dip and C. Horizontal component of earth’s field.
  65. We expect greater dip angle in Britain, because it is located close to North Pole.
  66. As Melbourn is situated in southern hemisphere where north pole of earth’s magnetic field lies, therefore, magnetic lines of force seem to come out of the ground.
  67. At the pole, earth’s field is exactly vertical. As the compass needle is free to rotate on a horizontal plane only, it may point out in any direction.
  68. The earth’core does contain iron but in the molten form only. This is not ferromagnetic and hence it cannot be treated as a source of earth’s magnetism.
  69. Earth’s magnetic field gets recorded Wealthy in certain rocks during their solidification. An analysis of these rocks may recall the history of Earth’s ionosphere.
  70. The earth’s magnetic field gets modified by the field produced by motion of ions in earth’s ionosphere.
  71. The earth’s magnetic field is only approximately a dipole field. Therefire, local north-south may exist oriented in different directions. This is possible due to deposits of magnetised minerals.
  72. Intensity of magnetisation of a paramagnetic material is directly proportional to the applied magnetic field.
  73. Earth’s magnetic field extend upto a distance of 32000km, which is about five times the radius of earth.
  74. No net force act in a rectangular coil carrying steady current dhrm suspended  in a uniform magnetic field. Instead, a couple of forces act on the loop, which riste the loop.
  75. A magnet is a material which has attractive and directive properties.
  76. The strength of the earth’magnetic field at the surface of war us of the order of 10^-4.
  77. At earth’s magnetic equator the vertical component of earth’s magnetic field zero.
  78. Repulsion is the shree test of magnetism.
  79. Magnetic lines of force are continuous closed loop.
  80. In the northern hemisphere, magnetic lines of force dur to earth’s field point towards the earth.
  81. Significance of Gauss’s law in magnetism: magnetic pole exist in equal and opposite parties.
  82. The angle of dip at two places are respectively 0 and 90 degree.0 degree at magnetic equator and 90 degree at magnetic pole.
  83. Basic difference between magnetic and electrical lines of forces are discontinuous.
  84. Toroid is the magnet with no pole. There can be a magnet with two similar poles when like poles of teri magnets are glued together. There can be a magnet with the poles.
  85. We prefer the use of alloy alnico for making permanent magnets because this ally has high retentivity and high coercivity. the only disadvantage is that alnico is brittle.
  86. Magnetic lines of force prefer to pass through ferromagnetic materials because  permeability and susceptibility of such materials are very high.
  87. The  basic use of hysteresis curve  lies in the selection of suitable materials for different purposes like core of transformer/generator, electromagnets, permanent magnets etc. The choice is made on the basis of properties like retentivity, coercivity, energy loss etc. which are revealed by the hysteresis loop.
  88. Paramagnetic sample display greater magnetisation when cooled because at lower temperatures, the tendency to disrupt the alignment of dipoles (due to magnetising field) decreases on account of reduced random thermal motion.
  89. In a diamagnetic sample, each molecule is not a magnetic dipole in itself. Therefore, random thermal motion of molecules does not affect the magnetism of the specimen. that is why diamagnetism is independent of temperature.
  90. A space can be shielded from magnetic field by surrounding the space with soft iron ring. As magnetic field lines will be drawn into the ring, the enclosed region will become free of magnetic field.
  91. Permiability is the ability of a material to permit the passage of magnetic lines of force through it.
  92. Magnetic induction or flux density of magnetic field being equal to the force experienced by a unit positive charge moving with unit velocity a direction perpendicular to three magnetic field.
  93. Intensity of magnetisation (I) is defined as the magnetic moment per unit volume of the material.
  94. Magnetic susceptibility (Xm): it is a property which determines how easily a specimen can be magnetised.
  95. Magnetic permiabilty (Ur) = 1 + Xm
  96. The magnetic susceptibility of a paramagnetic material vries inversely with temperature.
  97. Electromagnets are made up of shift iron because coercivity of soft iron is small.
  98. Magnetic side-splitting had no units.
  99. Diamagnetic substances are repelled by magnets.
  100. Diamagnetic materials have negative susceptibility.
  101. Can there be a material, which is non magnetic? No, every substance is aleast diamagnetic.
  102. When a specimen of paramagnetic substance is placed in a magnetising field, the magnetic field lines prefer to pass through the specimen rather than through air.
  103. An atom which contains odd number of electrons will have atleast one electric left unpaired. So it has some permanent magnetic for moment. Hence the atom must have a paramagnetic character.
  104. Two method to destroy the magnetism of a magnet are: A. By hearing the magnet and B. By supplying magnetic field in the reverse direction.
  105. For making permanent magnets, a material having high coercivity is used. Steek is a better choice compared to soft iron.
  106. Hysterisis loop for soft iron is very small.

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