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Transcript
2 Voltage sources and effects of an electric current This chapter outlines how electrical energy is produced from other sources of energy. It also explains the effects of an electric current, and how these effects are used in everyday life. Some effects are not wanted, including the effect of an electric current on the human body, so methods of protecting against the unwanted effects are also given. Chapter outline 2.1 Producing a voltage 2.2 Effects of an electric current 2.3 Protection against effects of an electric current CHAPTER 2 Voltage sources and effects of an electric current 25 2.1 Producing a voltage As explained further in Chapter 4, electricity is a form of energy. A basic law, called the Conservation of Energy, says that energy cannot be created or destroyed. However, energy can be converted from one form to another. This means that electrical energy has to come from other sources of energy, by a conversion process. The most commonly used energy sources for this are: • mechanical • chemical • heat • light. Producing electricity often involves several processes, such as first converting heat to mechanical energy, then using the mechanical energy to produce electricity. The basic cycle is shown in Figure 2.1, where an energy source is converted to electrical energy, which is then converted by an electrical appliance into another form of energy, such as heat. Heat and light are both forms of ‘radiant’ energy FIGURE 2.1 Electrical energy is produced by converting other forms of energy As you saw in Chapter 1, a voltage is ‘electrical pressure’ and current is a flow of electrons. The current does the work (lights a lamp, makes a motor turn), as a voltage by itself is simply a pressure source. So, electrical energy is a combination of voltage and current. That is, electrical energy is used (or more correctly, transformed) only when current flows. This means that the energy source to produce the electrical energy is only being used when current flows. For instance, a battery that’s not connected to anything is not supplying electrical energy, even though there’s a voltage developed across its terminals. Here’s a brief look at the various ways of producing electricity. All these methods convert one form of energy into electrical energy. Mechanical to electrical Mechanical energy is movement. The three main ways to convert mechanical movement into electrical energy are: • friction between two materials • mechanical stress applied to a piezo-electric element • moving a conductor in a magnetic field. Friction Most of us have felt the effect of ‘static’ electricity. When you walk on a nylon carpet or slide over a fabric-covered car seat you can be ‘charged up’ to a high voltage. This is because electrons have either been transferred to you or taken from you. Static electricity is produced when clouds move in the atmosphere, because of friction between the cloud and the surrounding air. Once the voltage is high enough, an electric current will flow through the air between two charged clouds. Rubbing certain types of materials together can also produce static electricity. The available energy from static electricity is 26 E l e c t r i c a l P RINCI P LES rarely useful, although very high voltages can be produced for use in certain types of tests and experiments. A popular way of producing a high voltage using friction is with a Wimshurst machine, invented in the 1880s. This machine has two rotating glass discs fitted with metal sectors, with one disc made to rotate in the opposite direction to the other. Motive power is with a crank handle. Two cross bars support metallic brushes that rub lightly against the discs. When the voltage produced by the action of the rotating discs is high enough, a spark is produced between two metal spheres. Some machines, such as that shown in Figure 2.2, use Leyden jars (an early type of capacitor) to increase the accumulated spark energy. A typical Wimshurst machine can produce sparks up to 80 mm in length, depending on the diameter of the rotating discs. FIGURE 2.2 http://en.wikipedia.org/wiki/File:Wimshurst.jpg The Wimshurst machine is hand powered, and can produce high voltages but not much energy The Wimshurst machine led to the invention in 1929 by American physicist Robert J. Van de Graaff of a more advanced electrostatic generator which uses a moving belt to accumulate very high electrostatic voltages on a hollow metal globe on the top of a stand. Modern Van de Graaff generators can produce five million volts or more and are used in test laboratories. A Wimshurst machine produces a lower voltage, but can produce more current than a Van de Graaff generator. Getty Images/Science Photo Library/Adam Hart-Davis (a) Typical Van de Graaff generator, (b) lightning discharge from the large Van de Graaff generator at the Boston Museum of Science Getty Images/Science Photo Library/Peter Menzel FIGURE 2.3 (a) (b) CHAPTER 2 Voltage sources and effects of an electric current 27 Piezo-electric crystal The piezo-electric effect was discovered in 1880 by the Curie brothers. (Piezo is Greek for pressure.) They found that when mechanical stress was applied to crystals such as topaz, quartz, Rochelle salt and cane sugar, electrical charges appeared, and that this voltage was proportional to the applied stress. Materials that have this characteristic are called piezo-electric materials. The piezo-electric effect occurs when the charge balance within the crystal lattice of a piezo material is disturbed. When there is no applied stress on the material, the positive and negative charges are evenly distributed so there is no potential difference. When the lattice is changed slightly, the charge imbalance creates a potential difference, often as high as several thousand volts. However, the current is extremely small and only causes a small electric shock. Piezo elements are used in electronic cigarette lighters, gas lighters, guitar pick-ups, some kinds of microphones and various types of sensors, such as accelerometers. When used in electronic gas lighters, a trigger-operated mechanical mechanism is arranged to apply a short, sharp force to the piezeo element, which produces a voltage that is high enough to cause a spark, as in Figure 2.4. In a microphone, the variations in air pressure due to sound waves cause the piezo-electric crystal element in the microphone to produce a very small voltage that changes with the sound waves. FIGURE 2.4 Piezo-electric effect, where pressure is applied to opposite faces of a quartz crystal to produce a voltage. Pressure can be by compressing, flexing or distorting the piezo element. Magnetism Magnetism plays a big role in the electrotechnology industry, and the topics of magnetism and electromagnetism are covered in Chapters 11 and 12. In principle, if a conductor is moved through a magnetic field, a voltage is induced in the conductor. This effect was discovered by Michael Faraday in 1831, a discovery that led to the electrical power generation industry of today. Figure 2.5 shows a single conductor being moved through a magnetic field, although the voltage it produces will be very small (a few millionths of a volt). Faraday found that the faster the conductor moves through a magnetic field, the higher the voltage. It doesn’t matter whether the magnetic field or the conductor moves – a voltage is produced if either moves relative to the other. He also found FIGURE 2.5 Producing a voltage by using mechanical energy to move a conductor in a magnetic field 28 E l e c t r i c a l P RINCI P LES FIGURE 2.6 A simple alternator has a magnet that rotates inside a coil. When the magnet is rotated, its magnetic field intersects with the coil, producing a voltage. that a higher voltage would be produced by increasing the strength of the magnetic field and also by forming the single conductor into a coil. Figure 2.6 shows a simplified arrangement in which a rotating magnet provides the moving magnetic field which intersects with the conductor, now formed as a coil. This device is called an alternator, although the drawing shows the basic principles only. An alternator produces a voltage that regularly changes polarity, called an alternating voltage. The diagrams in Figure 2.7 show the principle, in which one turn of the magnet causes the voltage to go from zero to a positive value, back to zero, then to a negative value and finally back to zero. This is the type of voltage produced by power stations. If the magnet in Figure 2.7 is rotated at 3000 revolutions per minute (RPM), the alternating voltage will change polarity 50 times a second, the same as the voltage supplied by a power station. To get a DC voltage (one that doesn’t change polarity) an extra part called a commutator is added to the alternator, making it into a DC generator (described in Chapter 13). The alternator is covered in Chapter 24. In a practical alternator, rotation is provided by a prime mover, such as a diesel or petrol engine, flowing water, wind power, high pressure steam that turns a turbine and so on. Coal-fired power stations have large alternators driven by high pressure steam turbines. The steam is produced by burning coal which heats water under high pressure in a boiler. Because coal produces greenhouse gases, nuclear, geothermal and solar energy sources are being increasingly used to produce steam. FIGURE 2.7 The voltage changes polarity (goes from positive to negative) every revolution of the magnet Renewable energy sources to power alternators Coal, gas and nuclear power stations use non-renewable energy sources. Coal- and gas-fired power stations in particular produce carbon dioxide, a greenhouse gas that is now recognised as a factor leading to climate change. For this and many other reasons, renewable energy sources are being increasingly used around the world to produce the mechanical force that rotates an alternator to generate electricity. Here’s a brief look at some of these energy sources: • Geothermal energy is the heat contained within the Earth, and is used to heat water to produce the steam required to power a turbine driving an alternator. A hydrothermal system uses CHAPTER 2 Voltage sources and effects of an electric current naturally occurring steam, such as that produced by a geyser. Geothermal energy is relatively abundant in New Zealand, and produces around 10 per cent of the country’s electrical power. In Australia, an experimental power station powered by hot rocks (rocks heated by radioactive decay) is being constructed. Water is forced through the hot rocks, which are up to three kilometres below the Earth’s surface. This type of power generation is still being developed to reach a commercial scale. • Solar energy, in which the heat of the sun is focused onto specially shaped pipes, thereby heating the water flowing through the pipes and producing high pressure steam to power a turbine. A large baseload solar power station is now operating in Whyalla (South Australia). Solar energy is generally only available during sunlight, but in this power station, solar heat energy is captured by a chemical process, allowing it to be released at any time to ensure power is generated on a continuous basis. • Hydro-electric power stations have long been in use, in which flowing water from a dam or river turns the alternators. The best known example in Australia is the Snowy River Scheme which currently provides around 67 per cent of all renewable energy in eastern Australia. • Wind power is another increasingly popular source of energy. In 2010, around 2 per cent of Australia’s power needs were supplied by over 60 wind farms, nearly half of these in South Australia. The main components of a typical wind generator are the rotor, usually three blades up to 80 or more metres in diameter, a tower and the nacelle (an aerodynamically shaped housing), which houses the gearbox, alternator and yaw motors. These motors rotate the nacelle assembly to keep the blades facing the wind. 29 Renewable energy is sustainable energy F igure 2 . 8 Paul Anderson, UK (a) Components that make up a wind generator (b) Gearbox, rotor, shaft and brake assembly being raised into position (photo courtesy Paul Anderson, UK) While most of the electricity we use today is generated from the electromagnetic effect, there are other methods, all of which produce a DC voltage (one that always has the same polarity). Chemical to electrical The best known application of producing electricity by way of chemical energy is the torch battery. (Batteries are explained in more detail in Chapter 25.) The principle was first discovered by Allesandro Volta in 1800, who noticed that the legs of dead frogs twitched when they were touching two different metals. He went on to find that when two different metals are put into a liquid called an electrolyte (an acid or an alkali) a voltage is developed between the two metals. He had discovered the voltaic effect, or the electric cell, the first useful way of producing electricity. 30 E l e c t r i c a l P RINCI P LES The basic electric cell (shown in Figure 2.9) has three components: the positive electrode, the negative electrode and an electrolyte. The choice of the electrode metals and the electrolyte determine the voltage the cell produces. You can make an electric cell able to produce about 0.8 V by putting a galvanised nail and a piece of copper wire into a lemon, because lemon juice is an acid (citric acid). FIGURE 2.9 An electric cell converts chemical energy to electrical energy A number of different types of cells have been developed, which fall into two categories: those that can be recharged, and those that are disposed of when the cell’s chemical energy has been fully used. Rechargeable cells include lead-acid, nickelcadmium (nicad) and nickel-metal hydride (NiMH). Disposable cells include carbon-zinc (alkaline and conventional) and most types of ‘button’ cells. A carbon-zinc cell produces 1.5 V and a leadacid cell produces 2 V. To get a higher voltage, cells are connected (in series) so their voltages add together to give the required voltage. A 6 V lead-acid battery has three cells and a 12 V battery has six, connected either internally or externally with metal straps. A 9 V carbon-zinc battery has six cells, stacked on top of each other. FIGURE 2.10 The most common types of electric cells are the zinc-carbon and the lead acid Heat to electrical Although heat is used in coal-fired (and nuclear) power stations, it doesn’t directly produce electricity. Instead the heat is used to make the high pressure steam to drive a steam turbine. However, heat can be used to directly produce a voltage using a device called a thermocouple which is simply two different metal wires joined at a point called the junction. The principle was discovered in 1821 by German–Estonian physicist Thomas Seebeck, and is now known as the thermoelectric effect or Seebeck effect. If heat is applied to the junction as in Figure 2.11, a voltage is developed by the thermocouple. How much voltage depends on the type of materials used in the thermocouple and the difference in the temperature between the hot and cold ends of the thermocouple. CHAPTER 2 Voltage sources and effects of an electric current 31 FIGURE 2.11 A thermocouple is formed by two different metals joined at one point The metals used in the thermocouple wires depend on the temperature the junction will be exposed to. For example, an iron and constantan thermocouple is used for temperatures up to about 600 8Celsius (C), a chromel-alumel type for temperatures up to about 1300 8C. The terms ‘chromel’, ‘alumel’ and ‘constantan’ are manufacturers’ names for the metals the thermocouple wires are made of. These metals are special alloys and are made by combining a number of different metals. The voltage output of a thermocouple is only a few thousandths of a volt (millivolts). An ironconstantan thermocouple with a temperature difference of 500 8C between both ends of the thermocouple produces a voltage of around 30 millivolts (0.03 volts). While this voltage is too low to provide useful power, because the voltage produced by a thermocouple is proportional to temperature, it can measure temperature. In Figure 2.11, the voltmeter is calibrated in degrees Celsius. Thermocouples are also used in industrial temperature control systems, in which the thermocouple voltage is used to control heating elements. Light to electrical Courtesy Aussie Solar Installations A device that produces a voltage when exposed to light is called photo-voltaic. The best known photo-voltaic device is the solar cell. These are typically constructed with crystalline silicon or the more expensive gallium arsenide, which is produced exclusively for use in photo-voltaic (solar) cells. A single solar cell produces about 0.4 V and cells are combined to form a solar panel that typically produces a voltage of 12 V to 24 V DC and up to 200 watts of power. The power available from a solar panel installation depends on the number of panels used. Typical power ratings for a domestic installation range from 1000 watts (1 kilowatt) to 5000 watts (5 kilowatts). Commercial installations produce even more power. Solar panels are commonly used to charge batteries that power remotely located roadside signs, telephones and weather stations. Solar powered calculators are now being joined by solar powered mobile phones, computers and lamps. Satellites and the international space station are powered by banks of solar panels. Tracking mirrors are often used in commercial installations to direct the sunlight onto the solar panels at all times during the day. A solar cell doesn’t need any maintenance and produces electricity whenever light is present. FIGURE 2.12 Solar cells convert the sun’s light energy to electrical energy 32 E l e c t r i c a l P RINCI P LES Summing up… • Energy can neither be created nor destroyed, it can only be converted from one form to another. • Electrical energy is produced by converting other sources of energy, including mechanical, chemical, heat and light. • Most electrical power is produced by alternators driven by some form of mechanical energy. • High voltages can be produced with friction between two materials, or by applying force to a piezo-electric material. • Batteries produce electricity by converting chemical energy to electrical energy. • Photo-voltaic devices such as the solar cell produce electricity when exposed to light. • Renewable energy sources include geothermal, solar, hydro, biogas and wind power. 2.2 Effects of an electric current An electric current causes a number of effects which can be useful, or merely unwanted byproducts. Effects caused by an electric current include: • heating • magnetism • chemical • luminous • physiological. All of these effects are used in some way, although the physiological effect (the effect of an electric current on a living organism) is generally a safety hazard. Heating Whenever current flows in a conductor, heat is produced. This effect is used in electric heaters, frypans, cooking ranges and so on. Heat is produced by a conductor because of its resistance. All conductors have a certain amount of resistance, so heat is always produced when current flows. The higher the resistance, the greater the amount of heat produced (for a given value of current). Because of this, the resistance of conductors supplying power to electrical equipment should be as low as possible. If, however, heat is wanted, as in an electric radiator, the conductor used in the heating element needs to have a certain amount of resistance. The material used is therefore designed to have resistance, and to withstand the heat it produces without burning out. Materials for this purpose include nichrome (a combination of nickel and chrome), which is used in electric radiator elements. The material used to make the filament in an incandescent lamp is another example. FIGURE 2.13 Heat is developed when current flows through a resistance CHAPTER 2 Voltage sources and effects of an electric current 33 As further explained in Chapter 4, because heat is a form of energy, electrical energy is used to produce the heat. Unless it is wanted, heat in an electric circuit is actually a loss, as energy is needed to produce it (wasted energy). Efficiency is a measure of power in to power out, so the more unwanted heat that is generated, the lower the efficiency. For example, an electric motor usually gets hot after it has been running for a while. The more efficient the motor is, the less heat it will produce (at its rated load). Magnetism The magnetic effect caused by an electric current is perhaps the most useful of all. The effect was discovered in 1820 when Danish physicist Hans Orsted observed that a compass needle was deflected when current from a battery was flowing in a nearby wire. On hearing of this discovery, French physicist André-Marie Ampère presented a paper a week later that explained the effect, and thereby opened up the field of electromagnetism. This topic is covered in Chapter 11. A basic application of the magnetic effect is the electromagnet, which has a coil of wire wound around a soft iron core. When current flows in the coil, the iron core becomes magnetised. A common use for the electromagnet is in a device called a relay. A simple relay is shown in Figure 2.14. The moving part, called the armature, is spring-loaded and is pulled down by the magnetised core when the coil is connected to a voltage (energised). A metal contact is attached to the armature so that it makes an electrical connection with a fixed metal contact when the armature moves towards the coil. If these contacts are connected then they switch power to a load (heater, motor), and the load will turn on. FIGURE 2.14 A relay has electrical contacts that close when the relay coil is energised, thereby switching on the load controlled by the relay The main application of a relay is to switch loads such as motors, heating elements, lights and so on. A small amount of power is needed to operate the relay coil, but the relay contacts can switch large amounts of power. Relays are available in a range of sizes and ratings. Those shown in Figure 2.15 all have a low voltage coil rated at 12 V. The contacts are rated to handle 230 V AC and currents of 10 A or more. FIGURE 2.15 Relays used to control a 230 V AC load such as motors, lights and heating elements 34 E l e c t r i c a l P RINCI P LES Two other very common applications of the magnetic effect are the electric motor and the transformer. As described in Chapter 21, a transformer converts an alternating voltage from one value to another. Transformers are used extensively in electrical power distribution and transmission, and are widely used in electronics and telecommunications. Electric motors are also widely used in many forms of transport, machines and appliances, and are explained in Chapters 22 and 23. FIGURE 2.16 Motors and transformers are common applications of the magnetic effect of an electric current Other uses of the magnetic effect are magnetic strips on plastic credit cards, storing computer data on a hard disk and recording sound on a tape recorder. Sensors that detect magnetism, called Hall effect devices, are used extensively in car engines to give timing signals. Large electromagnets are used in metal scrap yards to lift heavy metal objects, or to sort magnetic materials from nonmagnetic materials. Chemical effect As explained in Chapter 1, current can flow in certain types of liquids called electrolytes. Salt water is an electrolyte as it contains salt (sodium chloride) dissolved in water. Acids and alkalies are also electrolytes. Current can flow, because an electrolyte contains ions, which are atoms with a positive or a negative charge. A positively charged ion has less electrons (negative charge) than protons (positive charge), and a negatively charged ion has more electrons than protons, as shown in Figure 2.17. In this diagram, each ion has four protons and three neutrons, but different numbers of electrons. FIGURE 2.17 Ions are atoms in a liquid that have lost or gained electrons CHAPTER 2 Voltage sources and effects of an electric current 35 In salt water, the sodium atoms become positive ions as they each lose one electron which is taken by the chlorine atoms, making the chlorine atoms (which are now ions) negative. However, the liquid isn’t charged because the number of positive ions equals the number of negative ions. Because the ions are charged, they can be attracted by a voltage. The rule is: like charges repel and unlike charges attract. If two metal electrodes connected to a voltage source are placed in an electrolyte, the positive ions will flow towards (be attracted by) the negative electrode (cathode) and the negative ions will flow towards the positive electrode (anode), as shown in Figure 2.18. As a result, an electric current flows in the liquid. In other words, the electrolyte behaves as a conductor. The more ions there are in the liquid, the better the liquid can conduct an electric current. If more salt is added to the liquid, more ions are created and the resistance of the liquid is less. This is why salt water is a better conductor than fresh water. FIGURE 2.18 Current flow in an electrolyte – the positive ions move to the cathode and negative ions move towards the anode, in this case to produce purified copper It’s important to understand that an electric current does not itself have a chemical effect, but that it causes a chemical effect when passed through an electrolyte. The two main useful chemical effects are electrolysis and, as already explained, the voltaic effect (the electric cell). Electrolysis In the electrical industry, an important use of electrolysis is to produce pure copper (greater than 99.95 per cent), called electrolytic copper. High purity is needed because small amounts of impurity metals can seriously reduce copper’s ability to conduct electricity. For example, 0.05 per cent of arsenic impurity reduces its conductivity by 15 per cent. Electric cables must be made of very pure copper to prevent electrical losses in them, which is particularly important when the cables run for long distances. To purify copper electrolytically, the impure copper metal is made the anode (the positive electrode) in an electrolytic cell, as in Figure 2.18. A thin sheet of previously purified copper is used as the cathode (the negative electrode). The electrolyte is a solution of copper sulphate and sulphuric acid. When current is passed through the cell, positively charged copper ions are pulled 36 E l e c t r i c a l P RINCI P LES out of the anode into the liquid, and are attracted to the negative cathode, where they lose their positive charges and stick tightly as neutral atoms of pure copper metal. As the electrolysis continues, the impure copper anode dissolves and pure copper builds up as a thicker and thicker coating on the cathode. Impurity metals such as iron, nickel, arsenic and zinc leave the anode, but go into the solution. Impurities such as platinum, silver and gold are also released from the anode, but as they are not soluble, they fall to the bottom forming a valuable sludge, which can often cover the cost of the large amount of electricity used. Another use of electrolysis is the production of hydrogen, in which water (H2O) is decomposed into hydrogen (H2) and oxygen (O) by passing an electric current through the water. About 4 per cent of hydrogen gas produced worldwide is created by electrolysis, mainly as a by-product of passing current through salt water to produce chlorine gas. Aluminium is produced by electrolysis, in which powdered alumina, derived from bauxite, is contained within a large vessel (pot) lined with carbon or graphite, and fitted with a carbon electrode (anode). The carbon lining of the vessel is the cathode. When an electric current passes through the mixture, the carbon of the anode combines with the oxygen in the alumina, producing metallic aluminium and carbon dioxide. The aluminium settles to the bottom of the vessel where it is syphoned off into crucibles while the carbon dioxide escapes. A DC current of up to 200 000 amperes per pot is required, which is why aluminium smelters are located near affordable power sources. Electroplating The main use of electroplating is to protect metals against corrosion. It is also used to build up a worn metal surface. This is a process in which a metal object (e.g. a bumper bar) is coated by electrolysis with a thin layer of a different type of metal (e.g. chrome). The part to be plated becomes the cathode of the circuit (has a negative potential). Typically, the anode is made of the metal to be plated onto the part. Both components are immersed in an electrolyte containing dissolved metal salts and other ions that allow the flow of electricity. The current flowing through the solution causes the anode to release atoms through oxidisation into the electrolyte. The dissolved metal ions in the electrolyte are attracted by the negative potential of the part being plated (cathode) and are deposited onto the surface of the part. The anode is dissolved during the process at the same rate as the cathode is being plated. Sometimes electroplating uses a non-consumable anode, such as lead. In this case the electrolyte supplies the metal ions, which need to be periodically replenished as they are drawn out of the solution. Galvanic corrosion Most metals corrode in some way due to being exposed to oxygen, which causes the surface to oxidise. Rust is the formation of iron oxide, and, over time, iron in the presence of moisture will corrode away completely. Other metals such as copper and aluminium form an oxide layer, which helps protect the metal from further oxidisation. Another form of corrosion is galvanic corrosion, caused by electrolysis. This occurs when two different types of metals are in electrical contact and are exposed to an electrolyte, such as rain or sea spray. Corrosion occurs because of the voltaic effect, in which the dissimilar metals and the electrolyte form an electric cell. The current produced by the cell causes the metals to corrode, with one of the two metals corroding before the other. An understanding of this process led to the development of cathodic protection, a technique to control the corrosion of a metal surface by making it work as the cathode of an electrochemical cell. This is achieved by placing the metal in contact with another more easily corroded metal that CHAPTER 2 Voltage sources and effects of an electric current 37 acts as the anode of the cell. The first application of this occurred in 1824 when Sir Humphry Davy succeeded in protecting copper sheathing on a ship against corrosion from seawater by the use of iron sacrificial anodes. Today, most storage hot water heaters (gas and electrical) have an internal sacrificial anode to help prevent the boiler from corroding. FIGURE 2.19 Sacrificial anodes in use and the electrochemical series listing metals in the order of their behaviour as a sacrificial anode Galvanising is a common example of cathodic protection, in which steel sheets are covered with a thin layer of zinc. The zinc not only coats the steel, preventing it from oxidising (rusting), it forms a galvanic cell when the zinc is scratched, protecting the exposed steel by acting as a sacrificial anode. That is, the zinc corrodes before the steel, until eventually the zinc is completely corroded away, allowing the steel to rust. Figure 2.19 lists the commonly used metals in their electrochemical series, which shows that zinc is below iron, hence zinc will corrode in preference to iron. Rechargeable batteries Charging a battery is another example of the chemical effect, in which the charge current reverses the chemical action caused when the battery is discharged. In effect, electrical energy is stored in the battery during charging, and released by chemical action during discharge. This applies to rechargable batteries like the lead-acid type used in cars, and to nickel cadmium cells, but not to carbon-zinc batteries, which are not designed to be recharged. See Chapter 25 for more information about batteries. Luminous effect One of the first applications of electricity was the development of the arc lamp. This device has two carbon electrodes in open air, arranged so an electric arc is established between the electrodes when they are brought in contact with each other, then separated. While this type of lamp can produce high amounts of light, it also generates high levels of ultraviolet light and a large amount of heat. The principle of the arc lamp led to the development of the discharge lamp, which has a glass envelope filled with a gas, and contains two metal electrodes. Gases used include neon, argon, xenon, sodium, metal halide and mercury. Light is produced by passing an electric current through the gas, which ionises the gas, creating a plasma. A starting circuit provides a high voltage to cause the lamp to light, and a device (sometimes called a ballast) limits the current flowing through the lamp. The fluorescent lamp is the best known example of a gas-discharge lamp in which the ultraviolet radiation created by the current flowing through the gas (mercury vapour) causes a fluorescent 38 E l e c t r i c a l P RINCI P LES LED lights are energy efficient and are now used in homes for general lighting purposes coating inside the tube to glow. These lamps are far more efficient than incandescent lamps, where light is produced by heating a filament. Compact fluorescent lamps are now being increasingly used because of their high efficiency. The light emitting diode (LED) is a solid state device that produces light by way of the effect called electroluminescence, which is the result of electrons releasing energy in the form of photons (light). LED lighting is efficient, as not much heat is generated. The first practical visible-spectrum (red) LED was developed in 1962. Today, LEDs can produce red, green, blue, yellow, white, infrared and ultraviolet light. High power LEDs (over one watt) are used in traffic lights and vehicle lights, and are gaining acceptance in general lighting applications. LEDs operate from a low DC voltage, generally around two volts, they have a long life and maintain a consistent light output even in cold temperatures, unlike traditional lighting methods. FIGURE 2.20 LEDs operate from a low DC voltage and are being increasingly used in domestic and industrial lighting applications Physiological effect F igure 2 . 2 1 Blood is a good conductor of electricity and is carried through the body by the capillary system This effect is the result of passing an electric current through a living organism, such as a human or an animal. It has a few medical uses, mainly to stimulate a heart by applying a short burst of current that will shock the heart out of ventricular fibrillation (explained shortly). Otherwise, this effect is limited to stun-guns, cattle prods, electric fences and to those countries that still use the electric chair. The purpose of these items is either to produce a short, sharp electric shock, or in the case of an electric chair, to cause death. So it’s important you understand this effect as your life may depend on it! Life is kept going by two important body functions: breathing (lungs) and blood circulation (heart-beat). If either of these stop for more than a few minutes, the brain is starved of oxygen, and there will be so much damage that death soon follows. If an electric current passes through the body, the current can interfere with the tiny electrical impulses that travel through the body’s nervous system to control the heart and lung muscles. The amount of interference depends on the value of the current, how long it flows and where it flows. Current which flows through the brain, or the chest region where CHAPTER 2 Voltage sources and effects of an electric current the heart and lungs are, is more dangerous than a current passing through two fingers on the same hand. If breathing stops after an electric shock, the heart can go into what is called ventricular fibrillation. This is when the electrical impulses from the brain controlling the heart are confused enough to be out of step, making the heart quiver and stop pumping blood. An electric current can also cause terrible burns and make muscles tighten severely. Death and injury can not only result directly from electric shock, but indirectly. Electric shocks have caused people to fall off ladders and power poles, flung them into dangerous chemicals, and made them drop something heavy on themselves, such as the live appliance they were carrying at the time. The value of the current causing the shock can determine whether it’s lethal. Table 2.1 shows the effects of various values of current. Remember that a milliamp (mA) is one-thousandth of an ampere, so the current values are quite small. A current as low as three or four-thousandths of an ampere (3 to 4 mA) can cause pain and a current over 20 mA can stop the lungs from functioning. These values of current will hardly even light a lamp, but could kill you. Skin has a relatively high resistance, but blood is a good conductor. If your skin is wet, this reduces its resistance, and current can flow through your skin to the low resistance path of the arteries. Effects of an electric current passing through the body Current Effect up to 2 mA barely perceptible 2 mA to 8 mA sensation becomes obvious and more painful 8 mA to 12 mA muscle spasms and greater pain 12 mA to 20 mA unable to let go the conductor, can’t control muscles 20 mA to 50 mA if passing through the chest, breathing might stop 50 mA to 100 mA if near the heart, causes ventricular fibrillation 100 mA to 200 mA heart stops beating above 200 mA severe burns as well! The value of current that flows during an electric shock depends on two things: • the value of the voltage causing the shock • the resistance your body offers to the current. There is no such thing as a ‘safe’ voltage. However, voltages less than 50 V are unlikely to deliver a lethal shock. The type of voltage also has different effects. Because an alternating (AC) voltage regularly changes its polarity, it doesn’t have the same effect on the muscles as a direct (DC) voltage. A shock with a DC voltage causes the muscles to contract, making it almost impossible to let go the live conductor. An AC voltage could allow you to let go, although this would depend on the value of the current. Resistance is mainly that of the skin. Wet, sweaty hands have less resistance than dry skin. Standing on a wet floor allows current to flow through you to the ground by way of the feet. Standing barefoot is obviously very dangerous – always wear rubber-soled shoes when working with electricity. Wet clothes are another hazard when working with electricity, as wet cloth can conduct an electric current. The resistance of the cloth depends on the type of liquid soaked up by the material. Remember that tap water with all its usual impurities will conduct electricity. As we’ve said before, electricity and water is a dangerous combination! Here are some basic rules for avoiding an electric shock: • Think of any appliance, conductor, terminal or equipment as live, until you have proved otherwise. • Don’t work on an electrical appliance or circuit with wet hands. TABLE 2.1 39 40 E l e c t r i c a l P RI N CI P L E S • A wet floor is a good conductor of electricity – be extra careful. • Don’t wear metal jewellery (rings, watch) when working with electricity. • Remember that ‘familiarity breeds contempt’. Always have respect for electricity – it has no respect for you! 2.3 Protection against effects of an electric current FIGURE 2.22 Direct contact with a live conductor can cause current to flow through your body to ground The effects of an electric current can sometimes cause dangerous situations, the most obvious being the danger of an electric shock. The Australian and New Zealand wiring rules (AS/NZS 3000) specify ways of minimising these dangers. In regard to the physiological effect, the standards identify two situations that must be protected against: 1 Direct contact under normal service conditions with a live terminal, conductor or part of an electrical installation or circuit. Protection against this is achieved by a combination of insulation, barriers or enclosures and placing exposed electrical contacts out of reach. To protect the public, appliances are required to pass safety tests, and electrical wiring installations must be to the standards given in AS/NZS 3000. Those working on live electrical circuits are the most likely to receive an electric shock. Normally a circuit is isolated from the electrical supply before doing work, but in some cases this is not possible (e.g. when working on power lines or a live fuse box). Under these conditions, you need to wear protective gloves and other items of personal protective equipment, as well as understand the risk and take steps (control measures) to prevent being electrocuted. 2 Indirect contact that can occur under a fault condition. This might happen if you touch a metal-cased appliance that, because of an internal fault, is now live. Other faults are exposed live conductors because the insulation is missing, water reaching live terminals or electrical parts; any fault that could cause an electric shock to the user. Protection against these dangers is usually achieved by automatic disconnection of the electrical supply by way of fuses, circuit breakers and a unit called a residual current device (RCD), generally known as a safety switch. This is in combination with a system of earthing, in which metal parts of an appliance are connected to ground via the earth wire. CHAPTER 2 Voltage sources and effects of an electric current 41 FIGURE 2.23 Faults that can cause an electric shock include touching a live appliance or exposed live conductors faulty appliance making it live frayed lead exposing bare conductors The AS/NZS 3000 standards also require electrical installations to provide protection against the thermal effects of an electric current. For example, electrical equipment that produces a lot of heat must be properly ventilated and fire-proof enclosures should be used where appropriate. The magnetic effect must also be protected against. The problems caused by the chemical effect include corrosion and the possibility of creating an electric cell by joining two dissimilar metals. In the case of connecting aluminium conductors, the standards require specially designed components to make the connection, after first removing the aluminium oxide film on the connecting surfaces. Corrosion of outdoor metal fittings must be prevented by using specified coatings, such as galvanising. As already explained, other methods to prevent corrosion include the use of sacrificial anodes. Chapter summary • Electrical energy is produced by converting other forms of energy (mechanical, chemical, heat and light). • Electrical energy is converted to the required form of energy (e.g. heat, light or mechanical energy) by the electric load. • Large alternators driven by steam turbines (mechanical energy) generate most of Australia’s electrical energy. • An alternator has coils of wire arranged so they intersect with a moving magnetic field, thereby producing electricity. • Renewable energy sources that can drive an alternator include geothermal, solar, hydro, biogas and wind power. • Solar panels are photo-voltaic devices that convert light energy to electrical energy. • Friction can be used to produce high voltages. Examples are the Wimshurst machine and the Van de Graaff generator. • The electric cell produces electricity by converting chemical energy to electrical energy. • Charging a battery (a group of cells) reverses the chemical action that took place when the battery was being discharged. • Electric motors, transformers and electromagnets work by way of the magnetic effect of an electric current. • Electrolysis is the process of passing current from one electrode to another through an electrolyte. Electroplating is an example of electrolysis. • Discharge lamps are filled with a gas that produces light directly, or in the case of the fluorescent lamp, ultraviolet light that excites phosphors inside the tube. • Physiological effects of an electric current are an unwanted by-product, and pose dangers to all forms of biological life, including muscle contraction, ventricular fibrillation, burns and death. • Effects caused by an electric current include heating, magnetism, chemical, luminous and physiological. • Heat is produced in a conductor because of its resistance. 42 E l e c t r i c a l P RINCI P LES • Current flowing in a conductor creates a magnetic field around the conductor. Coiling the conductor makes an electromagnet. • Chemical effects include electrolysis and voltaic effect (electric cell). • Galvanic corrosion is caused by electrolysis when two different metals are in electrical contact, creating an electric cell. • A metal surface can be protected from corrosion with sacrificial anodes that corrode in preference to the metal being protected. • Fluorescent lamps are an example of the luminous effect on an electric current. • LED lighting is an example of electroluminescence. Self-check Answer these questions then check your answers at the back of the book. 1 What are three forms of energy that can be converted to electrical energy? 2 How can static electricity be produced? Name two machines that operate on this principle. 3 Give a common application of the piezoelectric effect. 4 How does a simple alternator work? 5 Name two non-renewable and two renewable energy sources that are used to produce steam to power alternators in a power station. 6 Apart from steam turbines, what are two renewable energy sources commonly used to power the prime mover that rotates an alternator? 7 What are the three components of a basic electric cell? 8 What is the main application of a thermocouple? 9 What is the name given to devices that produce a voltage when exposed to light? 10Name four situations where solar panels are used to provide electrical power. 11List four useful effects of an electric current. 12What causes heat to be produced in a conductor when current is flowing? 13List three commonly used electrical devices that work by the magnetic effect of an electric current. 14Name the two main chemical effects of an electric current. 15Which electrode are negative ions in an electrolyte attracted to, the anode or the cathode. Why is this so? 16Give three applications of electrolysis. 17What is meant by ‘cathodic protection’? 18What are the main components of a discharge lamp? 19Why is it important to be extra careful when working with electricity in damp surroundings? 20What two things determine the value of current flowing through your body when you get an electric shock? Online resources For more information on the contents of this chapter, visit these websites: • http://www.australiancoal.com.au/ (click on ‘Coal and its uses’ on the home page, then on ‘Electricity Generation’ for an overview of producing electricity in a coalfired power station) • http://www.suzlon.com/index.aspx (wind power generators) • http://science.howstuffworks.com/ wind-power1.htm • http://science.howstuffworks.com/ electromagnet.htm Also try Googling the following search terms: • electricity (Wikipedia gives an overview of some of the content of this chapter) • power stations in Australia (Wikipedia lists all power stations in Australia that are currently operating or planned, including wind farms, coal-fired and so on) • electrolytic cell (Wikipedia gives an overview of the terms electrolysis and the electric cell) • renewable energy (numerous websites cover this topic) • piezo-electric (numerous websites give information, in particular Wikipedia).