SIAEC 1 Electron Theory PDF

UNCONTROLLED COPY - FOR TRAINING PURPOSE ONLY Revision Service Will Not Be Provided To The Holder TRAINING MAN UAL For Traini ng Purpose Only ELECTRON THEORY STRUCTURE OF MATTER Matter is defined as anything that occupies space and has mass. Thus,...

UNCONTROLLED COPY - FOR TRAINING PURPOSE ONLY Revision Service Will Not Be Provided To The Holder TRAINING MAN UAL For Traini ng Purpose Only ELECTRON THEORY STRUCTURE OF MATTER Matter is defined as anything that occupies space and has mass. Thus, the objects in the environment that we live in, such as air and water, are all various forms of matter. Even the aircraft that you maintain are also a form of matter. The law of matter states that matter cannot be created or destroyed. However, the characteristics of the matter can be changed. All matter consists of elementary particles, e.g. electrons, protons and neutrons. The forces that bind these particles together to create matter are the same forces that create electrical power. Every aircraft generator, alternator and battery and virtually all electrical components function according to the electron theory. The electron theory describes specifically the internal molecular forces of matter as they pertain to electrical power. The electron theory is therefore a vital foundation upon which to build an understanding of electricity and electronics. ATOMS The concept and meaning of an atom arise on the foundation of ancient Greek natural philosophers who started asking interesting questions as: What is stuff composed of? What is the structure of material objects? Is there a basic unit from which all objects are made? As early as 400 B.C., some Greek philosophers believed that mater could not be continuously broken down and divided indefinitely. Thus, there must be an existence of a basic unit or form that was indivisible and foundational to the bigger structure. They named these basic forms as atomos (Atomos in Greek means indivisible). This indivisible building block of which all matter was composed became known as the atom. Atoms are the smallest particles of an element that can exist, either alone or in combination with other atoms. An element is a single substance that cannot be separated into different substances except by nuclear disintegration. Common elements include iron, oxygen, aluminium, hydrogen, copper, lead, gold, silver, which are all listed in the periodic table. The smallest particle of any of these elements would still have that element’s properties. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 1 TRAINING MAN UAL For Traini ng Purpose Only PROTONS AND NEUTRONS Each atom consists of a nucleus containing positively charged protons and electrically neutral neutrons. These protons and neutrons are not removable easily. It would require some form of high-energy nuclear occurrence to disturb the nucleus and subsequently dislodge its positively charged protons. Structure of an Atom ELECTRONS Surrounding the vast space outside the nucleus and travelling at high speed in orbits are electrons, each of which is negatively charged and weakly bounded to the atom. Each electron weighs about 1/1845 as much as a proton. The charges carried by the electrons and protons are equal but opposite in nature. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 2 TRAINING MAN UAL For Traini ng Purpose Only EXAMPLES The hydrogen atom is the simplest atom and has one electron and one proton while a helium atom has two electrons, two neutrons and two protons. Hydrogen Atom Helium Atom A lithium atom has three protons, three electrons and three neutrons. Lithium Atom B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 3 TRAINING MAN UAL For Traini ng Purpose Only MOLECULES AND COMPOUNDS When atoms bond together, they form a molecule. However, there are some molecules that could exist as single atoms. Two examples in aircraft maintenance are helium and argon. A molecule is the smallest particle; in which any compound can be divided and still retain its identity. If the molecule is further divided, atoms are formed. A compound is a chemical combination of two or more different elements, and the smallest particle of a compound is a molecule. A water molecule (H2O) contains two hydrogen atoms and one oxygen atom while a carbon dioxide molecule contains a carbon atom and two oxygen atoms. Water Molecules Different compounds have distinctly different properties. Materials are composed of atoms and molecules of these elements and compounds, thus providing different materials with different electrical properties. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 4 TRAINING MAN UAL For Traini ng Purpose Only IONS Positive electrical forces outside an atom tend to attract electrons from an atom’s outer ring, which results in an unbalanced electrostatic condition. The atom thus becomes charged and charged atoms are called ions. If an atom has an excess of electrons, it is said to be negatively charged and is called a negative ion. Conversely, if it has a shortage of electrons, it will be positively charged and is called a positive ion. Charged molecules are also called ions. Note that protons remain within the nucleus while electrons are added or removed from an atom, thus resulting in a positive ion or a negative ion. Examples of Molecule Ions B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 5 TRAINING MAN UAL For Traini ng Purpose Only ATOMIC STRUCTURE AND FREE ELECTRONS The path which an electron travels around the nucleus of an atom describes an imaginary sphere or shell. Simpler atoms like Hydrogen and Helium only has one shell but the more complex atoms like oxygen has a number of shells. The atomic structure of oxygen is illustrated in the following figure. Oxygen Atom When an atom has more than two electrons, it must have more than one shell as the first cell can only accommodate two electrons. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 6 TRAINING MAN UAL For Traini ng Purpose Only CONDUCTORS, SEMICONDUCTORS AND INSULATORS The atomic structure of a substance determines its conductivity. An element is a conductor, semiconductor or an insulator based on the number of electrons in the valence orbit (shell) of the material’s atoms. The valence orbit of any atom is the outermost shell of the atom. The electrons in this valence orbit are known as the valence electrons. All atoms desire to have their valence orbit full of electrons. The fewer number of valence electrons in an atom, the easier it will give up those electrons. Valence Electrons and Shell The electrons that move from one atom to another are called free electrons. They moved from the outer shell of one atom to the outer shell of another atom. However, the movement of these free electrons does not always result in electrical current flow. Under a power supply, for example a battery, a potential difference is created between the two ends of a conductor and the free electrons will then move in the same direction as the electrical current, thereby producing a useful electron flow. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 7 TRAINING MAN UAL For Traini ng Purpose Only CONDUCTORS Some elements, mainly metals, allow current to flow through them easily. They are known as conductors. These elements have fewer than half of their valence electrons and tend to readily accept the moving electrons of an electric current flow. Examples of the best conductors are gold and silver as their valence orbits contain only one electron each. However, we normally use cheaper alternative materials to reduce costs and increase workability. Common conductors used are copper and aluminium. The following figure illustrates the structure of a copper conductor. It must be noted electron movement can only occur for a conductor when there is an external force in addition to the molecular forces residing in the conductor’s atoms. In the case of aircraft, these external forces are usually supplied by the battery or the generator. An electric potential applied across a conducting material will cause electrons to become detached from their atoms and move (as they are negatively charged) towards the positive potential. As an electron leaves its atom, that atom loses part of its negative charge and becomes a positively charged ion. This makes it attractive to another electron which takes the place of the one just attracted away, and so on. Thus a flow of electrons moves, atom by atom towards the positive terminal, being replaced by an excess of electrons at the negative terminal. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 8 TRAINING MAN UAL For Traini ng Purpose Only SEMICONDUCTORS Materials with exactly half of their valence electrons are semiconductors. Semiconductors have very high resistance to current flow in their pure state. However, when some electrons are added or removed, the material will offer very low resistance to electric current flow. The two most common semiconductors are germanium and silicon, with 4 valence electrons in their valence orbits. The following figure shows the structure of atomic silicon. Atomic Structure of Silicon In a semiconductor the bond between the valence electrons and their nucleus is much stronger than in a conductor, thus lesser electrons are free to move when a potential is applied. When current does flow, the chances of a collision between the electrons moving due to the electric potential, and randomly moving electrons due to heat, is much less. When the semiconductor material is heated, it frees electrons that are previously held by their atoms. These electrons are then free to add to the current flow. Although the total current flow is much less than in a conductor, the amount by which current flow increases due to heat is proportionally greater. It is thus important to keep components made from semiconductor materials, such as transistors, cool. Otherwise an effect called thermal runaway can occur. This is when an increase in temperature causes an increase in current, which in turn causes a further increase in temperature, leading to increasing current. A process that can escalate until the material passes so much current it is destroyed. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 9 TRAINING MAN UAL For Traini ng Purpose Only INSULATORS Materials that have more than half of their valence electrons are insulators. Insulators will not easily accept extra electrons. Two of the best insulators are neon and helium. Diamond is also a good insulator. Due to cost and increased workability, common insulators we used include air, plastic, fibreglass and rubber. The following figure shows the structure of Diamond. Diamond An insulator has all its electrons tightly bonded to the nucleus and so it takes very large forces of either heat or potential to dislodge them. Insulators do not normally pass current. However, under extreme conditions such as high temperatures or with very high voltages applied, some insulating materials will conduct. In these circumstances the insulating material is said to have "Broken down" and usually the structure of the material is permanently damaged. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 10 TRAINING MAN UAL For Traini ng Purpose Only In some insulators (glass for example) heating the material to a high temperature will vibrate the atoms so violently that it will shake free enough electrons for conduction to occur. Cooling the material once more stops conduction. In most insulators however, conduction in a normally insulating material, whether caused by excessive heat or by excessive voltage will permanently destroy the material. For this reason insulating materials for electrical insulators, each have a safe working limit quoted by the manufacturer using the material for both voltage and temperature. In general, atoms with four valence electrons are semiconductors; atoms with fewer than four valence electrons are conductors, while those with more than four valence electrons are insulators. A summary is presented in the following table. Conductor Semiconductor Insulator No of Valence Less than half Exactly half More than half Electrons Examples Gold, Silver, Germanium, Neon, Helium, Copper, Silicon air, plastic, Aluminium fibreglass, rubber, diamond B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 11 UNCONTROLLED COPY - FOR TRAINING PURPOSE ONLY Revision Service Will Not Be Provided To The Holder TRAINING MAN UAL For Traini ng Purpose Only STATIC ELECTRICITY AND CONDUCTION INTRODUCTION TO STATIC ELECTRICITY There are basically two types of electricity, namely current and static. In current electricity, useful work is performed when a magnetic field is created by electrons’ movement through a circuit or heat is generated through a resistance. Static electricity refers to electric charges that are at rest. A material with atoms containing equal number of electrons and protons is electrically neutral. The material is left with a static charge when the number of electrons in that material increases or decreases. An excess of electrons would result in a negatively charged material while a loss of electrons would produce a positively charged body. This excess of deficiency of electrons can be generated by friction between two dissimilar substances or by contact between a neutral body and a charged body. For example, when a glass rod is rubbed with fur, the rod becomes negatively charged. However, when it is rubbed with silk, it becomes positively charged. TRIBOELECTRIC SERIES When we rub two different materials together, which becomes positively charged and which becomes negative? Scientists have ranked materials in order of their ability to hold or give up electrons. This ranking is called the triboelectric series. A list of some common materials is shown in the following figure. Under ideal conditions, if two materials are rubbed together, materials nearer the top of the graph will tend to have a positive charge, while materials nearer the bottom of the array will tend to have a negative charge. However, it should be noted that the order will change under the influence of environmental conditions, the state of the contact surfaces, and other factors. B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 1 TRAINING MAN UAL For Traini ng Purpose Only Triboelectric Series B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 2 TRAINING MAN UAL For Traini ng Purpose Only When a non-conductor is charged by rubbing with a dissimilar material, the charge remains at the points where the friction occurs because the electrons are not able to move through the material. However, when a conductor is charged, it must be insulated from other conductors or the charge will be lost. Electrostatic Charges B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 3 TRAINING MAN UAL For Traini ng Purpose Only DISTRIBUTION OF ELECTROSTATIC CHARGES Distribution of electrostatic charges varies depending on the surface. When a body having a smooth or uniform surface is electrically charged, the charge distributes evenly over the entire surface. For a rough or irregular shaped surface, charges are concentrated at points or areas having the sharpest curvature. This property is illustrated in the static dischargers, which provide points for the dissipation of static charges into the air before a high potential builds on the aircraft control surface. STATIC DISCHARGERS Precipitation static (P-Static) charges are build-up on the aircraft structure with the air stream particle friction during flight. The charge results mainly from the high-speed impact or frictional passage of these airborne particles and the charge rate is particularly high when, for example, ice crystals precipitate out from a cold-moist atmosphere (hence the expression “precipitation static”). There are numerous protuberances on an aircraft causing distribution of the electric field of the static charge in a way that concentrates it at the tip of the protuberance, with consequent higher field intensity in the atmosphere immediately at the tip. As a result, this portion of the atmosphere could reach such excessive voltage gradients that charge leakage could start and, after ionisation, a complete breakdown could occur. Negative charges are left behind on the surfaces while positive charges flow into the air stream. In addition, charges of the electrostatic type can be induced into the aircraft when flying into electric fields created by certain types of cloud formation. Such conditions can lead up to a lightning discharge. The accumulated static is discharged continuously in a natural attempt to equalise the potentials of the charges in the atmosphere and the aircraft. On glass reinforced plastic surfaces, the reduction or removal of accumulated charges is possible by a special paint application producing a conductive surface. In a study conducted, it was reported that a 70 thousand volt threshold of a tested B707 aircraft to have been reduced to 7.8 thousand volts by use of the dischargers. B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 4 TRAINING MAN UAL For Traini ng Purpose Only To minimise radio interference, static dischargers (in the form of static wicks or null field dischargers) are mounted at trailing edges of ailerons, elevators and rudders. They provide an easier exit path for the charges into the atmosphere. A - Rod-Type Static Discharger B - Wedge-Type Discharger Mounting attachment is bonded to structure and discharger elements are quickly attached with a single screw holding them in place. Wedge-type dischargers are installed along end of stabiliser and rod- type dischargers are fitted to trailing edge of fin tip and rudder. B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 5 TRAINING MAN UAL For Traini ng Purpose Only AIRPLANE GROUNDING The primary purpose of grounding an airplane during ground operations is to bleed off static charge due to atmospheric conditions or operations such as fuelling or cleaning. As some static charge is always liable to remain on an aircraft resulting in potential difference between aircraft and the ground, this is a hazard as it can cause an electric shock to personnel entering or leaving the aircraft as well as cause spark discharge between aircraft and external ground equipment being coupled to it. In addition to the use of static ground straps, the following methods that can be used to reduce this potential difference by leaking the charge to ground are: 1. Through the nose or tail wheel tyre Tyre being impregnated with carbon (a resistance to ground of 10-50 Kilo-ohms each) 2. Static discharge wick or similar device on some older aircraft, trailed from a landing gear assembly to provide ground contact on landing Stringent precautions are also observed during aircraft refuelling as the aircraft may be charged due to the fuel flowing through the hose generating electrical potentials, as well as for the fuel tanker. To prevent such potential difference build-up resulting in spark generation and ignition of flammable vapours, provide bonding connection between aircraft and tanker as well as for the aircraft and tanker. The hose nozzle is also bonded to a point specially provided on the aircraft. During refuelling, physical contact between the hose nozzle and tank filler is always maintained. The risk of fire or explosion is thus minimised. B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 6 TRAINING MAN UAL For Traini ng Purpose Only STATIC GROUNDING POINTS (B747-400) B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 7 TRAINING MAN UAL For Traini ng Purpose Only DISTRIBUTION OF CHARGES ON A CONDUCTOR The charges reside on the outer surface of a conductor. The reason for charges to reside on a conductor surface can be explained by the repulsion of like charges, which move outwards from the middle of a conductor until the surface is reached. Faraday demonstrated this property of charged conductor by charging a butterfly net, made of conducting material before testing the interior and exterior for charges. It was observed that the outside was charged, but no charge could be detected on the inside. He then turned the same net inside out by pulling a silk thread at the corner of the net, and found that the charge now appeared on the new outside, and that there was no charge on the new interior of the net. Faraday’s “Butterfly Net” B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 8 TRAINING MAN UAL For Traini ng Purpose Only TEST OF CHARGE DISTRIBUTION The distribution of charge on a charged conductor can be tested by means of a proof-plane, which consists of a small metal disc M, at the end of a long insulating handle H, made of ebonite or glass. Proof Plane Suppose that X is a positively pear-shaped charged conductor such that the curvature of its surface varies from place to place, as illustrated in the following figure. Variation of Charge To find the charge round A, the proof-plane is placed on X so that the whole of the disc M makes contact with the conductor round this point. The charged disc is then removed and placed inside a can on the top of a gold-leaf electroscope. By touching the inside, the whole of the charge on M is transferred to the can and electroscope, and the divergence of the leaf is noted. The experiment is then repeated with the whole of the disc M of the proof-plane touching different parts of the conductor X, and in this way the variation of the charge per unit area, or surface-density of charge, of X can be roughly measured. B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 9 TRAINING MAN UAL For Traini ng Purpose Only It is found that the pointed area of the conductor has the greatest surface-density of charge; that the parts of the surface, which have a much smaller curvature than the pointed area, have a much smaller surface-density, and that the almost plane portions of the conductor have a very small surface-density. In general, the part of the surface of a conductor, which has the greatest curvature, has the greatest density of charge. A charge metal sphere has a constant surface-density, as the curvature is the same all over. ACTION AT A POINT As shown previously, the charge on a conductor concentrates at the parts of its surface having the greatest curvature. In particular, the concentration of electricity is very great at the pointed part of a conductor. In a static discharger mounted on the aircraft, the air molecules in contact with the point of the discharger gain some charge, and are repelled from the discharger by the similar charge left on the point. Other air molecules then become charged by contact and move away from the point, with the result that the air molecules carry a stream of negative electricity; thereby the charge accumulated on the surface of the aircraft is dissipated through the discharger. B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 10 TRAINING MAN UAL For Traini ng Purpose Only LAWS OF ELECTROSTATIC ATTRACTION AND REPULSION We cannot see static electricity but we can observe its effects through the use of glass rod with another different material. If a glass rod is rubbed with silk the glass becomes positively charged and silk gets negatively charged. If the glass rod is brought close to a similar charged glass rod repulsion occurs. If now ebonite is rubbed with woollen cloth ebonite becomes negatively charged. If this rod is now placed next to the charged glass rod attraction occurs. The force created between two charged bodies is called the electrostatic force, which can be repulsive or attractive, depending on the objects’ charges. In summary LIKE CHARGES REPEL UNLIKE CHARGES ATTRACT Attraction and Repulsion between Electrostatic Charges The strength of repelling and attracting forces is inversely proportional to the square of the distance between the two bodies. If the distance between the two dissimilar objects is doubled, the force of attraction is reduced to one-fourth its original value. Conversely, if the distance is reduced by half, the force between them increases by a factor of four. B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 11 TRAINING MAN UAL For Traini ng Purpose Only COULOMB’S LAW In physics, Coulomb's law is an inverse-square law indicating the magnitude and direction of electrical force that one stationary, electrically charged object of small dimensions (ideally, a point source) exerts on another. Coulomb's Law may be stated as follows: "The magnitude of the electrostatic force between two point charges is directly proportional to the magnitude of each charge and inversely proportional to the square of the distance between the charges." where (in SI units): is the magnitude of the force exerted, in newtons is the charge on one body, in coulombs is the charge on the other body, in coulombs is the distance between them in metres UNITS OF CHARGE The unit of quantity for electricity is the coulomb (C), named after Charles A. Coulomb, a French physicist who conducted many experiments with electric charges. One coulomb is the amount of electricity that will cause 0.001118 g of silver to be deposited on one electrode when it is passed through a standard silver nitrate solution. It is also defined as 6.28 x 1018 electrons, i.e. 6.28 billion billion electrons. B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 12 TRAINING MAN UAL For Traini ng Purpose Only INDUCTION Consider a negatively charged ebonite rod R near to two uncharged metal spheres A, B, suspended by silk insulating threads, and suppose A, B, are in contact with each other as shown below. Induction Under the influence of the negative electricity on R, electrons are repelled from A to B, leaving A with a surplus positive charge. Thus A and B have equal positive and negative charges. While in the neighbourhood of R, the two charges can be separated by means of the insulating threads, and their magnitudes and nature compared by a charged goldleaf electroscope. The two charges on A and B are obtained without R touching the spheres and they are hence known as induced charges. The phenomenon is known as induction. Therefore it can be summarised that When a negatively charged object is placed near object B, it will induce a positive charge on object B. If the negatively charged object touches object B then negative charges will be transferred to object B Induction by Negatively Charged Object B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 13 TRAINING MAN UAL For Traini ng Purpose Only ELECTRICITY CONDUCTION Electrical conduction is the movement of electrically charged particles through matter. An electric current can be formed due to the movement in response to an electric field. SOLIDS In solid conductors made of various metals, silver being the best, closely followed by gold, aluminium, copper to name just the most common ones, we have a large number of free electrons readily available. When electricity travels through the metal, as shown in figure, it quickly knocks these free electrons from their outer orbits and starts a domino effect, with electrons jumping from atom to atom, so that the electricity reaches the other end of the conductor at the speed of light, even though the individual electrons may have only travelled a short distance. This is called conduction current. Solid Conductor An electron leaves the negative terminal of the battery and knocks an electron out of an atom in the conductor. This free electron knocks an electron out of another atom and takes its place. This continues through the circuit until a displaced electron enters the source at the positive terminal. Electricity is conducted through solids via movement of electrons. Copper is the material most commonly used as a conductor. Brass, aluminium and carbon are also commonly used, and although silver and gold are better conductors than these they are only used in special applications because of their high cost. The descending order of conductivity is silver, copper, gold, aluminium, brass and carbon. B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 14 TRAINING MAN UAL For Traini ng Purpose Only Although not as good as most of the above mentioned metals, all metals are reasonably good conductors. Many are rarely produced specifically as conductors, but when they are part of the assemblies manufactured for some other primary purpose, they may also be used as conductors. Examples of this are the use of steel, aluminium and other metals in aircraft and car structures and engines as conducting mediums, particularly as an earth or return path. In practice it is more usual to think of resistance in conductors and components rather than their conductance. The accompanying table lists the relative resistance of some solid conducting materials. Copper, with a datum of 1 is used as a comparison for the others. Carbon is the only non-metal solid that has significance as a conductor. Although carbon has a resistance 2000 – 3000 times that of copper, its self lubricating quality makes it the most suitable material for a rubbing connection with commutators or slip rings (which are usually manufactured from copper) of generators and alternators. Relative Resistances of Some Solid Conducting Materials B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 15 TRAINING MAN UAL For Traini ng Purpose Only LIQUIDS (ELECTROLYTES) When current is passed through a liquid it creates what we call ions. This is an atom of liquid or gas, which has either gained or lost an electron. If it gains an electron, it has an overall negative charge, (more electrons than protons) and is called a negative ion. If it loses an electron, it has an overall positive charge (less electrons than protons) and is called a positive ion. In a liquid it is not so much atoms, which are ionised, but rather groups of atoms called molecules. Negatively charged ions move in one direction and positively charged ions move the opposite way. The charges they carry are given up at the appropriate electrodes. Ionic substances are made of charged particles - ions. When the ionic solid is dissolved in water the ionic lattice breaks up and the ions become free to move around in the water. When you pass electricity through the ionic solution, the ions are able to carry the electric current because of their ability to move freely. A solution conducts by means of freely moving ions. If positive and negative electrodes are placed in an electrolyte, the positive ions and negative ions will naturally be attracted to the opposite polarity electrodes. The ionic transfer of electric charge is a conduction of current. When ions reach the electrodes, depending on their polarity, they either give or receive electrons, to thus contribute to the electron transfer of charge through the solid conducting circuit external to the electrolyte. B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 16 TRAINING MAN UAL For Traini ng Purpose Only SOLUTION CONDUCTIVITY Pure Water (distilled) Non Tap Water Poor Pool or Sea Water Weak Soluble Salt Solution (NaCl) Strong Strong Acids (HCL) Strong Weak Acids Weak Strong Bases Strong Weak Bases (Ammonia) Weak Molecular Compounds (sugar solution) Non GASES AND PLASMAS Electrical conductivity in neutral gases is very low. Gases act as a dielectric or insulator until the electric field reaches a breakdown value, when the electrons are free from the atoms in an avalanche process and plasma is formed. This plasma provides mobile electrons and positive ions, acting as a conductor, which supports electric currents and forms a spark, arc or lightning. In ordinary air below the breakdown field, the dominant source of electrical conduction is via mobile ions produced by radioactive gases and cosmic rays. B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 17 TRAINING MAN UAL For Traini ng Purpose Only VACUUM Conduction of current in a vacuum is much more difficult to achieve, because there are no gas molecules which can liberate free electrons. It requires a cathode (negative electrode) to be forced to release electrons either by heating it up, (thermionic emission) or using a high voltage to strip electrons out of it, as in a thermionic valve (radio valve) or cathode ray tube of television or oscilloscope type. This stream of electrons will then pass across the tube to the anode, which has a high positive potential (25,000 volts is not uncommon in TV sets). Electrons are emitted from Cathode by thermionic emission (heating of conductor) and stream across to the plate (anode) because of electrostatic attraction. THERMIONIC EMISSION Thomas Edison discovered the principle of thermionic emission as he looked for ways to keep soot from clouding his incandescent light bulb. Edison placed a metal plate inside his bulb along with the normal filament. He left a gap, a space, between the filament and the plate. He then placed a battery in series between the plate and the filament, with the positive side toward the plate and the negative side toward the filament. This circuit is shown. When Edison connected the filament battery and allowed the filament to heat until it glowed, he discovered that the ammeter in the filament- plate circuit had deflected and remained deflected. He reasoned that an electrical current must be flowing in the circuit - EVEN ACROSS THE GAP between the filament and plate. B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 18 TRAINING MAN UAL For Traini ng Purpose Only Edison could not explain exactly what was happening. At that time, he probably knew less about what makes up an electric circuit than you do now. Because it did not eliminate the soot problem, he did little with this discovery. However, he did patent the incandescent light bulb and made it available to the scientific community. Let's analyse the circuit in figure. You probably already have a good idea of how the circuit works. The heated filament causes electrons to boil from its surface. The battery in the filament-plate circuit places a POSITIVE charge on the plate (because the plate is connected to the positive side of the battery). The electrons (negative charge) that boil from the filament are attracted to the positively charged plate. They continue through the ammeter, the battery, and back to the filament. You can see that electron flow across the space between filament and plate is actually an application of a basic law you already know - UNLIKE CHARGES ATTRACT. Remember, Edison's bulb had a vacuum so the filament would glow without burning. Also, the space between the filament and plate was relatively small. The electrons emitted from the filament did not have far to go to reach the plate. Thus, the positive charge on the plate was able to attract the negative electrons. The key to this explanation is that the electrons were floating free of the hot filament. It would have taken hundreds of volts, probably, to move electrons across the space if they had to be forcibly pulled from a cold filament. Such an action would destroy the filament and the flow would cease. The application of thermionic emission that Edison made in causing electrons to flow across the space between the filament and the plate has become known as the EDISON EFFECT. You will remember that metallic conductors contain many free electrons, which at any given instant are not bound to atoms. These free electrons are in continuous motion. The higher the temperature of the conductor, the more agitated are the free electrons, and the faster they move. A temperature can be reached where some of the free electrons become so agitated that they actually escape from the conductor. They "boil" from the conductor's surface. The process is similar to steam leaving the surface of boiling water. Heating a conductor to a temperature sufficiently high causing the conductor to give off electrons is called THERMIONIC EMISSION. B-M3 ELECTRICAL FUNDAMENTALS Static Electricity & Conduction 19 UNCONTROLLED COPY - FOR TRAINING PURPOSE ONLY Revision Service Will Not Be Provided To The Holder TRAINING MAN UAL For Traini ng Purpose Only ELECTRICAL TERMINOLOGY The following are some common electrical terms, their units and factors affecting them. WATER ANALOGY The following figure shows an analogy using water to illustrate the electrical terms and their significance. Consider two Tanks A and B with different water levels which are interconnected as shown in the figure. Water will flow from the tank with the higher level to the other tank. The presence of the higher water level in one tank has created a difference in water pressure. Water Analogy Tank A - -ve Charged Body Tank B - +ve Charged Body Water flow- current flow B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 1 TRAINING MAN UAL For Traini ng Purpose Only POTENTIAL DIFFERENCE As seen from the figure the difference in water levels creates a pressure, which makes water flow. Therefore using the same analogy for an electric circuit, when two points are connected by a conductor, there will be a flow of current from the point with a large number of electrons to the other point with a smaller number of electrons. This results in a potential difference between the two points. When there is a potential difference between the two points, it simply means that a field of force is present which tends to move the electrons from one point to another. The unit for potential difference is Volt (V). A potential difference of 1V exists between 2 points of a conductor when it is carrying a constant current of 1A when the power dissipated between these points is equal to 1W. The following figure illustrates the application of potential difference between two points. B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 2 TRAINING MAN UAL For Traini ng Purpose Only ELECTROMOTIVE FORCE (EMF) Electromotive force (emf) or sometimes called electron-moving force is the driving force that causes the current to flow through a conductor. The unit for emf is Volt (V). It is generated by a battery, or by the magnetic force according to Faraday's Law. Emf is commonly generated by electrochemical reaction (e.g., a battery or a fuel cell), absorption of radiant or thermal energy (e.g., a solar cell or a thermocouple), or electromagnetic induction (e.g., a generator or an alternator). Electromagnetic induction is a means of converting mechanical energy, i.e., energy of motion into electrical energy. The emf generated in this way is often referred to as motional emf. Emf can also be considered electrical potential or pressure. The term voltage, which is measured in volts, is typically used instead of emf. Electromotive force is typically symbolised by the letter E and voltage is symbolised by the letter V. VOLTAGE Potential difference, electrical potential and electromotive force are measured in volts, leading to the commonly used term voltage and the symbol V (sometimes E is used for voltage). B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 3 TRAINING MAN UAL For Traini ng Purpose Only DISTINCTION BETWEEN EMF AND POTENTIAL DIFFERENCE The following example illustrates the distinction between emf and potential difference. The circuit has an emf of 4V. The voltmeter reads the potential drop of 3V between A and B. The potential difference between points B and C is 1 volt. Therefore the potential difference is a voltage difference between two points in a circuit. The emf is the voltage generated by the battery. CHARGE The earlier water analogy has shown that water molecules flow from A to B. By using the same analogy in an electrical circuit the electrons flow. These electrons constitute the charge. The unit for electrical charge is coulomb (C). It is the total charge Q of 6.21 x 1018 electrons. Thus a single electron has a charge of 1.61 x 10-19 C. CURRENT In the water analogy, the rate of flow of water is measured in litres/minute, cm3 /sec etc. Therefore using the same analogy, the current can be defined as the rate of flow of electric charge at a point in a circuit. The unit of current is Ampere (A). B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 4 TRAINING MAN UAL For Traini ng Purpose Only Electric current flow in coulombs/sec = amperes. 1 coulomb of electrical charge flow past any point in a conductor in 1 second constitutes a current of 1 ampere. Current is normally denoted by the letter I. RESISTANCE The resistance of a constriction in a large pipe is so great that essentially all the pressure drop will appear across the resistance. If the Water pipe is constricted (narrowed), the constriction will oppose the flow of water than the remainder of the pipe system. Likewise a resistor in an electric circuit will generally will oppose the flow of current than the wire of the circuit. Resistance is the property of a material to oppose the flow of current and to convert electrical energy into heat. Its magnitude depends on factors such as the nature of conductor material, its physical state, dimensions, temperature and thermal properties. The unit for resistance is Ohms ( ). One ohm is the electrical resistance between two points of a conductor when a constant p.d. of 1V, applied to these points, produces in the conductor a current of 1A, the conductor not being the source of any emf V R I where V is the voltage and I is the current. B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 5 TRAINING MAN UAL For Traini ng Purpose Only CONDUCTANCE Conductance is the ability of a material to conduct electricity. Conductance is the inverse of resistance. A material that has a low value of conductance will not conduct electricity as well as a material that has a high conductance and vice versa. The unit of electrical conductance is Siemens (S). The following table shows the relative conductance of some common metals. Metal Relative Conductance (Copper = 1) Silver 1.06 Copper (annealed) 1.00 Copper (Hard Drawn) 0.97 Aluminium 0.61 Mild Steel 0.12 Lead 0.08 CONVENTIONAL CURRENT FLOW Conventional current flow is from positive to negative. As the early discoverers had no knowledge of electron flow, they based their laws of electricity on the behaviour of electric circuits i.e. they could only consider the effects. Conventional Current Flow B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 6 TRAINING MAN UAL For Traini ng Purpose Only A discharged to B therefore A is considered to be positively charged with respect to B. In fact B was charge with electrons and more negative than A so electrons flowed from B to A. The convention was too well established to alter when the truth was discovered so it remains the convention today. ELECTRON FLOW Electron current flow is from negative to positive. It shows the “true” direction of current flow. Electron Flow B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 7 UNCONTROLLED COPY - FOR TRAINING PURPOSE ONLY Revision Service Will Not Be Provided To The Holder TRAINING MAN UAL For Traini ng Purpose Only GENERATION OF ELECTRICITY INTRODUCTION Energy cannot be created or destroyed, but it can be converted from one form to another. Some sources of electricity are listed below: Light Heat Friction Pressure Chemical Action Magnetism Motion B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 1 TRAINING MAN UAL For Traini ng Purpose Only LIGHT When certain photoemissive materials such as selenium are struck by light, light energy is absorbed and electrons are discharged. The electrons are then channelled through a conductor to an electrical circuit. A photoemissive material emits electrons when struck by light One of the applications is in solar powered calculators where the electrical current is produced by light. Although photoelectric devices are limited in use in the modern aircraft, spacecraft and satellites rely heavily on photocells and the sun as a source of electric power. Solar Powered Calculators B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 2 TRAINING MAN UAL For Traini ng Purpose Only HEAT Heat can also be used to produce electricity by subjecting two junctions of dissimilar metals to different temperatures. This is called the thermoelectric effect. A thermocouple is a loop of two wires made of dissimilar metals that are joined in two places. Electrical current flows as there is a temperature difference between the two junctions. Examples of thermocouple are iron/constantan, chromel/alumel and copper/zinc. Electrons flow in a thermocouple Thermocouples are used in many electronic temperature sensors in the aircraft. Some examples are the exhaust gas and cylinder head temperature sensors, electronic equipment temperature monitors and some fire detectors. In a cylinder head temperature measuring sensor, one of its junctions is held tightly against a hot engine cylinder head by a spark plug while the other junction is mounted in an area where the temperature is kept relatively constant. Cylinder Head Temperature (CHT) Thermocouple B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 3 TRAINING MAN UAL For Traini ng Purpose Only FRICTION Friction can produce static electricity by simply rubbing two dissimilar substances together. Static electricity is however, not a typically useful form of power. In fact, most static electricity found on the aircraft creates problems for both communication and navigation systems as well as advanced electronic devices. When an airplane flies through the air it accumulates a static charge, especially on the aircraft control surfaces. This is even more apparent when flying through any kind of precipitation or even worse, volcanic ash. Static wicks attached to the trailing edges of control surfaces are designed to help dissipate the static charge to the surrounding air. They act as a protection to the flight instruments, radio equipment as well as the flight surfaces. Without the static wicks attached, the static charge on the surface would try to “jump” along the un-conductive control hinges to the rest of the aircraft. This “jump” or arc could cause permanent damage to the surface itself if the static charge had the opportunity to build sufficiently. To further protect against this damaging “jump”, manufacturers also attach conductive bonding strips to keep the static build-up to a minimum. Bonding Strap on an aircraft B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 4 TRAINING MAN UAL For Traini ng Purpose Only PRESSURE Pressure is another electricity source. Piezoelectricity means electricity created by applying pressure to certain types of crystals. Since only small amounts of electricity are produced, applications are limited. The piezoelectric effect is used in radio communication microphones to convert sound waves into electrical power. Most piezoelectric devices use crystalline materials such as quartz to produce charge. When a force is applied to certain axis, their molecular structure distorts and electrons are emitted into a conductor. Quartz subjected to pressure An electrical charge builds across the faces of these crystals when they are bent or otherwise subjected to mechanical pressure. The crystal vibrates at its natural frequency and produces alternating voltage with specific frequency when excited by pulses of electric energy. B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 5 TRAINING MAN UAL For Traini ng Purpose Only CHEMICAL ACTION Chemical action is often used to produce electricity for aircraft systems. When materials of opposite charges are connected, immersed in an electrolyte and connected through external load, an electron flow is created. A carbon rod immersed in a paste-like electrolyte enclosed in a zinc container can form an alkaline battery. Chemical reaction occurs between the electrolyte and zinc, which changes the zinc into zinc chloride. During this process, electrons are released by zinc and current flows through a wire connecting a light bulb into the carbon rod. Most aircraft contain a battery used for emergency procedures and other functions like engine starting. Electrons flow between two dissimilar materials when they are connected by a conductor and immersed in an electrolyte Batteries on board the aircraft produced electricity through chemical action for the purposes of engine starting and emergency procedures. B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 6 TRAINING MAN UAL For Traini ng Purpose Only MAGNETISM Magnetism is one of the most effective ways of producing electricity and is used to produce most electrical power. Electromagnetic induction produces voltage when a conductor is moved through a magnetic field. Most aircraft use generators or alternators to produce electricity by this method. Electricity generated by Electromagnetic Induction The amount of electricity induced is dependent on the rate at which the lines of flux are cut. Thus, the rate can be increased through the increase of the number of flux lines with a stronger magnet or by moving the conductor through the lines faster. B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 7 TRAINING MAN UAL For Traini ng Purpose Only MOTION Motion can also be used to generate electricity. By using fossil fuels such as oil, coal and natural gas, mechanical motion is produced to drive generators, which in turn produce electricity. For example, in a gas turbine power plant, fuels are burned to create hot gases that go through a turbine, spinning and turning the copper armature inside the generator and generating an electric current. Gas Turbine Power Plant In the case of a nuclear power plant, nuclear reactions create heat to turn water into steam. The steam goes through a similar turbine, which spins and turns the copper armature inside the generator and generating an electric current. Nuclear Power Plant B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 8 TRAINING MAN UAL For Traini ng Purpose Only In a wind turbine, the wind pushes against the turbine blades, causing the rotor to spin and turn the copper armature inside the generator and generating an electric current. Wind Turbine In a hydroelectric turbine, flowing (or falling) water pushes against the turbine blades, causing the rotor to spin and turn the copper armature inside the generator and generating an electric current. Hydroelectric Turbine B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 9