# Earths magnetism

19. Jul 2021
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### Earths magnetism

• 1. Earth's Magnetism Part 2 Class 12th Priyanka jakhar Physics lecturer
• 2. The earth's magnetism or Terrestrial magnetism or geomagnetism --- The branch of Physics which deals with the study of Earth's magnetic field is called Terrestrial magnetism or geomagnetism . Evidences of Earth's magnetism 1) Freely suspended magnetic needle always stay in north south direction 2) Small iron nail buried in the earth becomes magnet 3) Natural points are obtained
• 4. Magnetism-- The property of any object by virtue of which it can attract a piece of iron or steel is called magnetism. The Earth’s Magnetism---The earth behave likes a huge bar magnet. Strength of the earth’s magnetic field varies from place to place on the earth’s surface. The magnitude of the earth’s magnetic field is of the order of 10–5 T. Causes of the Earth’s Magnetism --Exact causes the earth’s magnetism is not clear yet. There are some Earth Magnetism Theory which are as follows: • The magnetic field of the earth is thought to arise due to electrical currents produced by convective motion of metallic fluids (consisting mostly of molten iron & nickel) in the outer core of the earth. This effect is also known as the dynamo effect. • In outer layers of earth’s atmosphere, various gases are in ionized state. Due to rotation of the earth about its axis, strong electric current are set up due to movement of charged ions. Basic features of the Earth’s Magnetism Magnetic field lines of the earth resemble with a (hypothetical) giant magnetic dipole located at the centre of the earth. The axis of the dipole does not coincide with the axis of rotation of the earth.
• 5. • The axis of the dipole is titled by approximately 11.3º with respect to the later. • The magnetic poles are located where the magnetic field lines due to the dipole enter or leave the earth. • North magnetic pole is located at a latitude of 79.74º N and a longitude of 71.8º W (in north Canada) • The magnetic south pole is located at 79.74º S, 108.22º E (in Antarctica) • The pole near the geographic north pole of the earth is called the north magnetic pole. • The pole near the geographic south pole of the earth is called the south magnetic pole. • Nomenclature of the poles is confusing and one should not get confuse. If we look at the magnetic field lines of the earth (as shown in figure given above), we observe that unlike in the case of a bar magnet, the field lines go into the earth at the north magnetic pole (Nm) and come out from the south magnetic pole (Sm). • The convention arose because the magnetic north was the direction to which the north pole of a magnetic needle pointed; the north pole of a magnet was so named as it was the north seeking pole. • The north magnetic pole behaves like the south pole of a bar magnet inside the earth and vice versa.
• 6. Dynamo effect --The dynamo effect is a geophysical theory that explains the origin of the Earth's main magnetic field in terms of a self-exciting (or self- sustaining) dynamo. Though there are many theories on earth’s magnetic field, dynamo effect seems to be most accepted one. •The earth consists of core, mantle and crust •In the outer core, there is molten iron and nickel •The convective motion of these metallic fluids, results in electrical currents •The magnetic field is due to this electrical currents
• 8. Magnetic fields around planets behave in the same way as a bar magnet. But at high temperatures, metals lose their magnetic properties. So it’s clear that Earth’s hot iron core isn’t what creates the magnetic field around our planet. Instead, Earth’s magnetic field is caused by a dynamo effect. The effect works in the same way as a dynamo light on a bicycle. Magnets in the dynamo start spinning when the bicycle is pedalled, creating an electric current. The electricity is then used to turn on the light. This process also works in reverse. If you have a rotating electric current, it will create a magnetic field. On Earth, flowing of liquid metal in the outer core of the planet generates electric currents. The rotation of Earth on its axis causes these electric currents to form a magnetic field which extends around the planet. The magnetic field is extremely important to sustaining life on Earth.
• 9. T errestrial Magnetism: i) GeographicAxis --is a straight line passing through the geographical poles of the earth. It is the axis of rotation of the earth. It is also known as polar axis. ii) Geographic Meridian --at any place is a vertical plane passing through the geographical north and south poles of the earth. iii) Geographic Equator --is a great circle on the surface of the earth, in a plane perpendicular to the geographic axis. All the points on the geographic equator are at equal distances from the geographic poles. iv) Magnetic Axis --is a straight line passing through the magnetic poles of the earth.It is inclined to Geographic Axis nearly at an angle of 17°. v) Magnetic Meridian --at any place is a vertical plane passing through the magnetic north and south poles of the earth. vi) Magnetic Equator --is a great circle on the surface of the earth, in a plane perpendicular to the magnetic axis. All the points on the magnetic equator are at equal distances from the magnetic poles. vii)The true North and South Pole --The true North( 𝑵𝒈) and South (𝑺𝒈) Pole lies at the ends of the polar axis and are defined by the axis of rotation of the earth. The position of the geographical poles are fixed.
• 10. viii) The Earth's magnetic poles--- The Earth's magnetic poles 𝑵𝒎 and 𝑺𝒎 are not true geographical poles. The position of the magnetic poles are not fixed. Magnetic Map Magnetic map is obtained by drawing lines on the surface of earth. which passes through different places having same magnetic elements. The main lines drawn on earth’s surface are given below (i) Isogonic Line -- A line joining places of equal declination is called on isogonic line. (ii) Agonic Line -- A line joining places of zero declination is called an agonic line (iii) Isoclinic Line --A line joining places of equal inclination or dip is called an aclinic line, (iv) Aclinic Line --A line joining places of zero inclination or dip is called an aclinic line. (v) Isodynamic Line --A line joining places of equal horizontal component of earth’s magnetic field (H) is called an isodynamic line.
• 11. Magnetic Element of the Earth’s Magnetic Field These are the quantities which completely describe magnitude and direction of the earth’s magnetic field at a place. There are three magnetic elements of the earth: • Magnetic Declination • Horizontal component of the earth’s magnetic field • Angle of dip or Magnetic inclination
• 12. Declination (θ): θ Magnetic Meridian Geographic Meridian The angle between the magnetic meridian and the geographic meridian at a place is Declination (D) at that place. It varies from place to place. Measure by horizontal compass. Lines shown on the map through the places that have the same declination are called isogonic line. Line drawn through places that have zero declination is called an agonic line. At higher latitudes, the declination is greater whereas near the equator, the declination is smaller. In India, declination angle is very small and for Chennai, magnetic declination angle is -1º 8’ (which is negative (west)
• 13. Declination is positive when magnetic north is east of true north. Declination is negative when it is to the west.
• 15. Dip or Inclination (δ): The angle between the horizontal component of earth’s magnetic field and the earth’s resultant magnetic field at a place is Dip or Inclination( I) at that place. It is zero at the equator and 90° at the poles. For Chennai, inclination angle is 14o 16’. Lines drawn up on a map through places that have the same dip are called isoclinic lines. The line drawn through places that have zero dip is known as an aclinic line. It is the magnetic equator. Measure by vertical compass known dip circle. Angle of dip will have more value at northern hemisphere than at southern hemisphere. In the north magnetic pole, the angle of dip is considered as positive dip ( + ). In south magnetic pole, the angle of dip is considered as negative dip ( - ) . At a point, the positive values of dip indicate that the earth's magnetic
• 16. δ BH B δ BV At a point, the negative values of dip indicate that the earth's magnetic field is pointing upward.
• 17. Horizontal Component of Earth’s Magnetic Field (BH ): The total intensity of the earth’s magnetic field does not lie in any horizontal plane. Instead, it lies along the direction at an angle of dip (δ) to the horizontal. The component of the earth’s magnetic field along the horizontal at an angle δ is called Horizontal Component of Earth’s Magnetic Field. BH = B cos δ ----1 BV = B sin δ ------- 2 B = √ 𝑩𝑯 𝟐 + Similarly Vertical Component is such that On dividing equ 2 by equ 1 θ δ BV BH B Magnetic Meridian Geographic Meridian 𝑩𝑽 𝟐 BV = B sin δ BH = B cos δ BV BH = tan δ
• 18. (i) At magnetic equator The Earth’s magnetic field is parallel to the surface of the Earth (i.e., horizontal) which implies that the needle of magnetic compass rests horizontally at an angle of dip, I = 0º 𝑩𝑯 = BE 𝑩𝑽 = 0 This implies that the horizontal component is maximum at equator and vertical component is zero at equator. (ii) At magnetic poles The Earth’s magnetic field is perpendicular to the surface of the Earth (i.e., vertical) which implies that the needle of magnetic compass rests vertically at an angle of dip, I = 90º 𝑩𝑯 = 0 𝑩𝑽 = BE This implies that the vertical component is maximum at poles and horizontal component is zero at poles.
• 19. Atom If we cut the magnet in to smaller pieces, each piece will still acts as a small magnet because the point from which the magnetism begins is from the smallest particles of the material called atoms. Atoms are the main building blocks of matter, atom has nucleus in its center and electrons are orbiting the nucleus. A nucleus contains protons and neutrons, protons are positively charged particle and neutrons have no charge. Electrons are negatively charged particles orbiting the nucleus. The magnetic field is produced by the electrons that are orbiting the nucleus. In 1800 Andre-Marie Ampere suggested that whenever electric charges or electrons are in motion magnetic field is produced. The direction in which electron spin and orbit determine the direction of magnetic field. one electrons orbit and spin in clockwise direction. The remaining electrons orbit and spin in anti-clockwise direction. The strength of magnetic field is called as magnetic moment.
• 22. When two electrons in the same atom spinning and orbiting the nucleus in opposite direction, then the magnetic field strength of this atom is zero because the opposite spins of the electrons causes their magnetic fields to cancel each other. Therefore, no net magnetic moment or magnetic field strength exist. But if more number of electrons in a atom are spinning in clock wise direction and less number of electrons are spinning in anti-clock wise direction, then the atom has some magnetic field strength. For example, five electrons are spinning in clock wise direction and two electrons are spinning in anti-clock wise direction, the magnetic fields of two electrons spinning in clock wise and anti-clock wise direction will cancel their magnetic fields each other and remaining three electrons which are spinning in clock wise direction exhibits magnetic field strength. Paired electrons :The electrons occupy the same orbital of an atom but orbiting and spinning in opposite direction is called paired electrons. Unpaired electron :The electron occupy the orbital of an atom singly rather than a pair is called as unpaired electron.
• 23. The magnetic properties of a solid are the result of the magnetic property of the atoms or ions of these solids. More specifically the magnetism and magnetisation of a solid will depend on the movement of electrons in an atom. It can thus be said that each electron of an atom behaves like a magnet, lending the whole solid its magnetic property. This magnetic behaviour of the electrons of an atom is due to the movement patterns. They have specifically two types of movements, •Electrons revolve around the nucleus of the atom •Electrons also spin on their own axis, spins in opposite sides are labelled with + and – signs. These two motions of the electrons give the atom and the substance their magnetic power. These constant motions make an electric field around the electrons, almost like a loop of current which lends it its magnetic property.
• 24. Diamagnetic materials-----Diamagnetism was discovered and named in September 1845 by Michael Faraday. Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. The electronic configuration in an atom of a diamagnetic material is such that the vector sum of the orbital and spin magnetic moments of all the electrons is zero. Thus, the atomic magnetic moment is zero. Hence, a diamagnetic material has no intrinsic magnetic moment associated with it. or The orbital motion of electrons around the nucleus produces a magnetic field perpendicular to the plane of the orbit. Thus each electron orbit has finite orbital magnetic dipole moment. Since the orbital planes of the other electrons are oriented in random manner, the vector sum of magnetic moments is zero and there is no resultant magnetic moment for each atom.
• 25. The cause of magnetization: for these substances is the orbital motion of electron in which velocity of the electron is affected by the external magnetic field. Effect of magnetic field :When a diamagnetic material is placed in a magnetic field, the orbital motion of electrons changes due to the external magnetic field. In the presence of a uniform external magnetic field-- some electrons are speeded up and some are slowed down. The electrons whose moments were anti-parallel are speeded up according to Lenz’s law and this produces an induced magnetic moment in a direction opposite to the field. The induced moment disappears as soon as the external field is removed. When placed in a non-uniform magnetic field--the interaction between induced magnetic moment and the external field creates a force which tends to move the material from stronger part to weaker part of the external field. It means that diamagnetic material is repelled by the field. This action is called diamagnetic
• 26. The properties of diamagnetic materials are i) Magnetic susceptibility is negative. ii) Relative permeability is slightly less than unity. iii) The magnetic field lines are repelled or expelled by diamagnetic materials when placed in a magnetic field. iv) Susceptibility is nearly temperature independent. It has a small negative value. v) Diamagnetic Materials experiences a repelling force when brought near the pole of a strong magnet. vi)When placed in a non uniform magnetic field they have a tendency to move. away from the field from the stronger part to the weaker part of the field. They get magnetised in a direction opposite to the field. vii)When suspended freely in a uniform magnetic field, they set themselves perpendicular to the direction of the magnetic field. vii)As soon as the magnetizing field is removed, it loses its
• 27. Paramagnetic materials--- In some magnetic materials, each atom or molecule has net magnetic dipole moment which is the vector sum of orbital and spin magnetic moments of electrons. Due to the random orientation of these magnetic moments, the net magnetic moment of the materials is zero. Effect of magnetic field --- In the presence of an external magnetic field, the torque acting on the atomic dipoles will align them in the field direction. As a result, there is net magnetic dipole moment induced in the direction of the applied field. The induced dipole moment is present as long as the external field exists. When placed in a non-uniform magnetic field, the paramagnetic materials will have a tendency to move from weaker to stronger part of the field. Materials which exhibit weak magnetism in the direction of the applied field are known as paramagnetic materials.
• 28. Examples:--- Aluminium, Platinum, Chromium and Oxygen etc. The properties of paramagnetic materials are: i) Magnetic susceptibility is positive and small. ii) Relative permeability is greater than unity. iii) The magnetic field lines are attracted into the paramagnetic materials when placed in a magnetic field. iv) Susceptibility is inversely proportional to temperature.
• 29. Ferromagnetic materials --- When ferromagnetic materials are placed in the strong external magnetic field, it gets strongly magnetized. Cause of ferromagnetism The phenomenon of ferromagnetism arises due to both the interaction between the neighbouring atomic dipoles and the alignment of the permanent dipoles in atoms that result from unpaired electrons in the outer shells. An atom or a molecule in a ferromagnetic material possesses net magnetic dipole moment as in a paramagnetic material. A ferromagnetic material is made up of smaller regions, called ferromagnetic domains . Within each domain, the magnetic moments are spontaneously aligned in a direction. This alignment is caused by strong interaction
• 30. Each domain has net magnetisation in a direction. However the direction of magnetisation varies from domain to domain and thus net magnetisation of the specimen is zero. Effect of Temperature: Ferromagnetism depends upon temperature. As the temperature of a ferromagnetic material is increased, the domain structure starts distorting because the exchange coupling between neightbouring moments weakens. At a certain temperature, depending upon the material, the domain . structure collapses totally and the material behaves like paramagnetic material. The temperature at which a ferromagnetic material transforms into a paramagnetic substance is called Curie temperature (Tc ) of that material.
• 31. Effect of external magnetic field-- four domains and their probable direction of magnetic moments are shown in the figure. When the substance is placed in external magnetic field the resultant magnetic moment can be produced in two different ways i) By the displacement of the boundaries of domain--- The domain which are oriented along the direction of magnetic field their size increase and size of other domains Oriented in other direction decrease. ii) By the rotation of domains---- all the domains align themselves along the direction of external magnetic field. If external magnetic field is weak, magnetisation of substance is by displacement of boundary. In weak magnetic field magnetization process is reversible. On removing the field domains come to original state and the substances demagnetised. But in strong magnetic field magnetization is by rotation of domain.
• 32. In a strong magnetic field the magnetization is Irreversible therefore even on removal of failed the substance remains magnetized . When placed in a non-uniform magnetic field, the ferromagnetic materials will have a strong tendency to move from weaker to stronger part of the field. Materials which exhibit strong magnetism in the direction of applied field are called ferromagnetic materials. Examples: Iron, Nickel and Cobalt. The properties of ferromagnetic materials are: i) Magnetic susceptibility is positive and large. ii) Relative permeability is large. iii)The magnetic field lines are strongly attracted into the ferromagnetic materials when placed in a magnetic field. iv)Susceptibility is inversely proportional to temperature. v) The magnetic susceptibility decreases with increase of temperature. That’s why the ferromagnetism decreases with rise of temperature. Maximum at absolute zero of temperature and drops to zero at Curie
• 33. vi) When it is placed in a magnetic field, it develops strong induced magnetism. vii) With the removal of the magnetizing field, it does not lose its magnetisation. viii) Its permeability is extremely large compared to that of free space. Hence B >> H. ix) When placed in a magnetic field, it is strongly magnetized in the direction of the magnetic field.
• 34. Domain theory:--- In 1907, Weiss proposed domain theory to explain ferromagnetism. In ferromagnetic materials there is a strong interaction called exchange coupling or exchange interaction between neighbouring magnetic dipole moments. Due to this interaction, small regions are formed in which all the atoms have their magnetic moments aligned in the same direction. Such a region is called a domain and the common direction of magnetic moment is called the domain axis. Domain size can be a fraction of a millimeter(𝟏𝟎−𝟔 - 𝟏𝟎−𝟒 m) and contains about 𝟏𝟎𝟏𝟎 - 𝟏𝟎𝟏𝟕 atoms. The boundary between adjacent domains with a different orientation of magnetic moment is called a domain wall. In unmagnetized state, the domain axes of different domains are oriented randomly, resulting in the net magnetic moment of the whole material to be zero.
• 35. even if the magnetic moments of individual domains are nonzero. In nonuniform magnetic field ferromagnetic material tends to move from weaker part to stronger part of the field. When the strong external magnetic field is completely removed, it does not set the domain boundaries back to original position and the net magnetic moment is still nonzero and ferromagnetic material is said to retain magnetization. Such materials are used in preparing permanent magnets.
• 36. Hard and soft magnetic materials Based on the shape and size of the hysteresis loop. Ferromagnetic materials are classified as soft magnetic materials with smaller area and hard magnetic materials with larger area.
• 37. When temperature is increased, thermal vibration will upset the alignment of magnetic dipole moments. Therefore, the magnetic susceptibility decreases with increase in temperature. According to the Curie's Law, the magnetization which is present in a paramagnetic material is said to be directly proportional to the applied field of magnetic. If the object which we have used is heated then the magnetization is viewed to be temperature which is inversely proportional. Curie temperature ---- The minimum temperature at which a ferromagnetic substance is converted into paramagnetic substance is defined as Curie temperature. Curie temperature is the temperature above which the magnetic materials lose their ferromagnetic properties. Curie’s Law:--Magnetic susceptibility of a material varies inversely with the absolute temperature. I α H / T or I / H α 1 / T χ α 1 / T χ = C / T (where C is Curie constant) H / T I
• 38. Curie temperature for iron is 1000 K, for cobalt 1400 K and for nickel 600 K. At lower temperatures, the magnetic dipoles are aligned. Above the curie temperature, random thermal motions cause misalignment of the dipoles. Curie point ----Curie point, also called Curie Temperature, temperature at which certain magnetic materials undergo a sharp change in their magnetic properties. This temperature is named for the French physicist Pierrie Curie, who in 1895 discovered the laws that relate some magnetic properties to change in temperature.
• 39. Curie Weiss Law: The Curie–Weiss law describes the magnetic susceptibility χ of a ferromagnet in the paramagnetic region above the Curie point. Mathematically, it is written as Χ = C/ (T-TC), Here C is a material specific Curie constant T is the absolute temperature measured in Kelvin Tc is the Curie temperature measured in Kelvin.
• 40. Property Diamagnet substances Paramagnetic substances Ferromagnetic substances 1. Effect of magnets They are feebly repelled by magnets. They are feebly attracted by magnets. They are strongly attracted by magnets. 2. In external magnetic field Acquire feeble magnetisation in the opposite direction of the magnetising field. Acquire feeble magnetisation in the direction of the magnetising field. Acquire Strong magnetisation in the direction of the magnetising field. 3. In a non-uniform magnetic field Tend to move slowly from stronger to weaker parts of the field. Tend to move slowly from weaker to stronger parts of the field Tend to move quickly from weaker to stronger parts of the field 4. In a uniform magnetic field A freely suspended diamagnetic rod aligns itself perpendicular to the field. A freely suspended paramagnetic rod aligns itself parallel to the field. A freely suspended ferromagnetic rod aligns itself parallel to the field.
• 41. 5. Susceptibility value (χm ) Susceptibility is small and -ve Susceptibility is small and positive., where is a small number. Susceptibility is very large and positive. 6. Relative permeability value(μr ) Slightly less than 1 Slightly greater than 1 Of the order of 7. Permeability value / / / 8. Effect of temperature Susceptibility is independent of temperature. Susceptibility varies inversely as temperature: Susceptibility decreases with temperature in a complex manner. 9. Removal of magnetizing field Magnetisation lost as long as the magnetizing field is applied. As soon as the magnetizing field is removed magnetisation is lost. Magnetisation is retained even after the magnetizing field removed. 𝟏 < μr 𝟎 ≤ μr < 1 −𝟏 ≤ χm < 𝝐 χm > 1000 𝝁 > 𝛍𝐨 Χm ∝ 𝟏 𝑻 Χm ∝ 𝟏 T−Tc (T > Tc ) 𝝁 < 𝛍𝐨 𝟏 << μr 𝑩 < 𝐁𝐨 𝑩 > 𝐁𝐨 𝝁 ≫ 𝛍𝐨 𝑩 ≫ 𝐁𝐨
• 42. 10. Variation of M with H M changes linearly with H. M changes linearly with H and attains saturation at low temperature and in very strong fields. M changes with H non- linearly and ultimately attains saturation. 11. Hysteresis effect B-vector shows no hysteresis. B-vector shows no hysteresis. B-vector shows hysteresis. 12. Curie point No Curie point No Curie point Have Curie point At a certain temperature called Curie Point, they lose ferromagnetic properties and behave like paramagnetic substances. 13. Curies law dont obeys curies law obeys curies law obeys curies law
• 43. 14. Physical state of the material Solid, liquid or gas. Solid, liquid or gas. Normally solids only. 15. Examples Bi, Cu, Pb, Si, N2(at STP), H2O, NaCl Al, Na, Ca, O2(at STP), CuCl2 Fe, Ni, Co, Gd, Fe2O3, Alnico.
• 44. Magnetisation: Magnetisation is the process by which a magnetic substance atta ns magnetism temporarly or permanently. The methods used to magnetise a magnetic substance are •Single touch method •Double touch method •Eiectrical method of magnetization •Use direct current Demagnetisation: Demagnetisation is the process of removing the magnetic property o a magnet. The magnetism of a magnet can be totally or partially destroyed in the following •By rough handling •By heating •By induction •By passing electricity •Use alternating current
• 46. Differences between soft and hard ferromagnetic materials S.No. Properties Soft ferromagnetic materials Hard ferromagnetic materials 1 When external field is removed Magnetisation disappears Magnetisation persists 2 Area of the loop Small Large 3 Retentivity Low High 4 Coercivity Low High 5 Susceptibility and magnetic permeability High Low 6 Hysteresis loss Less More 7 Magnetization Easy Difficult 8 Demagnetization Easy Difficult 10 Uses Solenoid core, transformer core Permanent magnets and electromagnets 11 Examples Soft iron, Mumetal, Stalloy etc. Carbon steel, Alnico, Lodestone etc.
• 48. The Hysteresis Loop and Magnetic Properties Hysteresis ---The term "hysteresis" is derived from , an ancient Greek word meaning "deficiency" or "lagging behind". It was coined around 1890 by Sir James Alfred Ewing to describe the behaviour of magnetic materials. A great deal of information can be learned about the magnetic properties of a material by studying its hysteresis loop. A hysteresis loop shows the relationship between the induced magnetic flux density B / intensity of magnetization I and the magnetizing force H. It is often referred to as the B-H loop. The loop is generated by measuring the magnetic flux B of a ferromagnetic material while the magnetizing force H is changed. When a ferromagnetic material is kept in a magnetising field, the material gets magnetised by induction. An important characteristic of ferromagnetic material is that the variation of magnetic induction B and intensity of magnetization I with magnetising field H is not linear. It means that the ratio B H = µ is not a constant. A ferromagnetic material (example, Iron) is magnetised slowly by a magnetising field H .
• 49. The magnetic induction B and intensity of magnetization I of the material increases from point A with the magnitude of the magnetising field and then attains a saturation level. Saturation magnetization is defined as the maximum point up to which the material can be magnetised by applying the magnetising field. Intensity of Magnetisation (I) increases with increase in Magnetising Force (H) initially through OA and reaches saturation at A. When H is decreased, I decreases but it does not come to zero at H = 0. B is measured for various values of H and if the results are plotted in graphic forms then the graph will show a hysteresis loop. •The magnetic flux density (B)/intensity of magnetization I is increased when the magnetic field strength(H) is increased from 0 (zero). •With increasing the magnetic field there is an increase in the value of magnetism and finally reaches point A which is called saturation point where B is constant. •With a decrease in the value of the magnetic field, there is a decrease in the value of intensity of magnetization I . But at point B ,H is equal to zero, substance or material retains some amount of magnetism is called retentivity or residual magnetism.H = 0 , I ≠ 𝟎 . OB is called Retentivity.
• 50. •In order to demagnetise the material ,there is a decrease in the magnetic field H towards the negative side, intensity of magnetization I also decreases. At point C the substance is completely demagnetized .I = 0 , H≠ 𝟎. OC is called coercivity. •Coercivity: The magnetizing field (H) needed to demagnetize the magnetic material completely is known as its coercivity. Also called the magnetic coercivity, coercive field or coercive force. •The force required to remove the retentivity of the material is known as Coercive force (C). The material (like iron) having thin loop is used for making temporary magnets and that with thick loop (like steel) is used for permanent magnets.
• 52. •In the opposite direction, the cycle is continued where the saturation point is D, retentivity point is E and coercive force is F. •Due to the forward and opposite direction process, the cycle is complete and this cycle is called the hysteresis loop. The loop ABCDEFA is called Hysteresis Loop. Hysteresis loss--- During the magnetisation of the specimen through a cycle, there is loss of energy in the form of heat. This loss is attributed to the rotation and orientation of molecular magnets in various directions. The area of the loop gives the information of loss of energy per unit volume due to the one cycle of magnetisation and demagnetisation and is dissipated in the form of heat. More area of the loop means more loss of energy per unit volume. Eg. --- permanent magnet , steel etc. Less area of the loop means less loss of energy per unit volume. Eg.---soft iron core etc.
• 53. From the hysteresis loop, a number of primary magnetic properties of a material can be determined. 1.Retentivity - It is a material's ability to retain a certain amount of residual magnetic field when the magnetizing force is removed after achieving saturation. (The value of B at point B on the hysteresis curve.) 2.Residual Magnetism or Residual Flux - the magnetic flux density that remains in a material when the magnetizing force is zero. Note that residual magnetism and retentivity are the same when the material has been magnetized to the saturation point. However, the level of residual magnetism may be lower than the retentivity value when the magnetizing force did not reach the saturation level. 3.Coercive Force - The amount of reverse magnetic field which must be applied to a magnetic material to make the magnetic flux return to zero. (The value of H at point C on the hysteresis curve.) 4.Permeability, m - A property of a material that describes the ease with which a magnetic flux is established in the component. 5.Reluctance - Is the opposition that a ferromagnetic material shows to the establishment of a magnetic field. Reluctance is analogous to the resistance in an electrical circuit.
• 56. Applications of hysteresis loop The significance of hysteresis loop is that it provides information such as retentivity, coercivity, permeability, susceptibility and energy loss during one cycle of magnetisation for each ferromagnetic material. i) Permanent magnets: The materials with high retentivity, high coercivity and low permeability are suitable for making permanent magnets. Examples: Carbon steel and Alnico ii) Electromagnets: The materials with high initial permeability, low retentivity, low coercivity and thin hysteresis loop with smaller area are preferred to make electromagnets. Examples: Soft iron and Mumetal (Nickel Iron alloy). iii) Core of the transformer: The materials with high initial permeability, large magnetic induction and thin hysteresis loop with smaller area are needed to design transformer cores. Examples: Soft iron
• 57. Magnetic Shielding :----- When a soft ferromagnetic material is kept in a uniform magnetic field, large number of magnetic lines crowd up inside the material leaving a few outside. If we have a closed structure of this material, say a spherical shell of iron kept in magnetic field, very few lines of force pass through the enclosed space. Most of the lines will be crowded into the iron shell .This effect is known as magnetic shielding. The instrument which need to be protected from magnetic field is completely surrounded by a soft ferromagnetic substance. This technique is being used in space ships. Some scientific experiments require the experiment to be protected from magnetic field in the laboratory. There, high magnetic fields of magnets need to be shielded by providing a case made up of soft ferromagnetic material.
• 58. Electromagnet Electromagnet works on the principle of magnetic effect of electric current. It is formed when a strong magnetic field is produced inside a solenoid to magnetise a piece of magnetic material like soft iron. An electromagnet is a solenoid wrapped around a soft iron core. "Soft" meant that the domain will align with the magnetic field produced by the solenoid. Components of electromagnet Solenoid: Coils of Wire Soft Iron Core
• 59. Electromagnet Right Hand Rule This is different from the straight-line current right hand rule. Your thumb will now represent the north pole of an electromagnet. Your fingers are going to represent the direction of current. 1.Wrap your hand around the solenoid around the soft iron core with the curl of your hand in the direction of conventional current flow. This will be either clockwise or counterclockwise. 2.Your thumb will now be in the direction that will be the north pole of the electromagnet. The two ways to strengthen an electromagnet are: 1.Increase the number of coils of the solenoid 2.Increase the current (Increasing the voltage of the power source would result in this)