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Unit 51: Electrical Technology The Properties of Conductors Course Aims • NDGTA At the end of this course the learner will be able to describe… 1. Describe the electrical properties of conductors: conductivity; resistivity 2. Describe the mechanical properties of conductors: tensile strength; rigidity 3. Compare and contrast different conducting materials 4. Describe the conducting properties of liquids and gases So what is a Conductor? • • • NDGTA A conductor is a materials that contains moveable electrical charges. In a metal such as copper or aluminium these moveable electrical particles (charges) are electrons Materials that don’t allow electrons to flow freely are termed insulators Conductivity v Resistivity • • • • NDGTA Conductivity (σ) is a parameter that distinguishes the physical character of a material with respect to its ability to allow electrons to ‘flow’ i.e. the greater the conductivity the easier the electrons will flow. Conversely another way of looking at this is to consider how much resistance a material has with respect to a flow of electrons. This physical property is termed resistivity (ρ). Thus the greater the resistivity the harder it is for the electrons to flow. Thus conductivity and resistivity are ‘two sides of the same coin’. Thus σ = 1/ρ Conductivity v Resistivity • • • NDGTA For a given material, the resistance (R) that that material offer to the flow of electrons (i.e. an electric current) depends upon… – The length of the material (l) – The cross sectional area of the material (A) Thus R is proportional to the length (l) of the material i.e. the longer the material the more the electrons will have to ‘battle’ to get through the material R is inversely proportional to the area (A) of the material i.e. the smaller the area the harder it will be for the electrons to ‘squeeze’ into the area Conductivity v Resistivity • • • • • • NDGTA Thus R is proportional to l / A In mathematics we can replace a proportionality sign with an equal sign and a constant, thus… R = ρl/A ρ the constant of proportionality is known as the resistivity Rearranging give ρ = RA/l ρ is measured in Ω.m Conductivity v Resistivity NDGTA • • σ = 1/ρ Thus σ = l/RA • • • • • • The unit of conductivity is 1/(Ω.m) i.e. S/m or S.m-1 S (siemens) is the inverse of Ω (resistance) This resistance (R) is measured in Ohms (Ω) Conductance (G) is measured in siemens (S) Thus R = 1/G Thus G = σA/l Problems NDGTA • Given the conductivity of silver is 6.30 x 107 S.m-1 at 200C what is the resistance of the conductor whose length is 3 mm and diameter 0.5 mm? • Given the resistivity of lithium is 9.28 x 10-8 Ω.m at 200C what is the conductance of a length of material is 3 cm long with a cross sectional area of 5 mm x 12mm? Linear Thermal Expansion NDGTA • The linear thermal expansion coefficient relates the change in a material's linear dimensions to a change in temperature. It is the fractional change in length per degree of temperature change. Ignoring pressure, we may write: – αL=(1/L)(dL/dT) • where L is the linear dimension (e.g. length) and dL/dT is the rate of change of that linear dimension per unit change in temperature. Linear Thermal Expansion NDGTA • The change in the linear dimension can be estimated to be: – ΔL/L = αLΔT • This equation works well as long as the linear expansion coefficient does not change much over the change in temperature. If it does, the equation must be integrated. • Thus if lo is the initial length and l is the length recorded after the change in temperature then ΔL = l – lo Linear Thermal Expansion NDGTA • Thus ΔL/L = (l – lo)/lo • Thus ΔL/L = αLΔT becomes…(l – lo)/lo = αLT where T is the difference in temperature • Rearranging gives l = lo(1+αLT) • Given the coefficient of linear expansion of aluminium is 23 x 10-6 (/oC) at 20oC calculate the increase in the length of the 50 m length of the material used as a overhead power line if the heat radiated from the sun causes the daytime temperature to rise to 24oC. If the cross sectional area of the power line is 5 cm and its resistivity is 2.82 x 10-8 Ω.m calculate its conductance The effect of temperature on conductance NDGTA • In general when the temperature of a conductor increases the resistance increases. • If the resistance of a conductor increases then its conductance decreases. • (Note please don’t confuse thermal conductivity with electrical conductivity – thermal conductivity (W/(m.K) – watts per metre-Kelvin) is a measure of the ‘flow’ of heat through a conductor!) NDGTA Tensile Strength NDGTA • Ultimate tensile strength (UTS), is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract. • Tensile strength is the opposite of compressive strength • The UTS is usually found by performing a tensile test and recording the stress versus strain; the highest point of the stress-strain curve is the UTS. Tensile Strength Force pulling material apart ‘necking’ NDGTA Force pulling material apart Tensile Strength NDGTA It is an intensive property; therefore its value does not depend on the size of the test specimen. However, it is dependent on other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material. • Tensile strengths are rarely used in the design of ductile members, but they are important in brittle members. They are tabulated for common materials such as alloys, composite materials, ceramics, plastics, and wood Tensile Strength NDGTA Tensile strength is defined as a stress, which is measured as force per unit area. For some nonhomogeneous materials (or for assembled components) it can be reported just as a force or as a force per unit width. In the SI system, the unit is pascal (Pa) or, equivalently, newtons per square metre (N/m²). The customary unit is pounds-force per square inch (lbf/in² or psi), or kilo-pounds per square inch (ksi), which is equal to 1000 psi; kilopounds per square inch are commonly used for convenience when measuring tensile strengths. Tensile Strength Convert 14.5 psi to Pa. (N.m-2) NDGTA Tensile Strength NDGTA Tensile Strength • • • NDGTA Many materials display linear elastic behavior, defined by a linear stress-strain relationship, as shown in the figure up to point 2, in which deformations are completely recoverable upon removal of the load; that is, a specimen loaded elastically in tension will elongate, but will return to its original shape and size when unloaded. Beyond this linear region, for ductile materials, such as steel, deformations are plastic. A plastically deformed specimen will not return to its original size and shape when unloaded. Note that there will be elastic recovery of a portion of the deformation. For many applications, plastic deformation is unacceptable, and is used as the design limitation. After the yield point, ductile metals will undergo a period of strain hardening, in which the stress increases again with increasing strain, and they begin to neck, as the cross-sectional area of the specimen decreases due to plastic flow. In a sufficiently ductile material, when necking becomes substantial, it causes a reversal of the engineering stress-strain curve (curve A); this is because the engineering stress is calculated assuming the original cross-sectional area before necking. The reversal point is the maximum stress on the engineering stress-strain curve, and the engineering stress coordinate of this point is the tensile ultimate strength, given by point 1. The UTS is not used in the design of ductile static members because design practices dictate the use of the yield stress. It is, however, used for quality control, because of the ease of testing. It is also used to roughly determine material types for unknown samples. Conduction in Liquids NDGTA • An electrolyte is any substance containing free ions that make the substance electrically conductive • The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible • Commonly, electrolytes are solutions of acids, bases or salts. Furthermore, some gases may act as electrolytes under conditions of high temperature or low pressure. Conduction in Liquids NDGTA • Electrolyte solutions are normally formed when a salt is placed into a solvent such as water and the individual components dissociate due to the thermodynamic interactions between solvent and solute molecules, in a process called solvation. For example, when table salt, NaCl, is placed in water, the salt (a solid) dissolves into its component ions, according to the dissociation reaction – NaCl(s) → Na+(aq) + Cl−(aq) Electrolysis NDGTA • Video: Electric Circuit Experiments: Water as a Conductor | eHow.com Electrolysis - History • 1800 • 1807 • 1875 - • 1886 • 1886 • 1890 - NDGTA William Nicolson and Johann Ritter decomposed water into hydrogen and oxygen. Potassium, sodium, barium, calcuim and magnesium were discovered by Sir Humphrey Davy using electrolysis. Paul Emile Lecoq de Boisbaudran discovered gallium using electrolysis. Fluorine was discovered by Henri Moissan using electrolysis. Hall-Heroult process developed for making aluminium Castner-Kellner process Castner-Kellner process developed for making sodium hydroxide Source: Wikipedia Electrolysis NDGTA • Electrolysis is the passage of a direct electric current through an ionic substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at the electrodes and separation of materials. • The main components required to achieve electrolysis are : – An electrolyte : a substance containing free ions which are the carriers of electric current in the electrolyte. If the ions are not mobile, as in a solid salt then electrolysis cannot occur. – A direct current (DC) supply : provides the energy necessary to create or discharge the ions in the electrolyte. Electric current is carried by electrons in the external circuit. – Two electrodes : an electrical conductor which provides the physical interface between the electrical circuit providing the energy and the electrolyte Laws of Electrolysis NDGTA • Faraday's 1st Law of Electrolysis - The mass of a substance altered at an electrode during electrolysis is directly proportional to the quantity of electricity transferred at that electrode. Quantity of electricity refers to the quantity of electric charge, typically measured in coulomb • Faraday's 2nd Law of Electrolysis - For a given quantity of electricity (electric charge), the mass of an elemental material altered at an electrode is directly proportional to the element's equivalent weight. The equivalent weight of a substance is its molar mass divided by an integer that depends on the reaction undergone by the material Laws of Electrolysis NDGTA • Faraday 1st law can be summarized by – • where: – m is the mass of the substance liberated at an electrode in grams – Q is the total electric charge passed through the substance – F = 96,487 C mol−1 is the Faraday Constant – M is the molar mass of the substance – z is the valency number of ions of the substance (electrons transferred per ion). • Note 1: M/z is the same as the equivalent weight of the substance altered. • Note 2: m is proportional to Q Laws of Electrolysis NDGTA • For Faraday's second law, Q, F, and z are constants, so that the larger the value of M/z (equivalent weight) the larger m will be. • In the simple case of constant-current electrolysis, Q = It leading to Application of Electrical Conduction in Liquids 1. Electroplating of metals. 2. Electro-refining of metals. 3. Extraction of metals or Electrometallurgy. 4. Battery NDGTA Electroplating of Metals NDGTA Electroplating is a process whereby a thin coating of desired material is applied on a required material. This is mostly done on stainless steel to prevent rusting, or on some decorative items, so that they look attractive. On stainless steel, generally nickel-chromium plating is done. On decorative items, such as spoons, plates, jewelry items, silver, gold or other plating is done. Electroplating is cheap and cost effective. It enhances the life of the object and makes it look better in appearance. Electroplating of Metals • • • • • NDGTA First the item to be electroplated is smoothened and cleaned thoroughly. It should not have any oily or dirt marks on it. An electrolyte is selected whose ions are required to be deposited on the item. Direct current is preferred to alternating current, as alternating current may result in non-smooth deposit. The item to be electroplated forms the anode or cathode of the electrolytic cell. This is the drawback of the electroplating process. The item has to be electrically conducting, or has to be made electrically conducting. For a smooth coating, the electrolytic process has to be optimized for time, temperature and current in the cell. Extraction of Metals (Electro-metallury) • NDGTA Extraction of metals by the process of electrolysis is known as electro-metallurgy. This process is used in case highly reactive metals such as sodium. An ore containing sodium is used in a molten form. This forms the electrolyte. Anode and cathodes are generally carbon rods or steel. The Na atoms get attracted to the cathode of the cell and then the entire cathode with its coating is stored for further use. Battery • • NDGTA All batteries that we come across in our day to day use, including car batteries, dry cells used in torches, calculators, hand-sets, etc. are all examples of an electrolytic cell. But in this case the reverse of an actual electrolytic process is being used. The chemicals inside the cells produce current (and voltage) which is utilized. In a car battery, for example, two grids are used as anode (Pb) and cathode (PbO2). The solution is H2SO4 of generally about 6 M in concentration. Conduction in a Gas (or Plasma) • • • NDGTA In air and other ordinary gases below the breakdown field, the dominant source of electrical conduction is via a relatively small number of mobile ions produced by radioactive gases, ultraviolet light, or cosmic rays. Since the electrical conductivity is low, gases are dielectrics or insulators. However, once the applied electric field approaches the breakdown value, free electrons become sufficiently accelerated by the electric field to create additional free electrons by colliding, and ionising, neutral gas atoms or molecules in a process called avalanche breakdown. The breakdown process forms a plasma that contains a significant number of mobile electrons and positive ions, causing it to behave as an electrical conductor. In the process, it forms a light emitting conductive path, such as a spark, arc or lightning. Plasma • • • NDGTA Plasma is the state of matter where some of the electrons in a gas are stripped or "ionized" from their molecules or atoms. A plasma can be formed by high temperature, or by application of a high electric or alternating magnetic field. Due to their lower mass, the electrons in a plasma accelerate more quickly in response to an electric field than the heavier positive ions, and hence carry the bulk of the current. Applications of Electrical Conduction in Gases • NDGTA Electrical conduction in gases have a few but significant commercial and scientific applications. These include… – – …thyratrons, gaseous rectifiers, ignitrons, glow tubes, and gas-filled phototubes. These tubes are used in power supplies, control circuits, pulse production, voltage regulators, and heavy-duty applications such as welders. …In addition, there are gaseous conduction devices widely used in research problems. Some of these are ion sources for mass spectrometers and nuclear accelerators, ionisation vacuum gauges, radiation detection and measurement instruments, and thermonuclear devices for the production of power