Basic Principles of Electromagnetism

CHAPTER`4C h a p t e r 4 | 87BASIC PRINCIPLES OFELECTROMAGNETISM4.0 INTRODUCTIONThis chapter is explaining about the relationship between current flows in conductor,classify factors that affect electromagnetic strength and understand the characteristicsof magnetic quantities in electromagnet. The learning outcomes for this chapter arethe students should be able to explain clearly the relationship between current flowand magnetism.4.1 MAGNETMagnet can be defined as material that can attract piece of iron or metal. Magnet hastwo poles north and south. Material that attracted by the magnet is known as magneticsubstances. The ability to attract the magnetic substances is known as magnetism.4.1.1 Principles Of MagnetMagnet has a magnetic field around the magnet itself. Magnetic field is theforce around the magnet which can attract any magnetic material around it.Flux magnet is the line around the magnet bar which form magnetic field.Figure 4.1: Magnetic FieldBasic Principles of Electromagnetism

C h a p t e r 4 | 884.1.2 Basic Magnet LawMagnetic flux lines have a direction and pole. The direction of movementoutside of the magnetic field lines is from north to south. The strongestmagnetic fields are at the magnetic poles. Different poles attract each other andsame magnetic poles will reject each other. Flux form a complete loop andnever intersect with each other. Flux will try to form a loop as small aspossible.Figure 4.2: Magnetic Flux4.1.3 Types Of MagnetThere are two types of magnet known as pure magnet and manufacturemagnet.i) Pure MagnetPure magnet is a magnet stone. The stone originally have the naturalmagnetic. Basically the stone is found in the form of iron ore.ii)Manufacture MagnetThere are two types of manufacture which is permanent magnet andtemporary magnet.a) Permanent MagnetThe ability of the magnet to kept its magnetism. There are fivebasic shape of permanent magnet as shown in Figure 4.3. Basicallypermanent magnet is used in a small device such as speakers, meterand compass. Permanent magnet can be obtained by naturally ormagnetic induction and placing a magnet into a coil then suppliedwith a high electrical current. The basic types of permanent magnetare U Shape, Horseshoe, Rod, Cylinder and Bar.Basic Principles of Electromagnetism

C h a p t e r 4 | 89b) Temporary magnetAn electric current can be used for making temporary magnetsknown as electromagnet. It has magnetic properties when subjectedto magnetic force and it will be lost when power is removed.Typically the temporary magnet is used in electrical componentsuch as relay and small devices such as electrical bell.4.2 ELECTROMAGNETElectromagnet is a magnetic iron core produced when the current flowing through thecoil. Thus, the magnetic field can be produced when there is a current flow through aconductor. The direction of the magnetic field produced by the current in the solenoidcan be determined using two methods:i) Right Hand Grip Ruleii) Maxwell's Screw Law4.2.1 Right Hand Grip RuleRight Hand Grip Rule is a physics principle applied to electric current passingthrough a solenoid, resulting in a magnetic field. By wrapping the right handaround the solenoid, thumb is pointing in the direction of the magnetic northpole and fingers in the direction of the conventional current. This rule can alsobe applied to electricity passing through a straight wire. The thumb points inthe direction of the conventional current from positive to negative. Meanwhilethe fingers points of the magnetic lines of flux.Figure 4.3: Right Hand Grip Rule4.2.2 Maxwell's Screw LawAnother way to determine the direction of the flux and current in a conductoris to use Maxwell's screw rule. A right-handed screw is turned so that it movesforward in the same direction as the current, its direction of rotation will givethe direction of the magnetic field from south to north.Basic Principles of Electromagnetism

C h a p t e r 4 | 90Figure 4.4: Maxwell’s Screw Law4.3 ELECTROMAGNETIC EFFECTA flow of current through a wire produces a magnetic field in a circular path aroundthe wire. The field patterns of a current flow in a conductor can de determine usingboth rules of right hand grip or Maxwell’s Screw. Note that, conventional currentflow towards or inside the conductor is marked by cross (X) and current flow away oroutside the conductor is marked as dot ( · ).4.3.1 Single ConductorThe direction of the field pattern in going and out going produced by a currentflowing through a single conductor can be determine by applying both rules asillustrated in Figure 4.5.Source: Fundamental Electrical & Electronic Principles by Christopher R RobertsonFigure 4.5: Current flow (a) in going (b) out goingBasic Principles of Electromagnetism

C h a p t e r 4 | 914.3.2 Two ConductorsIf the two conductors where the current flow in the same direction, themagnetic flux pattern will produce around both conductors and combine tocreate attraction between them as shown in Figure 4.6 (a). If the current inconductors flow in opposite direction, the field pattern will repulse each other.The effect is shown in Figure 4.6(b).(a)(b)Source: Pengajian Kejuruteraan Elektrik dan Elektronik,Cetakan Pertama 2000 by Abd.Samad HanifFigure 4.6: Two closed current-carrying conductors flow(a) in same direction (b) in opposite direction4.4 ELECTROMAGNETIC STRENGTHThere are 4 factors that affect electromagnetic strength:1. Number of turnsThe strength of the electromagnet is directly proportional to the number of turn inthe coil. By varying the number of turns in its coil can produce very strongmagnetic fields and its strength.2. Current strengthThe strength of the electromagnet is directly proportional to the current flowing inthe coil. Greater the current flow through the coil, stronger will be the magneticfields produced.Basic Principles of Electromagnetism

C h a p t e r 4 | 923. Length of coilThe strength of the electromagnet is directly proportional to the length of the coil.By coil up the wire can increasing the length and increase the force of magneticfield.4. Types of conductorDepend on the nature of the core material. The use of soft of core can producesthe strongest magnetism.4.5 ELECTROMAGNETIC INDUCTIONWhen a conductor is move across a magnetic field, an electromagnetic force (emf) isproduced in conductor. This effect is known as electromagnetic induction. The effectof electromagnetic induction will cause induced current. There are two laws ofelectromagnetic induction:i. Faraday’s lawii. Lenz’z Law4.5.1 Faraday’s lawFaraday’s law is a fundamental relationship which comes from Maxwell’sequations. It is a relative movement of the magnetic flux and the conductorthen causes an emf and thus the current to be induced in the conductor.Induced emf on the conductor could be produced by two methods i.e. flux cutsconductor or conductor cuts flux.a. Flux cuts conductorFlux cut conductor is when the magnet is move towards the coil as shown inFigure 4.9, a deflection is noted on the galvanometer showing that a currenthas been produced in the coil.Source: Pengajian Kejuruteraan Elektrik dan Elektronik, Cetakan Pertama 2000 by Mohd. Isabin IdrisFigure 4.7: Flux cuts conductorBasic Principles of Electromagnetism

C h a p t e r 4 | 93b. Conductor cuts fluxConductor cut flux is when the conductor is moved through a magnetic fieldas shown in Figure 4.8. An emf is induced in the conductor and thus a sourceof emf is created between the ends of the conductor. This is the simple conceptof AC generator. This induced electromagnetic field is given bywhereE = Blv volts [4.1]B = flux density, Tl = length of the conductor in the magnetic field, mv = conductor velocity, m/sIf the conductor moves at the angle θ° to the magnetic field, thenE = Blv sinθ° volts [4.2]Source: Pengajian Kejuruteraan Elektrik dan Elektronik, Cetakan Pertama 2000 oleh Mohd.Isa bin IdrisFigure 4.8: Conductor cuts fluxExample 4.1A conductor 300mm long moves at a uniform speed of 4m/s at right-angles toa uniform magnetic field of flux density 1.25T. Determine the current flowingin the conductor when(a)(b)its ends are open-circuitedits ends are connected to a load of 20 Ω resistance.Basic Principles of Electromagnetism

C h a p t e r 4 | 94Solution 4.1When a conductor moves in a magnetic field it will have an emf induced in itbut this emf can only produce a current if there is a closed circuit. Induced emfE = Blv = (1.25)(300/1000)(4)(a)If the ends of the conductor are open circuit, no current will flow eventhough 1.5 V has been induced.(b)From Ohm’s lawE 1.5I = = 75mAR 204.5.2 Lenz’z LawThe direction of an induced emf is always such that it tends to set up a currentopposing the motion or the change of flux responsible for inducing that emf.This effect is shown in Figure 4.9.Source: Pengajian Kejuruteraan Elektrik dan Elektronik, Cetakan Pertama 2000 by Mohd. Isabin IdrisFigure 4.9: Bar magnet move in and move out from a solenoidBasic Principles of Electromagnetism

C h a p t e r 4 | 954.6 MAGNETIC QUANTITY CHARACTERISTICSThere are many magnetic quantities in the System International (SI) unit. This chapteris only going to discuss on magnetomotive force, magnetic field strength, magneticflux, flux density, permeability and reluctance.4.6.1 Magnetiomotive Force, F mMagnetomotive force is a cause of the existence of magnetic flux in amagnetic circuit. The total flux produced is depends on the number of turn (N)made in the circuit. It is also proportional to the current (I) passing through thecoil. Then, the magnetomotive force is the product of current and the numberof turns.F m = NI ampere turn [4.3]4.6.2 Magnetic Field Strength, HMagnetic field strength or magnetizing force is defined as magnetomotiveforce, F m per meter length of measurement being ampere-turn per meter.HF NI= m= ampere turn / meter [4.4]l lwhereF m - magnetomotive forceN - number of turnsI - currentl - average length of magnetic circuitExample 4.2A current of 500mA is passed through a 600 turn coil wound of a toroid ofmean diameter 10cm. Calculate the magnetic field strength.Basic Principles of Electromagnetism

C h a p t e r 4 | 96Solution 4.2I = 0.5AN = 600d = π x 10 x 10 -2 mNI600 × 0.5H = ampereturn / metre = = 954.81 AT/ml0.31424.6.3 Magnetic Flux and Flux DensityMagnetic flux is the amount of magnetic filed produced by a magnetic source.The symbol for magnetic flux is phi (Φ). The unit for magnetic flux is theweber, Wb. Magnetic flux density is the amount of flux passing through adefined area that is perpendicular to the direction of flux:Magnetic flux density =magnetic fluxareaΦB = Tesla [4.5]AThe symbol for magnetic flux density is B. The unit of magnetic flux densityis the tesla, T, and the unit for area A is m 2 where 1 T = 1 Wb/m.Example 4.3A magnetic pole face has rectangular section having dimensions 200mm by100mm. If the total flux emerging from the the pole is 150µWb, calculate theflux density.Solution 4.3Magnetic flux, Φ = 150 µWb = 150 x 10 -6 WbCross sectional area, A = 200mm x 100mm = 20 000 x 10 -6 m2−6Φ 150 × 10Flux density, B = =−6A 20000 × 10= 7.5 mTBasic Principles of Electromagnetism

C h a p t e r 4 | 974.6.4 PermeabilityPermeability is the measure of the ability of the material to allow the magneticfield to exist in it. Absolute permeability, µ of a material is the ratio of the fluxdensity to magnetic field strength.µ = µ 0µr[4.6]If the magnetic fields exist in the vacuum, the ratio of the flux density to themagnetic field strength is a constant called the permeability of free space. Forair or any other non-magnetic medium, the ratio of magnetic flux density toBmagnetic field strength is constant , = a constant. The equation forHpermeability of free space in non-magnetic medium is as shown in equation4.7 below.BH= µ 0[4.7]The permeability of free space, µ 0 is equal to 4π x 10-7 H/m. In the air or anynon-magnetic material µr = 1, this is the same magnetic properties as avacuum as shown in equation 4.7. µ r is relative permeability and it isconsidered when the different type of material is used. µr is defined as theratio of the flux density produced in the material that produced in the air or asdefine in equation 4.8.flux density in materialµr=[4.8]flux density in vacuumµr varies with the type of magnetic material. The approximate ranges ofrelative permeability for some common magnetic materials are as follows:Cast iron: µ r = 100 – 250Mild steel: µ r = 200 – 800Cast steel: µ r = 300 – 900Therefore the permeability for all media other than free space in a magneticmedium or material is as shown in equation in 4.9 below.BH= µ 0µ[4.9]rBasic Principles of Electromagnetism

C h a p t e r 4 | 98Example 4.4A flux density of 1.2 T is produced in a piece of cast steel by a magnetizingforce of 1250 A/m. Find the relative permeability of the steel under theseconditions.Solution 4.4B = µ 0µ HrB 1.2µr= == 764−2µ H (4π× 10 )(1250)04.6.5 ReluctanceReluctance, S is the magnetic resistance of a magnetic circuit to presence ofmagnetic flux. The equation for reluctance is as equation 4.10 belowFmHl l 1S = = = =[4.10]Φ BA ( B / H ) A µ 0µ ArThe unit for reluctance is 1/H or H -1 or A/Wb. The ferromagnetic materialshave low reluctance and can be used as magnetic screens to prevent magneticfields affecting materials within the screen.Example 4.5Determine the reluctance of a piece of metal with length 150mm, when therelative permeability is 4 000. Find the absolute permeability of the metal.Solution 4.5Reluctance,S =1µ 0µ rA=(4π× 10−7−3150 × 10)(4000)(1800 × 10−6)= 16 580 H -1Basic Principles of Electromagnetism

C h a p t e r 4 | 99Absolute permeability,= µ µµ0r−7= (4π× 10 )(4000)= 5.027 x 10 -3 H/mREFERENCERobertson, C.R (2008) “Fundamental Electrical and Electronic Principles 3 rdNewnes, New YorkEdition”,PROBLEMS1. Find the magnetic field strength applied to a magnetic circuit of mean length 50 cmwhen a coil of 400 turns is applied to it carrying a current of 1.2 A.(960AT/m)2. A current of 2.5A when flowing through a coil produces an mmf of 675 At. Calculatethe number of turns on the coil(270 turns)3. A magnetizing force 8000 A/m is applied to a circular magnetic circuit of meandiameter 30 cm by passing a current through a coil wound on the circuit. If the coil isuniformly wound around the circuit and has 750 turns, find the current in the coil.(10.05 A)4. The maximum working flux density of a lifting electromagnet is 1.8 T and the effectivearea of a pole face is circular in cross-section. If the total magnetic flux produced is 353mWb, determine the radius of the pole.(0.196m)5. A coil of 300 turns is wound uniformly on a ring of non-magnetic material. The ringhas a mean circumference of 40 cm and a uniform cross-sectional area of 4cm 2 . If thecurrent in the coil is 5 A, calculate (a) the magnetic field strength, (b) the flux densityand (c) the total magnetic flux in the ring.(3750AT/m, 4.712mT, 1.885µWb)6. At what velocity must a conductor 75mm long cut a magnetic flux of density 0.6T if ane.mf. of 9V is to be included in it? Assume the conductor, the field and the direction ofmotion are mutually perpendicular.(200m/s)7. A conductor of length 15cm is moved at 750mm/s at right-angles to a uniform fluxdensity of 1.2 T. Determine the emf induced in the conductor.(0.135V)Basic Principles of Electromagnetism

C h a p t e r 4 | 1008. Find the speed that a conductor of length 120mm must be moved at right angles to amagnetic field of flux density 0.6 T to induce in it an emf of 1.8 V.(25m/s)9. A 25 cm long conductor moves at a uniform speed of 8 m/s through a uniform magneticfield of flux density 1.2T. Determine the current flowing in the conductor when (i) itsends are open-circuited (ii) its ends are connected to a load of 15 ohms resistance.(0, 0.16A)10. A conductor of length 0.5 m situated in and at right angles to a uniform magnetic fieldof flux density 1 wb/m 2 moves with a velocity of 40 m/s. Calculate the emf induced inthe conductor. What will be the emf induced if the conductor moves at an angle 60º tothe field.(20V, 17.32V)Basic Principles of Electromagnetism