MAGNETIC MATERIALElectricity is not a fun but easy tolearn.
SR. VERY IMPORTANT QUESTIONSNO.1 Diamagnetism, Para magnetism and Ferromagnetism2 Hard and Soft magnets3 Magnetic Parameters4 Soft and Hard magnets5 Ferrites6 Hysteresis Loop
MAGNETISM 1. Bar Magnet and its properties 2. Current Loop as a Magnetic Dipole and Dipole Moment 3. Current Solenoid equivalent to Bar Magnet 4. Bar Magnet and it Dipole Moment 5. Coulomb’s Law in Magnetism 6. Important Terms in Magnetism 7. Tangent Law 8. Properties of Dia-, Para- and Ferro-magnetic substances 9. Curie’s Law in Magnetism 10. Hysteresis in Magnetism 11. Hard and soft magnets 12. Etc ……………..Created by, Vijay Balu Raskar (Electrical Engineer – Mumbai University)
Magnetism:- Phenomenon of attracting magnetic substances like iron, nickel, cobalt, etc.• A body possessing the property of magnetism is called a magnet.• A magnetic pole is a point near the end of the magnet where magnetism isconcentrated.• Earth is a natural magnet.•The region around a magnet in which it exerts forces on other magnets andon objects made of iron is a magnetic field.Properties of a bar magnet:1. A freely suspended magnet aligns itself along North – South direction.2. Unlike poles attract and like poles repel each other.3. Magnetic poles always exist in pairs. i.e. Poles can not be separated.4. A magnet can induce magnetism in other magnetic substances.5. It attracts magnetic substances.Repulsion is the surest test of magnetisation: A magnet attracts iron rod as wellas opposite pole of other magnet. Therefore it is not a sure test of magnetisation.But, if a rod is repelled with strong force by a magnet, then the rod is surelymagnetised.
Representation of Uniform Magnetic Field: x x x x x x x x x x x x x x x x x x x x x x x x x Uniform field Uniform field perpendicularUniform field on the perpendicular & into the & emerging out of the planeplane of the diagram plane of the diagram of the diagramCurrent Loop as a Magnetic Dipole & Dipole Moment: Magnetic Dipole Moment is A M=IA n B SI unit is A m2. TIP: When we look at any one side of the loop carrying current, if the current is in anti-clockwise direction then that side of the loop behaves like I Magnetic North Pole and if the current is in clockwise direction then that side of the loop behaves like Magnetic South Pole.
Bar Magnet: Geographic Length1. The line joining the poles of the magnet S P M P N is called magnetic axis. Magnetic Length2. The distance between the poles of the magnet is called magnetic length of the magnet.3. The distance between the ends of the magnet is called the geometrical length of the magnet.4. The ratio of magnetic length and geometrical length is nearly 0.84.Magnetic Dipole & Dipole Moment:A pair of magnetic poles of equal and opposite strengths separated by afinite distance is called a magnetic dipole.The magnitude of dipole moment is the product of the pole strength m andthe separation 2l between the poles.Magnetic Dipole Moment is M = m.2l. l SI unit of pole strength is A.m The direction of the dipole moment is from South pole to North Pole along the axis of the magnet.
Magnetic Intensity or Magnetising force (H):i) Magnetic Intensity at a point is the force experienced by a north pole of unit pole strength placed at that point due to pole strength of the given magnet. H=B/μii) It is also defined as the magnetomotive force per unit length.iii) It can also be defined as the degree or extent to which a magnetic field can magnetise a substance.iv) It can also be defined as the force experienced by a unit positive charge flowing with unit velocity in a direction normal to the magnetic field.v) Its SI unit is ampere-turns per linear metre.vi) Its cgs unit is oersted.Magnetic Field Strength or Magnetic Field or Magnetic Induction or Magnetic Flux Density (B):i) Magnetic Flux Density is the number of magnetic lines of force passing normally through a unit area of a substance. B = μ Hii) Its SI unit is weber-m-2 or Tesla (T).iii) Its cgs unit is gauss. 1 gauss = 10- 4 Tesla
MAGNETIC PARAMETERS1.Magnetic Permeability (μ):The property of material by virtue of which it allows itselfto be magnetized.It is the ratio of magnetic field density to magnetic fieldstrength.It accept magnetization.It varies material to material depending on temperature,saturation.It is the degree or extent to which magnetic lines of forcecan pass enter a substance.A good magnetic material should have high value ofpermeability.Magnetic field density is directly proportional to fieldintensity.Its SI unit is T m A-1 or wb A-1 m-1 or H m-1
Permeability Concept• For some materials, the net magnetic dipole moment per unit volume is proportional to the H field M m H • the units of both M and magnetic H are A/m. susceptibility (dimensionless) 9
• Assuming that M m H we have B m0 H M m0 1 m H m H• The parameter m is the permeability of the material. 10
2.Magnetic Susceptibility (Xm ):i) It is the property of the substance which shows how easily a substance can be magnetised.ii) It can also be defined as the ratio of intensity of magnetisation (I) in a substance to the magnetic intensity (H) applied to the substance.iii) Xm =I/Hiv) Susceptibility has no unit.v) When –ve , solid is dimagnetic.vi) When small range of +ve, Solid is paramagnetic.vii) If large range value of +ve, Solid is ferromagnetic.viii)It may be positive or negative.
3. Intensity of Magnetisation: (I):i) It is the degree to which a substance is magnetised when placed in a magnetic field.ii) It can also be defined as the magnetic dipole moment (M) acquired per unit volume of the substance (V).iii) It can also be defined as the pole strength (m) per unit cross-sectional area (A) of the substance.iv) I = M / Vv) I = m(2l) / A(2l) = m / Avi) SI unit of Intensity of Magnetisation is A m-1.vii) Magnetic field gets redistributed when placed in solid and no field.4. Curie Temperature (C):-i) The temperature above which ferromagnetic material looses their magnetic properties.ii) Above Ctemp, domain structure for gets destructed and domain looses their alignment.Relation between Magnetic Permeability (μr) & Susceptibility (cm ): μr = 1 + c m
5. Magnetic Dipole Moment:- (μm) i) It is the product of pole strength (m) and distance between poles(d). ii) It is a vector quantity. iii) It is directed from South pole to North pole. iv) Its direction is given by right hand screw rule. Relation between B and H: B=μH (where μ is the permeability of the medium)Magnetic Flux (Φ):i) It is defined as the number of magnetic lines of force passing normally through a surface.ii) Its SI unit is weber. Relative Magnetic Permeability (μr): It is the ratio of magnetic fluxB or μ = μ / μ μ = B / density in a material to that in vacuum. r 0 r 0 It can also be defined as the ratio of absolute permeability of the material to that in vacuum.
Comparison of Dia, Para and Ferro Magnetic materials: DIA PARA FERRO1. Diamagnetic Paramagnetic substances Ferromagnetic substancessubstances are those are those substances are those substancessubstances which are which are feebly attracted which are stronglyfeebly repelled by a by a magnet. attracted by a magnet.magnet. Eg. Aluminium, Chromium, Eg. Iron, Cobalt, Nickel,Eg. Antimony, Bismuth, Alkali and Alkaline earth Gadolinium, Dysprosium,Copper, Gold, Silver, metals, Platinum, Oxygen, etc.Quartz, Mercury, Alcohol, etc.water, Hydrogen, Air,Argon, etc.2. When placed in The lines of force prefer to The lines of force tend tomagnetic field, the lines of pass through the crowd into the specimen.force tend to avoid the substance rather than air.substance. N S S N S N
2. When placed in non- When placed in non- When placed in non-uniform magnetic field, it uniform magnetic field, it uniform magnetic field, itmoves from stronger to moves from weaker to moves from weaker toweaker field (feeble stronger field (feeble stronger field (strongrepulsion). attraction). attraction).3. When a diamagnetic When a paramagnetic rod When a paramagnetic rodrod is freely suspended in is freely suspended in a is freely suspended in aa uniform magnetic field, it uniform magnetic field, it uniform magnetic field, italigns itself in a direction aligns itself in a direction aligns itself in a directionperpendicular to the field. parallel to the field. parallel to the field very quickly. N S N S N S
4. If diamagnetic liquid If paramagnetic liquid If ferromagnetic liquidtaken in a watch glass is taken in a watch glass is taken in a watch glass isplaced in uniform placed in uniform placed in uniformmagnetic field, it collects magnetic field, it collects magnetic field, it collectsaway from the centre at the centre when the at the centre when thewhen the magnetic poles magnetic poles are closer magnetic poles are closerare closer and collects at and collects away from and collects away fromthe centre when the the centre when the the centre when themagnetic poles are magnetic poles are magnetic poles arefarther. farther. farther.
5. When a diamagnetic When a paramagnetic When a ferromagneticsubstance is placed in a substance is placed in a substance is placed in amagnetic field, it is magnetic field, it is magnetic field, it isweakly magnetised in the weakly magnetised in the strongly magnetised indirection opposite to the direction of the inducing the direction of theinducing field. field. inducing field.6. Induced Dipole Induced Dipole Moment Induced Dipole MomentMoment (M) is a small (M) is a small + ve value. (M) is a large + ve value.– ve value.7. Intensity of Intensity of Magnetisation Intensity of MagnetisationMagnetisation (I) has a (I) has a small + ve value. (I) has a large + ve value.small – ve value.8. Magnetic permeability Magnetic permeability μ Magnetic permeability μμ is always less than is more than unity. is large i.e. much moreunity. than unity.
9. Magnetic susceptibility Magnetic susceptibility cm Magnetic susceptibility cmcm has a small – ve value. has a small + ve value. has a large + ve value.10. They do not obey They obey Curie’s Law. They obey Curie’s Law. AtCurie’s Law. i.e. their They lose their magnetic a certain temperatureproperties do not change properties with rise in called Curie Point, theywith temperature. temperature. lose ferromagnetic properties and behave like paramagnetic substances.Curie’s Law:Magnetic susceptibility of a material varies inverselywith the absolute temperature. IαH/T or I/Hα1/T I Xm α 1 / T Xm = C / T (where C is Curie constant) H/TCurie temperature for iron is 1000 K, for cobalt 1400 Kand for nickel 600 K.
Hysteresis Loop or Magnetisation Curve:Intensity of Magnetisation (I) increases with increase I Ain Magnetising Force (H) initially through OA and Breaches saturation at A.When H is decreased, I decreases but it does notcome to zero at H = 0.The residual magnetism (I) set up in the material C O F Hrepresented by OB is called Retentivity.To bring I to zero (to demagnetise completely), Eopposite (negative) magnetising force is applied. DThis magetising force represented by OC is calledcoercivity.After reaching the saturation level D, when themagnetising force is reversed, the curve closes tothe point A completing a cycle.The loop ABCDEFA is called Hysteresis Loop.The area of the loop gives the loss of energy due tothe cycle of magnetisation and demagnetisation andis dissipated in the form of heat.The material (like iron) having thin loop is used formaking temporary magnets and that with thick loop(like steel) is used for permanent magnets. Animating Hysteresis Loop: End of Magnetism Courtesy - Website
HARD MAGNETS SOFT MAGNETSMaterials which retain their magnetism Soft magnetic materials are easy toand are difficult to demagnetize are called magnetize and demagnetize.hard magnetic materials.These materials retain their magnetism These materials are used for makingeven after the removal of the applied temporary magnets.magnetic field. Hence these materials areused for making permanent magnets.They have large hysteresis loss due to They have low hysteresis loss due tolarge hysteresis loop area. small hysteresis area.Susceptibility and permeability are low. Susceptibility and permeability are high.Coercivity and retentivity values are Coercivity and retentivity values are less.large.Magnetic energy stored is high. Since they have low retentivity and coercivity, they are not used for making permanent magnets.They possess high value of BH product. Magnetic energy stored is less.The eddy current loss is high. The eddy current loss is less because of high resistivity.
HARD MAGNETS SOFT MAGNETSExample ExampleCarbon Steel IronTungsten – Steel Si – SteelCobalt – steel Alloy steelCopper nickel iron alloy Soft ferritesEtc Etc1) Used in measuring instruments 1) Construction of transformer core2) Magnetic detectors 2) Core of electrical machines3) Permanent Magnet 3) Core of reactors4) Etc 4) Making electromagnets 5) Etc
Ferrites• Ferrites are the most useful ferrimagnetic materials.• Ferrites are ceramic material containing compounds of iron.• Ferrites are non-conducting magnetic media so eddy current and ohmic losses are less than for ferromagnetic materials.• Ferrites are often used as transformer cores at radio frequencies (RF).• Low dielectric loss.• Low coersive force.• They are mechanically hard, brittle and difficult to machined.• Very high resistivity.• Types of Ferries are, 1) Soft, 2) Hard,3) Rectangular loop and 4) Microwave Ferrites.• Some material have hysterisis loop, rectangular in shape.• Dielectric constant is of the order of 10 to 12. 22