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MAGNETISM

1. Magnetism:
      Magnetism is the property displayed by magnets and produced by the movement of electric charges, which results in objects being attracted. Magnet is a piece of iron or other materials that can attract iron containing objects and that points north and south when suspended.

 

2. History of Magnetism:
       In 1830 Michael Faraday (British) and Joseph Henry (American) independently discovered that a changing magnetic field produced a current in a coil of wire. Faraday, who was perhaps the greatest experimentalist of all time, came up with the idea of electric and magnetic “fields.” He also invented the dynamo (a generator), made major contributions to chemistry, and invented one of the first electric motors.
     In the 19th century James Clerk Maxwell, a Scottish physicist and one of the great theoreticians of all times, mathematically unified the electric and magnetic forces. He also proposed that light was electromagnetic radiation. In the late 19th century Pierre Curie discovered that magnets lose their magnetism above a certain temperature that later became known as the Curie point.

 

Commonly known ideas of magnetism:
* The earth behaves as a magnet with the magnetic field pointing approximately from the geographic south to the north.
                                   

* When a bar magnet is freely suspended, it points in the north-south direction. The tip which points to the geographic north is called the north pole and the tip which points to the geographic south is called south pole of the magnet.
* There is a repulsive force when north poles of two magnets are brought close together. Conversely, there is an attractive force between the north pole of one magnet and the south pole of the other.
* We cannot isolate the north, or south pole of magnet. If a bar magnet is broken into two halves, we get two similar bar magnets with somewhat weaker properties. Unlike electric charges, isolated magnetic north and south poles known as magnetic monopoles do not exist.
* It is possible to make magnets out of iron and its alloys.

 

3. Properties of Magnetism:
     A magnet is characterized by the following two properties

 

i. Attractive Property:
    A magnet attracts magnetic substance like iron, cobalt, nickel and some of their alloys like magnetite.
    Scientific form of magnetite is (Fe3O4).

 

ii. Directive Property:
     When a magnet is freely suspended, it aligns itself in the geographical north south direction.
* A magnet may be i. Natural ii. Artificial
* Natural magnet is oxide of iron. But due to irregular shape, weak magnetism and high brittleness, natural magnets find no use in the laboratory.
* The magnets made by artificial methods are called artificial magnets or man made magnets. They may be of different types like bar magnet, horse shoe magnet, Robinson’s ball ended magnet, magnetic needle, electromagnet.
* The two points near the two ends of a magnet where the attracting capacity is maximum are called magnetic poles. When a magnet is freely suspended, its one pole always directs towards the north. This pole is called south pole.
* The imaginary line joining the two poles of a magnet is called magnetic axis of the magnet.
* Similar poles repel each other and dissimilar poles attract each other.
* When a magnetic substance is placed rear a magnet, it gets magnetized due to induction.

             

4. Magnetic Properties of Materials:
I. Diamagnetism:
   Diamagnetic materials create an induced magnetic field in a direction opposite to an externally applied magnetic field, and are repelled by the applied magnetic field.
                                                                        Or
    Diamagnetic material is a material tending to become magnetized in direction at 180° to the applied magnetic field.
Ex: Bismuth, Zinc, copper, Silver, Gold, diamond, Water, Mercury, Water…. Etc.
    When a material is put in a magnetic field, the electrons circling the nucleus will experience, in addition to their Coulomb attraction to the nucleus, a Lorentz force from the magnetic field. Depending on which direction the electron is orbiting, this force may increase the centripetal force on the electrons, pulling them in towards the nucleus, or it may decrease the force, pulling them away from the nucleus. This effect systematically increases the orbital magnetic moments that were aligned opposite the field, and decreases the ones aligned parallel to the field (in accordance with Lenz's law). This results in a small bulk magnetic moment, with an opposite direction to the applied field.

 

II. Para magnetism:
      Paramagnetic substances are substances which when placed in magnetic field acquire a feeble magnetism in the direction of the field.
Ex: Aluminum, platinum, Manganese, Sodium, Oxygen…..etc.
    Paramagnet is a body or substance that, placed in a magnetic field, possesses magnetization in direct proportion to the field strength; a substance in which the magnetic moments of the atoms are not aligned.
    Paramagnetic material is attracted by the poles of magnet, but not retaining any permanent magnetism.
     In a paramagnetic material there are unpaired electrons, i.e. atomic or molecular orbitals with exactly one electron in them. While paired electrons are required by the Pauli exclusion principle to have their intrinsic ('spin') magnetic moments pointing in opposite directions, causing their magnetic fields to cancel out, an unpaired electron is free to align its magnetic moment in any direction. When an external magnetic field is applied, these magnetic moments will tend to align themselves in the same direction as the applied field, thus reinforcing it.

 

III. Ferro magnetism:
     Ferromagnetic substances are those substance, which when placed in a magnetic field, are strongly magnetized in the direction of field.
Ex: Iron, Cobalt, Nickel…….etc.
     Ferro magnetism is a body as iron that below a certain temperature, the Curie point, can possess magnetization in the absence of an external magnetic field; noting or pertaining to a substance in which the magnetic moments of the atoms are aligned.
     A ferromagnet, like a paramagnetic substance, has unpaired electrons. However, in addition to the electrons' intrinsic magnetic moment's tendency to be parallel to an applied field, there is also in these materials a tendency for these magnetic moments to orient parallel to each other to maintain a lowered-energy state. Thus, even in the absence of an applied field, the magnetic moments of the electrons in the material spontaneously line up parallel to one another.
    Every ferromagnetic substance has its own individual temperature, called the Curie temperature, or Curie point, above which it loses its ferromagnetic properties. This is because the thermal tendency to disorder overwhelms the energy-lowering due to ferromagnetic order.
    Ferromagnetism only occurs in a few substances; the common ones are iron, nickel, cobalt, their alloys, and some alloys of rare earth metals.
    Atoms of Ferromagnetic substance have a permanent dipole moment i.e. they behave like a very small magnet. The atoms form a large no. of effective regions called domain in which 1018 to 1021 atoms have their dipole moment aligned in the same direction. The magnetism in ferromagnetic substance, when placed in a magnetic field, is developed due to these domain by
     a) To displacements of boundaries of the domains
     b) The rotation of the domains.

 

IV. Ferrimagnetism:
     Like ferromagnetism, ferrimagnets retain their magnetization in the absence of a field. However, like antiferromagnets, neighboring pairs of electron spins like to point in opposite directions. These two properties are not contradictory, because in the optimal geometrical arrangement, there is more magnetic moment from the sublattice of electrons that point in one direction, than from the sublattice that point in the opposite direction.
     Most ferrites are ferrimagnetic. The first discovered magnetic substance, magnetite, is a ferrite and was originally believed to be a ferromagnet; Louis Neel disproved this, however, after discovering ferrimagnetism.

 

5. Curie temperature:
       The Curie temperature is a physical constant and refers to a characteristic property of ferromagnetic materials. Above the Curie temperature, a material loses its ferromagnetic properties. In a ferromagnetic material, elementary dipoles are aligned into the so – called domains and the domains – through their arrangement – bring about the internal magnetic field of the material –magnetization. At temperatures above the Curie point the ordered state is destroyed, magnetic dipoles become chaotically disordered and the material no more exhibits ferromagnetic properties. This change comes about in an abrupt manner at reaching the Curie temperature.
       As temperature increases, the magnetic property of ferromagnetic substance decreases and above a certain temperature the substance changes into paramagnetic substance. This temperature is called Curie temperature.
     Permanent magnets are made of steel, cobalt steel, ticonal, alcomax and alnico. Electromagnets, cores of transformers, telephone diaphragms, armatures of dynamos and motors are made of soft iron, mu-metal and stalloy.

 

6. Magnetic Field:
     Region in space around a magnet where the magnet has its magnetic effect is called magnetic field of the magnet.
     Magnetic fields, like gravitational fields, cannot be seen or touched. We can feel the pull of the Earth’s gravitational field on ourselves and the objects around us, but we do not experience magnetic fields in such a direct way. We know of the existence of magnetic fields by their effect on objects such as magnetized pieces of metal, naturally magnetic rocks such as lodestone, or temporary magnets such as copper coils that carry an electrical current. If we place a magnetized needle on a cork in a bucket of water, it will slowly align itself with the local magnetic field. Turning on the current in a copper wire can make a nearby compass needle jump. Observations like these led to the development of the concept of magnetic fields.

 

7. Motion in Magnetic Field:
    In Magnetic field, the magnetic force is perpendicular to the velocity of the particle. So no work is done and no change in the magnitude of the velocity is produced. Though the direction of momentum may be changed.
   We shall consider motion of a charged particle in a uniform magnetic file. First consider the case of velocity ‘v’ perpendicular to magnetic field ‘B’. The perpendicular force ‘qvB’, acts as a centripetal force and produces a circular motion perpendicular to the magnetic field.
      If velocity has a component along B, this component remains unchanged as the motion along the magnetic field will not be affected by the magnetic field. The motion in a plane perpendicular to B is as before a circular one, thereby producing a helical motion.
    If ‘r’ is the radius of the circular path of a particle, then a force of mv2/r, acts perpendicular to the path towards the centre of the circle, and is called the centripetal force. If the velocity v is perpendicular to the magnetic field B, the magnetic force is perpendicular to both v and B and acts like centripetal force. It has a magnitude qvB. Equating the two expressions for centripetal force.
      mv2/r = qvB
      r = mv/qB
for the radius of the circle describe by the charged particle. The larger the momentum, the larger s the radius and bigger the circle described. If  is the angular frequency, then
       v = 
ω
r.
So 
ω
 = 2π v = qB/ m
     Which is independent of the velocity or energy. Here v is the frequency of rotation. The independence of v from energy has important application in the design of a cyclotron.
The time taken for the revolution is T = 2 π/ 
ω

8. Magnetic Moment:
      Magnetic Moment is defined as “The product of the pole strength and the distance between the poles.”
The SI unit of magnetic moment is “Torque”.
Torque = N.m/T
              = A.m2
              = J/T
Where A= ampere
            J = Joules
           T = Tesla.

 

9. Magnetic Flux:
     Magnetic Flux is defined as the quantity of magnetism, being the total number of magnetic lines of force passing through a specified area in a magnetic field.
     The SI unit of magnetic flux is Weber.
     The CGS unit of magnetic flux is Maxwell.
     It is denoted by the symbol 
ϕ
 or ϕ B.
     
ϕ = BA
     Where B = Magnetic Field
                  A = Area perpendicular to magnetic field B.

10. Magnetic Flux Density:
      Magnetic flux density of a point in a magnetic field is the force experienced by a north pole of unit strength placed at that point.
The SI unit of Magnetic flux density is “Tesla”
Tesla = newton /ampere
           = weber/meter2

 

11. Magnetic lines of Force:
    The magnetic lines of force are imaginary curves which represent a magnetic field graphically. The tangent drawn at any point on the magnetic lines of force gives the direction of magnetic field at that point.

                                  
Properties of Magnetic line of force:
* Magnetic lines of force are closed curves. Outside the magnet they are from north to south pole and inside the magnet they are from south to north pole.
* They are continuous through the body of magnet.
* Magnetic lines of force can pass through iron more easily than air.
* Two lines of force near intersect each other.
* They tend to contract longitudinally.
* They tend to expand laterally.
* If the lines of force are crowded, the field is strong.
* If the lines of force are parallel and equidistant, the field is uniform.

 

12. Terrestrial Magnetism:
     Our earth behaves as a powerful magnet whose south pole is near the geographical North pole and whose north pole is near the geographical south pole. The magnetic field of earth of a place is described in the terms of following three elements.
Declination: The acute angle between magnetic meridian and geographical meridian at a place is called the angle of declination at that place.
Dip or Inclination: Dip is the angle which the resultant earth’s magnetic field at a place makes with the horizontal. At poles and equator, dip is 90° and 0° respectively.
Horizontal component of earth’s magnetic field: At a place it is defined as the component of earth’s magnetic field along the horizontal in the magnetic meridian.
   Its value is different at places. Approximately its value is o.4 gauss or 0.4 × 10-4 tesla.

 

13. Magnetization:
      Magnetization is a process of making a substance temporarily or permanently magnetic, as by insertion in magnetic field.
Permanent Magnets and Electro Magnets:
    Substances which at room temperature retain their ferromagnetic property for a long period of time are called permanent magnets. Permanent magnets can be made in a veriety of ways. One can hold an iron rod in the north-south direction and hammer it repeatedly.
An efficient way to make a permanent magnet is to place a ferromagnetic rod in solenoid and pass a current. The magnetic field of the solenoid magnetises the rod.

 

14. Magnetic susceptibility:
      Magnetic susceptibility is the intensity of magnetization of a body placed in a uniform magnetic field of unit strength.

 

15. Put compass needles at the poles:
      A compass needle consists of a magnetic needle which floats on a pivotal point. When the compass is held level, it points along the direction of the horizontal component of the earth’s magnetic field at the location. Thus, the compass needle would stay along the magnetic meridian of the place. In some places on the earth there are deposits of magnetic minerals which cause the compass needle to deviate from the magnetic meridian. Knowing the magnetic declination at a place allows us to correct the compass to determine the direction of true north.
    If we take our compass to the magnetic pole, the magnetic field lines are converging or diverging vertically so that the horizontal component is negligible. If the needle is only capable of moving in a horizontal plane, it can point along any direction, rendering it useless as a direction finder. What one needs in such a case is a dip needle which is a compass pivoted to move in a vertical plane containing the magnetic field of the earth. The needle of the compass then shows the angle which the magnetic field make with the vertical. At the magnetic poles such a needle will point straight down.

Posted Date : 03-02-2021

 

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