Electrical and Magnetic Properties PDF

EERING ENGIN MATERALAS ENGG520  References Engineering Materials Technology W. Bolton The Science and Engineering of Materials Donald R. Askeland Foundation of Material science and engineering  Electrical properties: The prime objective of thi...

EERING ENGIN MATERALAS ENGG520  References Engineering Materials Technology W. Bolton The Science and Engineering of Materials Donald R. Askeland Foundation of Material science and engineering  Electrical properties: The prime objective of this lesson is to explore the electrical properties of materials, that is, their responses to an applied electric field. Electricity is concerned with the movement of charged particles in electrical circuits. Therefore in order to gain a basic understanding of electricity we must first understand what a charged particle is. We also need to know about the electrical properties of the materials that are used to make electrical circuits and this requires a general understanding of the structure of these materials. Electrical properties Electrical properties  Electrical resistance and Conductance  The resistance is a measurement how hard it is for an electrical current to flow through the component. In the quantitative sense, the resistance between two points can be defined as the voltage difference that is required to take a unit current across the defined two points. The resistance of an object is defined as the ratio of the voltage across the object to the current flowing through it.  resistance in a conductor depends on the amount of free electrons in the medium  The resistance is measured in Ohms  The conductance of a component is a measurement of how easily a current can flow through the component  The resistance of a component depends on various things. The length of the conductor, the area of the conductor, and the material of the conductor 1 G R  Resistivity and Conductivity  Resistivity is how hard it is for an electrical current to flow through specific material  Resistivity is an intrinsic property that quantifies how strongly a given material opposes the flow of electric current.  Conductivity is how easily it is for an electrical current to flow through specific material  It is an intrinsic property that quantifies how strongly a given material conduct and allow the flow of electric current.  The conductivity of a material can be defined as the conductance of a block having unit dimensions made out of the material. The conductivity of a material is the inverse of the resistivity. 1    Resistivity and Conductivity  L 1 Conductanc R e G A  1 R Resistance  L R  1 A  Conductivity  V (m)  1  R Resistivity I m L R  A  Resistivity and Conductivity  Like balls in a Newton's cradle, electrons in a metal quickly transfer energy from one terminal to another, despite their own negligible movement  Resistivity and Conductivity  A metal consists of a lattice of atoms, Like balls in a Newton's each with an outer shell of electrons cradle, electrons in a that freely dissociate from their parent metal quickly transfer atoms and travel through the lattice. energy from one terminal to another,  When an electrical potential difference despite their own (a voltage) is applied across the negligible movement. metal, the resulting electric field causes electrons to drift towards the positive terminal. The mechanism is similar to transfer of momentum of balls in a Newton's cradle.  Resistivity and Conductivity  Most metals have resistance. In simpler models, this can be explained by replacing electrons and the crystal lattice by a wave-like structure each.  When the electron wave travels through the lattice the waves interfere, which causes resistance.  The more regular the lattice is the less disturbance happens and thus resistance lowers.  The amount of resistance is thus caused by mainly two factors. Firstly it is caused by the temperature and thus speed of vibration of the crystal lattice. The temperature causes irregularities in the lattice. Secondly the impurity of the metal is relevant as different ions cause irregularities too  Resistivity and Conductivity  The amount of resistance is thus caused by mainly two factors:  Firstly it is caused by the temperature and thus speed of vibration of the crystal lattice. The temperature causes irregularities in the lattice.  Secondly the impurity of the metal is relevant as different ions cause irregularities too  Resistivity and Conductivity  The larger the cross-sectional area of the conductor, the more electrons per unit length are available to carry the current.  As a result, the resistance is lower in larger cross- section conductors. The number of scattering events encountered by an electron passing through a material is proportional to the length of the conductor.  The longer the conductor, therefore, the higher the resistance. Different materials also affect the resistance. L L R R  A A  The difference between Conductance and Conductivity  Conductance is a property of the component but conductivity is a property of the material.  Conductance depends on the dimensions of the conductor, but conductivity does not depend on the dimensions.   Difference Between Resistance and Resistivity  Resistance and resistivity are properties of conductors where resistance is an extrinsic property whereas resistivity is an intrinsic property  This means that resistivity of a conductor is always same and is independent upon its length or size  Resistance and resistivity of a material are related to each other through an equation which is as follows  Resistivity = resistance X area of cross-section/length RA   L  The Ohm’s law is the single most important law when the topic resistance is discussed.  It states that for a given temperature, the ratio of voltage across two points, to the current passing through those points, is constant. This constant is known as the resistance between those two points. V R I Conductivity The electrical resistivity ρ is a measure of the electrical resistance of material, being defined by R= resistance RA   A= cross-sectional area L L= Length The unit of resistivity is ( .m ) An insulator such as ceramic will have a very high resitivity typically in order of 1016 .m Or higher. An electric conductor will have a very low resistivity, typically of the order of 10-8 .m  The electrical conductance G of a length of material is the reciprocal of its resistance & has a unit of   1 1 G R  This unit is given a special name, the Siemens). The electrical conductivity   is the reciprocal of the resistivity 1 L LG     R. A A The unit of conductivity is thus   1 m-1 or S/m. Since the conductivity is the reciprocal of the resistivity, an electrical insulation will have a very low conductivity of order of 10-10 S/m. While an electrical conductor will have a very high conductivity of the order 10>8 S/m.  Conductance 1 G  Conductance R Resistance 1 V   Conductivity (m) 1  R Resistivity m I L R  A Ex: Using the valve of conductivity given in the table. Determine electrical conductance of a 2 m length of ni-chrome wire at 200c if it has a cross- section area of 1mm2. (Take electrical conductivity as 0.9 x10 6 (m)  1 L 1  With the conductance G Than RA, R LG  So A, 6 6 A 1.1x10 x10 1 G  0.45  L 2 L R  A Ex: 2 m Ni –Chrome wire, 4 mm in diameter has a conductivity of 0.9 x10 6 (m)  1 Determine electrical conductance, the electrical resistance, and resistivity. Resistivity: 1  Than  L 1  With the conductance G So RA, R LG  A, A 1.1x106 xArea G  0.55 S L 2  Calculate the expected conductance of a circular aluminium cable of diameter 16 mm and length 10 km. Take the resistivity of aluminium to be 2.8 × 10-8 Ω.m.  A  (d ) 2 1 4 G   (16) 2 2 x10  4 m 2 R 4 L R  A 1 G 0.714  1 1.4 10,000 8 R 2.8 x10 -4 1.4 2x10 What are the factors considered for the selection of cables  Cable size (length and diameter)  Cable operation (Voltage and current)  Environment.  Nature of material Environment  Nature of material  Aluminium conducts less well compared with copper - for a given size of cable copper will conduct better; but because aluminium actually has a better weight-to-conductivity ratio, aluminium wire with the same conductivity as copper wire would be physically thicker, but would still work out to be lighter, and probably cheaper too.  Due to this, many overhead power lines are made of aluminium - the lower weight is a benefit, and physical size isn't an issue.  Nature of material Advantages of Aluminum Wiring  Due to its lightweight nature, aluminum is fairly malleable and easy to work with. The lightweight nature of aluminum is beneficial when wiring is to be done over long distances as it makes the job less rigorous. Aluminum also reduces corona, an electric discharge associated with high power transmissions.  When it comes to cost, aluminum is more affordable than copper wire. With aluminum, you will require about half the amount you would need if copper wire were used instead. When extensive wiring needs to be done in your home, the difference between the two materials can give you significant savings Density Al = 2712 (kg/m3) Cu= 8940 (kg/m3) Fe=7850 (kg/m3)  Nature of material  Disadvantages of Aluminum Wiring  If not installed properly, aluminum wiring can raise the risk of house fires. When aluminum wire warms, it expands and when it cools, it contracts. The tightness of the wiring decreases with each progressive warm-cool cycle experienced, creating the phenomenon known as “cold creep." These loose connections can cause sparking which may result in fires. Wires progressively heat up and could even melt surrounding insulation and fixtures, triggering a fire.  Aluminum wires require higher maintenance than copper wiring. This is partly due to the high wear and tear rate as well as greater risk of fire. For example, aluminum also undergoes corrosion when it encounters certain metal compounds, and this oxidization gives the connection increased resistance. This adds to overall home maintenance costs.  Nature of material  Advantages of Copper Wiring  Copper has one of the highest electrical conductivity rates among metals, which allows it to be soldered with ease.  It also makes it possible for smaller conductors to be used to transmit power loads. Smaller conductors are easier to transport and install, and they cost less, which helps manage wiring costs. Copper doesn’t undergo the same extreme expansion and contraction cycles as aluminum so it is a more stable material to use.  Due to its high ductile properties, copper can be formed into very fine wire, making it more versatile.  Copper has a high tensile strength as well, so it can undergo extreme stress but show minimal signs of wear and tear. This makes the wiring more durable than aluminum. Due to its great resilience, high durability, low maintenance, and high performance, copper wiring also adds to home value.  Nature of material  Disadvantages of Copper Wiring  Copper wire costs much more than aluminum, so when extensive wiring is necessary, the overall costs may prove to be prohibitive. Copper is also heavier which can add to the difficulty in wiring. More supports are required to secure the heavier wire in place, which also adds to overall cost.  Nature of material  Fire Hazards  Homes and mobile homes built between 1965 and 1973 primarily used aluminum for conducting electricity because it was light and relatively inexpensive when compared to the rising cost of copper wire during that period. Although aluminum deteriorates quicker than copper does, and exhibits more defects over time, the real problems occur from the connectors and switches that connect electrical circuits together. Weak or deteriorating connectors -- and constantly or heavily loaded circuits -- usually contribute to the electrical fires associated with aluminum wire usage.  Nature of material Gold Gold is a worse conductor than copper and silver (and more expensive), but resists oxidation much better. As a result gold is used to plate connectors, whose connection may degrade if the surface is oxidized. Gold wire is also used in chips to bond the interconnects between the silicon and the chip package. Gold is used as it resists oxidation when bonding. Copper can also be used, but would need to be done in an inert atmosphere like nitrogen.  Nature of material Silver As you mentioned, silver has a higher conductivity than copper, but isn't used widely due to its cost. But it does have a few niche uses where extremely low resistance is desired, such as in sensitive scientific instruments; cryogenics (where it's desirable to have minimal heat generated in wires); and also in electrical contacts in switches, where the softness of silver and relatively good- conducting silver-oxide makes it a good choice for metal-to- metal connections.  Nature of material Tungsten Tungsten is used when you actually want the wire to have resistance, and also have it not melt even when white-hot, as is the case in incandescent lamps.  Nature of material Alloys There are many different alloys used in electrical conductors. Here are some examples: - Nickel-chrome (nichrome) wire, like tungsten, can survive high temperatures, and the higher resistance is desirable. Being cheaper than tungsten, is used in heater wires, where the wire doesn't need to get white-hot. - Solder, which is an alloy of tin and lead, or other mixes of metals in the case of lead-free solder, is used for bonding electrical components to copper pads on PCBs. The low melting temperature of tin-lead makes it suitable for this task. - Cryogenic wire is often phosphor bronze (copper, tin, phosphorus), where the resistance of this alloy doesn't change much at very low temperatures..  Cables and wires Insulations  The selection of cable insulation and finish covering is normally based on the type of installation, ambient operating temperature, service conditions, type of load served, and other criteria as applicable. In many installations unusual conditions may be prevalent, such as corrosive atmosphere, high ambient temperature, insect and rodent hazard, presence of oil and solvents, presence of ozone, and extreme cold.  Cable installation  Cables can be used for outdoor or indoor  installations depending upon the distribution system and the load served.  A good understanding of local conditions is essential to assure that the selected cable system will operate satisfactorily.  Many times cable insulation is damaged or weakened during installation by applying the incorrect pulling tensions. Magnetic properties  Magnetic properties  Magnetism is one of the aspect of Electromagnetic interactions.  The motion of charged particle creates a magnetic field.  Permanent magnets display the strongest magnetism due to unpaired electrons called Ferromagnetism, which is easily observable. Factors Determining Magnetic Properties  Electron orientation  Electrons can behave as tiny magnets, each with north (N) and south (S) poles. When an atom's electrons are lined up in the same orientation, with most having their N pole facing one direction, the atom becomes like a magnet, with N and S poles. It is also possible for the electrons to be in various directions, making the atom not magnetic Factors Determining Magnetic Properties  Moving electrons create magnetic field  The reason that electrons can behave like tiny magnets is the fact that when electrons move, they create a magnetic field. Placing a compass near a wire carrying DC electrical current can show that a magnetic field is created due to the electrons moving through the wire.  A magnetic field is also created when electrons rotate around a nucleus and when they spin while in orbit. Factors Determining Magnetic Properties  Spinning electrons  Electrons have a property called spin. This spinning creates a magnetic field with N and S poles, just as the spinning Earth has magnetic poles. Note that the N pole on an electron is really a North-seeking pole, just as in a magnet.  If electrons in the shells of an atom spin in the same direction, the atom will exhibit a magnetic field and will respond to the forces of a magnet. If half of the electrons spin one way and the rest spin the other way, they will neutralize each other and the material will not be affected by a magnetic field  Magnetic properties  Strong and weak electron alignments  Atoms such as iron have most of their electrons aligned in the same direction. Thus, iron or nickel would be attracted to a magnet.  Aluminum only has a few electrons aligned, and thus it is only weakly magnetic.  An element with half of its electrons oriented one way would not be attracted to a magnet  Magnetic properties  Atomic and molecular alignment  Although some atoms may be highly magnetic, they really need to be aligned to make a material magnetic. If magnetic atoms are facing different directions, their fields will cancel out each other  Magnetic properties  Domains  The final factor in a material being magnetic concern the orientation of its domains in a solid. A group of atoms in a metal may become aligned, but the various groups may be misaligned.  These groups are called domains.  It is necessary to line up many of the domains in a material like iron in order for it to become a magnet.  Magnetic properties  Domains Magnetic material with domains misaligned Aligned domains makes material highly magnetic  Magnetic properties  Domains  Diamagnetism  Diamagnetism refers to materials that are not affected by a magnetic field.  if the all electrons are paired which means that in an orbital an electron spins clockwise and another spins anticlockwise so the net magnetic moment would be zero.  repelled by both sides of the magnet kept near  Paramagnetism  Paramagnetism refers to materials like aluminum or platinum which become magnetized in a magnetic field but their magnetism disappears when the field is removed.  Paramagnetism is temporary magnetism caused due to arrangement of unpaired electrons under the influence of another magnet but revert back to normal alignment once the magnet is removed, or electricity switched off.  Unlike ferromagnetism it is weak, displayed by aluminium, platinum etc.   Ferromagnetism  Ferromagnetism refers to materials (such as iron and nickel) that can retain their magnetic properties when the magnetic field is removed.  Ferro is the Latin word for iron (this is the reason behind the atomic symbol of iron- Fe), a material which displays strong magnetic properties. Electrons produce a small magnetic field as they spin and orbit the nucleus of an atom. For many atoms, the combinations of electrons in their orbits cancel each other out. In ferromagnetic materials, however, the electron fields in the atoms do not cancel out, so they exhibit a long- range ordering phenomenon at the atomic level, which causes unpaired electron spins to line up parallel with each other in a region called a domain.  Hard Magnetic Materials  Materials which retain their magnetism and are difficult to demagnetize are called hard magnetic materials.  These materials retain their magnetism even after the removal of the applied magnetic field. Hence these materials are used for making permanent magnets. In permanent magnets the movement of the domain wall is prevented.  They are prepared by heating the magnetic materials to the required temperature and then quenching them.  Impurities increase the strength of hard magnetic materials.  Soft Magnetic Materials  Soft magnetic materials are easy to magnetize and demagnetize.  These materials are used for making temporary magnets. The domain wall movement is easy. Hence they are easy to magnetize. By annealing the cold worked material, the dislocation density is reduced and the domain wall movement is made easier. Soft magnetic materials should not possess any void and its structure should be homogeneous so that the materials are not affected by impurities.  Relation between magnetic field and Electrical field  A moving charge always has both a magnetic and an electric field, and that’s precisely the reason why they are associated with each other. They are two different fields with nearly the same characteristics. Therefore, they are inter-related in a field called the electromagnetic field. In this field, the electric field and the magnetic field move at right angles to each other. However, they are not dependant on each other. They may also exist independently. Without the electric field, the magnetic field exists in permanent magnets and electric fields exist in the form of static electricity, in absence of the magnetic field  Relation between magnetic field and Electrical field  The area around a magnet within which magnetic force is exerted, is called a magnetic field. It is produced by moving electric charges. The presence and strength of a magnetic field is denoted by “magnetic flux lines”. The direction of the magnetic field is also indicated by these lines. The closer the lines, the stronger the magnetic field and vice versa. When iron particles are placed over a magnet, the flux lines can be clearly seen. Magnetic fields also generate power in particles which come in contact with it. Electric fields are generated around particles that bear electric charge. Positive charges are drawn towards it, while negative charges are repelled.  Hard Magnetic Materials  They have large hysteresis loss due to large hysteresis loop area.  Susceptibility and permeability are low  Coercivity and retentivity values are large. Magnetic energy stored is high  The eddy current loss is high.  Soft Magnetic Materials  They have low hysteresis loss due to small hysteresis area.  Susceptibility and permeability are high.  Coercivity and retentivity values are less  Since they have low retentivity and coercivity, they are not used for making permanent magnets  Magnetic energy stored is less.  The eddy current loss is less because of high resistivity.