UNIT: ELECTRICAL PRINCIPLES | TOPIC 4: MAGNETISM AND ELECTROMAGNETISM INTRODUCTION > Magnetism plays an important role in Electrical and Electronic Engineering because without its components such as relays, solenoids, inductors, chokes, coils, loudspeakers, motors, generators, transformers, and electricity meters etc., would not work if magnetism did not exist > Then every coil of wire uses the effect of electromagnetism when an electrical current flow through it. DEFINATION OF MAGNETISM AND ELECTROMAGNETISM > Magnetism is a property of force produced ina magnet or magnetic materials. The magnet is an element which can attract magnetic materials, Normally a magnet can attract iron, cobalt, steel, nickel. Ifa magnetic piece is made free to rotate, it will always align along north and south. > Electromagnetism is a property of force produced when an electrical current flow through a simple conductor such asa length of wire or cable, and as current passes along the whole of the conductor then a magnetic field is created along the whole of the conductor. The small magnetic field created around the conductor has a definite direction with both the “North” and “South” poles produced being determined by the direction of the electrical current flowing through the conductor. > Magnets: Materials that attract other metals THE TYPES OF MAGNETS 1. Natural magnet: these are mined black stone like substances (loadstone) that can attract iron and faced along north and south. They are nowadays used as the direction indicator. 2._ Artificial magnet. The magnet can also be produced stfially. \ Artificial magnets are of two types a S i) Permanent magnet: this is an object that aré always surrounded by their own magnetic field, e.g bar magnets and horseshoe magnets. A permanent magnet can be produced from allayed steel. An iron piece can be made permanent magnet by directional rubbing of another magnet on it. An iron piece can also be made magnet by ‘winding conductor around the iron piece and by supplying current through the conductor. ) Temporary magnet: this is an object that becomes a magnet only when placed in a magnetic field. A ‘temporary magnet retains its own magnetic field for a short period of time, then loses it. You can make @ ‘temporary magnet from an object made of iron or one that has an iron content, such as an iron nail. MAGNETIC AND NON-MAGNETIC MATERIALS 1. Ferro Magnetic Materials: these materials are strongly attracted by a magnet. The examples of Ferromagnetic materials are iron, steel, nickel, cobalt, some metallic alloys. The relative permeability of these materials is very high. 2. Para/Semi Magnetic Materials: these materials are attracted by a magnet but not very strongly. The examples of paramagnetic materials are aluminum, tin, platinum, magnesium, manganese etc, The relative permeability of these materials is slightly more than one, 3. Dia/Non- Magnetic Materials: these materials are not at all attracted by any magnet. The relative permeability of these materials is less than one. The examples of diamagnetic materials are zinc, mercury, lead, sulfur, copper, silver etc. Prepared by Omondi Ferdinand — 1 Electrical Trainer - Seme TVCUNIT: ELECTRICAL PRINCIPLES | TOPIC 4: MAGNETISM AND ELECTROMAGNETISM THE CONCEPT OF MAGNETISM AND ELECTROMAGNETISM > Magnets can be found in a natural state in the form of a magnetic ore, with the two main types being Magnetite also called “iron oxide", (Fe:0.) and Lodestone, also called “leading stone”. If these two natural magnets are suspended from a piece of string, they will ake up a position in-line with the Earth's magnetic field always pointing north, Agood example of this effect is the needle of a compass. For most practical applications these natural ‘occurring magnets can be disregarded as their magnetism is very low and because nowadays, man-made artificial magnets can be produced in many different shapes, sizes and magnetic strengths. > Magnetism isa term used to describe any phenomenon that can be attributed toa magnetic field. Magnets can exert forces on other magnets or magnetic materials. A magnetic fleld is described as a region where magnets/magnetic materials experience a force. Magnets have poles, named “north poles” and “south poles". Like poles (north-north or south-south) repel and unlike poles (north-south) attract. Magnetic poles have never been observed alone (2 north pole is always accompanied by a south pole). > Magnetism comes from a property of electrons known as spin. Spin gives electrons a property called the magnetic moment. Usually, magnetic moments of nearby electrons are in opposite directions and so they cancel each other out. > Inmaterials that have been magnetized, the magnetic moments of electrons are aligned. The combined ‘magnetic moments are what allows a magnetized material to exert forces on other magnetic materials. When ‘you place @ material inside a magnetic field, the external field can cause the magnetic moments of electrons in the atoms of the material to line up, causing the materials to become magnetized. The degree to which a material becomes magnetized depends on bath the type of material and the strength of the external magnetic field. Some materials retain the alignment of magnetic moments even when the external magnetic field removed, and they become permanent magnets, > Electromagnetism is a term that describes plienomena that can be attributed to electric or magnetic forces. Electric and magnetic fields are interrelated, and they can be considered to be aspects of one electromagnetic force, as we will mention below. > Magnetism and Electromagnetism Magnetism is produced as a result of electrons spinning in thelr own axes while rotating about the nuclei of atoms. ‘THE MAGNETIC THEORY > When the material is Magnetized this random arrangement of the molecule’s changes and the tiny uneligned and random molecular magnets become “lined-up” in such @ way that they produce a series magnetic arrangement. This idea of the molecular alignment of ferromagnetic materials is known as Weber's Theory and is illustrated below. Magnetic Molecule Alignment of a Piece of iron and a Magnet Sef " es wo wo Figure (a) Unmagnetised material. Figure (b) Magnetised material ~ domain lined up in series Prepared by Omondi Ferdinand — 2 Electrical Trainer - Seme TVCUNIT: ELECTRICAL PRINCIPLES | TOPIC 4: MAGNETISM AND ELECTROMAGNETISM If a compassis placed in the magnetic field in various positions, the direction of the lines of flux may be determined by noting the direction of the compass pointer. The direction of a magnetic field at any point is taken as that in which the north-seeking pole of a compass needle points when suspended in the field. The direction of a line of flux is from the north pole to the south pole on the outside of the magnet and is then assumed to continue through the magnet back to the point at which it emerged at the north pole. Thus, such lines of flux always form complete closed loops or paths, they never intersect and always have a definite direction, > N s a? @ co) Figure 2 The laws of magnetic attraction and repulsion The laws of magnetic attraction and repulsion can be demonstrated by using two bar magnets, 1. Unlike Poles Attracts In Fig. 2 (a), with unlike poles adjacent, attraction takes place. Lines of flux are imagined to contract and the magnets try to pull together. The magnetic field is strongest in between the two magnets, shown by the lines of flux being close together. Ss 2. Like Poles Repels ee In Fig. 2 (b), with similar poles adjacent (i.e., two north oles), repulsion occurs, ie., the two north poles try to push each other apart, since magnetic fuxlins running side by siden the same direction repel Oo The Properties of The Magnets @ ‘Magnet always attracts magnetic materials. ‘Magnet always tries to align along north and south. ‘Similar magnetic poles repulse each other. If a magnet is heated, or hammered the magnet can lose its magnetism. If a magnet is divided into many pieces each of the pieces behaves as a complete magnet MAGNETIC QUANTITIES AND THEIR UNITS 1, Magnetic Flux and Flux Density © Magnetic flux is the amount of magnetic field (or the number of lines of force) produced by a magnetic source. The symbol for magnetic flux is © (Greek letter ‘phi’). The unit of magnetic flux is the weber, Wb. © Magnetic flux density is the amount of flux passing through a defined area that is perpendicular to the direction of the flux: magnetic flux area * The symbol for magnetic flux density is 8. The unit of magnetic flux density is the tesla, T, where 1 T= 1Wb/m2. Hence; Magnetic flux density B esa where A(m?) is the area Prepared by Omondi Ferdinand — 4 Electrical Trainer ~ Seme TVC