14. ENERGY 14.1 WHAT IS ENERGY?

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1 14.1 WHAT IS ENERGY? 14. ENERGY Everyone knows what energy is, but it is not so easy to explain! Look at the photographs below. The dancing children obviously have lots of energy; there is lots of energy in the other photos too. Scientists define energy as the capacity to do work. In science, work refers to almost anything that can be done! The children are using their energy to dance, and the musician is using her energy to sing and play the base. The energy of the waterfall makes the water move fast and the energy of the train enables it to pull a heavy load at high speed. In all the pictures, the energy is also making a lot of noise! Anything that gets moving, or makes noise or heat or light or electricity, is using energy and doing work. The energy is not the same thing as the work that is done; energy is simply the ability or readiness to do work. In this chapter we look at different kinds of energy. You will learn how they are classified and how one kind of energy can be changed into another. You will also learn about the first and second laws of thermodynamics that summarise what happens during energy changes. After that, we examine some of the ways that energy is harnessed in the modern world. You will learn about heat exchangers, heat pumps and air conditioners, the internal combustion engine (the engine in a car or truck), the electric motor and the power stations that generate electricity for our homes and factories. You will learn where all the energy that we use comes from, and where we may have to look for it in the future. 183

2 14.2 DIFFERENT KINDS OF ENERGY In this chapter we discuss seven forms of energy: mechanical energy (that is the energy associated with the mass of an object), electrical energy, sound energy, light energy, heat energy, chemical energy and nuclear energy. Forms of energy that involve motion are classified as kinetic energy and forms of energy that do not involve motion are classified as potential energy. Kinetic energy is the energy of moving things. Kinetic comes from the Greek word kinesis meaning motion. The term kinetic energy is used mainly for the mechanical energy of a moving object. However electricity, sound, light and heat all involve motion, so they are often regarded as forms of kinetic energy too. Mechanical kinetic energy is the energy of anything that moves and has mass. In the photos above, the racing cars have a lot of kinetic energy and so do the women athletes. All the photos in the previous module show examples of kinetic energy. The children and the singer are moving, so is the water in the waterfall and so is the train. Kinetic energy is associated with all kinds of motion, including rotation. The photo on the right shows the flywheel at a pumping station. This flywheel has a mass of more than 30 tonnes and when it is turning fast it has so much kinetic energy that it can drive the pumps on its own for quite a long time. Electrical energy is a stream of electrons flowing through a conductor (see Chapter 4 and Module 13.5). Electricity is a convenient form of energy because it can be delivered to homes and factories through wires, and it can easily be converted into other kinds of energy. The picture below shows electric wires delivering electricity to peoples homes and for an electric train. Sound energy is a pulse of vibrations travelling through the air (see Modules 7.13 and 14). The source of the sound vibrates and nudges the molecules in the air surrounding it. You can easily feel the vibrations from a loudspeaker like the one shown on the right. A pulse of backwards-and-forwards vibrations of the air molecules carries the sound energy through the air at a speed of about 330 metres per second. Light energy is an example of electromagnetic radiation. Electromagnetic radiation includes radio and TV waves, heat (infra-red) rays, light, X-rays and γ-rays. These rays all carry energy, but unlike sound waves (which must be carried by a medium such as air or water), they require no medium. They travel through space at a speed of 3 x 10 9 metres per second. In this chapter we 184

3 will focus mainly on light which provides energy for life on Earth through photosynthesis. Heat energy is part of the electromagnetic radiation that we receive from the sun. We also get heat energy from burning fuels and in other ways. As you learnt when you studied the kinetic theory (see Modules 9.2 and 3), heat energy is associated with the random motions of atoms and molecules. We can think of this as internal kinetic energy. When a solid is heated, the particles do not move from place to place, they jiggle about at random, faster and faster, nudging one another and conducting the heat all through the solid. When a fluid (a liquid or a gas) is heated, the particles move around all over the place, faster and faster in all directions at random. Convection currents quickly spread the heat all through the fluid (see Module 7.3). Heat is a low grade form of energy because a lot of it is always wasted as internal kinetic energy and therefore cannot do useful work. Potential energy is stored energy, or energy-in-waiting. Potential comes from the Latin word potentia meaning power or capability. The term potential energy is used mainly for the stored mechanical energy of a stationary object which is being acted on by a force. However, chemical and nuclear energy are also forms of stored energy. Mechanical potential energy is the stored energy of a stationary object that is being acted on by a force; an object that is poised, ready to move. In the photo on the right, a large rock is balanced high up on a cliff. It is being pulled down by the force of gravity but is supported by the rocks underneath it. If this rock slips, or if someone pushes it, it will crash down and damage anything that it hits. Any object above ground level has potential energy of this sort. In the photo on the left, the arrow is being pushed forward by the tension in the bowstring. When the archer lets go of the bowstring, it will shoot the arrow through the air towards its target. Any stationary object that is being pushed or pulled by a piece of elastic, or a spring, has potential energy of this sort. Chemical energy is the potential energy that is stored in substances that are ready to undergo chemical reactions. A good example is an electric cell or battery. When the terminals are connected and the circuit is switched on, chemical reactions in the cell will send a stream of electrons through the circuit to do their work. Other examples of chemical potential energy are explosives and fuels, including food which is the fuel for our bodies. Nuclear energy is the potential energy that is stored in the nuclei of atoms. This energy is released slowly during radio-active decay and fast during nuclear fission and nuclear fusion. Nuclear fission is the source of energy in nuclear power stations and atom bombs, and nuclear fusion is the source of energy of the stars including our sun. 1. List seven forms of energy and classify each of them as kinetic energy or potential energy. 2. Why is electricity a convenient form of energy? Explain how the potential energy of the rock and the arrow above can be described as energy-in-waiting. Describe two chemical examples of energy-in-waiting.

4 14.3 CHANGING ONE KIND OF ENERGY INTO ANOTHER Many activities involve changing one form of energy into another. In this module we will look at a few examples. When we summarise energy changes, we will use > to mean changes into. Explosions: The head of a match contains stored chemical energy. When we strike the match, the match head explodes (gently) and its chemical energy changes into heat and light energy. chemical energy > heat (and light) energy Industrial explosives such as dynamite contain a large amount of stored chemical energy. When dynamite is used to break rocks in a quarry or a mine, the chemical energy changes into kinetic energy as the rocks fly apart; the explosion makes a lot of heat and sound energy too! chemical energy > kinetic energy (plus heat and sound energy) Using a cell or battery: An electric cell or battery contains stored chemical energy (see Module 4.6). When a cell is connected to a light bulb, the stored chemical energy changes into electrical energy in the wires, and the electrical energy in the wires changes into light energy (and a bit of heat) in the bulb. chemical energy > electrical energy > light (and heat) energy electric motor pulley wheels axel In the diagram on the left, the chemical energy stored in the battery changes to electrical energy in the wires that lead to the small electric motor. The motor changes the electrical energy into mechanical kinetic energy in the form of a turning pulley wheel that drives a larger pulley wheel. The larger pulley wheel is attached to an axel and, as the axel turns, it winds up a string and lifts a mass. Lifting the mass changes kinetic energy from the motor into potential energy in the raised mass. chemical energy > electrical energy > kinetic energy > potential energy Using a dynamo: Many kinds of electric motor can convert energy in mass both directions. If an electric current flows through a motor of this kind, the axel of the motor turns; but if, alternatively, something makes the axel turn, then it produces an electric current! A motor that is used backwards like this, to make electricity, is called a dynamo. We will mention dynamos again later in this chapter. For now, we will just consider the energy changes that take place when a dynamo is used. Look again at the diagram on the left above. Now imagine that the battery is replaced by a light bulb and that everything happens backwards! When you let go of the weight, it will fall towards the ground; its potential energy will become kinetic energy. As it falls, it pulls the string which turns the axel and the pulley wheels. These wheels transfer the kinetic energy to the axel of the dynamo and make it turn quite fast. When the axel of the dynamo turns, it changes the kinetic energy into electrical energy in the wires; finally the bulb changes the electrical energy into light (and heat) energy. potential energy > kinetic energy > electrical energy > light (and heat) energy The diagrams below show three other ways to turn the axel of the dynamo and make electricity. Look at each and decide what energy changes are taking place (answers on the next page). steam engine burning fuel water tank coiled spring dynamo paddle wheel dynamo dynamo

5 Diagram 1: chemical energy (in the fuel) > heat energy (from the burning fuel) > kinetic energy (in the steam engine) > electrical energy (in the dynamo) > light energy (in the light bulb). Diagram 2: potential energy (in the water because it is raised above the ground) > kinetic energy (in the flowing water which turns the paddle wheel) > electrical energy (in the dynamo and the wires) > light energy (in the light bulb). Diagram 3: potential energy (in the coiled spring) > kinetic energy (as the spring uncurls and turns the pulley wheels) > electrical energy (in the dynamo) > light energy (in the light bulb). Using food: The energy that keeps us warm and keeps us moving, comes from the chemical energy stored in our food. Look at the photos of the cyclists. As they ride up the hill, they change the chemical energy stored in their food into kinetic energy in their legs and in the moving bicycles. As they climb higher and higher, some of this kinetic energy is gradually stored as potential energy. When they reach the top, they no longer need to use energy from their food to push the pedals around. The potential energy due to their height, gives them with all the energy they need to freewheel back to the bottom of the hill without pedalling! As they speed downhill, their potential energy is converted back into kinetic energy. The making of food and fuels: Plants are the producers at the start of all food chains (Module 2.8). In the process called photosynthesis (Module 6.12), they use light energy from the sun to make carbohydrates from carbon dioxide (in the air) and water (in the soil). After that, some of the carbohydrates are converted into lipids and proteins. In the end, all our food (Modules 12.3 and 13.11) and all our fossil fuels (Modules 13.12/13) come from plants. Now we can summarise all the energy changes that help to get the cyclists up to the top of the hill and down again: light energy (from sun) > chemical energy (stored in food) > kinetic energy (legs moving and bicycle moving uphill) > potential energy (stored as height) > kinetic energy (bicycle freewheeling downhill). bullet battery water tank metal case full of gunpowder paddle wheel 1. The pictures above show a woman firing a gun, and the cartridge that was loaded into the gun. Explain the energy changes that happen when the gun is fired. 2. Summarise the energy changes happening in diagram 4. 4 electric motor water 3. Almost all the energy we use comes from the sun. Explain why you agree or disagree with this statement. 187

6 14.4 THERMODYNAMICS Thermodynamics is the branch of science that deals with energy conversions, especially conversions that involve heat energy and mechanical energy. During the 18 th and 19 th centuries, scientists slowly learned how to measure energy. After many years they discovered two important generalisations that summarised all their measurements. These generalisations are called the first and second laws of thermodynamics. Simple versions of these laws are discussed below. The first law of thermodynamics states that when energy is changed from one form to another, the total quantity of energy remains the same. To put it another way, although the form of the energy may change, the quantity is always conserved. A popular version of this law is that energy can neither be 188 created nor destroyed. For example, when the cell in the diagram drives electricity through the wires, the quantity of electrical (and heat) energy in the wires, is equal to the quantity of chemical energy lost by the cell. In the same way, when electricity passes through the bulb, the quantity of light (and heat) energy produced by the bulb is equal to the quantity of energy lost by the electricity. Notice that, in both these examples, a bit of energy is wasted as heat. That brings us to the second law. The second law of thermodynamics states that when energy is changed from one form to another, some useful energy is always wasted. Most of this energy is lost to the surroundings as heat, that is to say, as internal kinetic energy associated with the random movement of atoms and molecules (see Module 14.2). This loss of useful energy, energy that is wasted simply to make particles jump about a bit faster, is referred to as the degradation of energy. A popular version of this law is that things always tend to run down. The second law of thermodynamics is important for engineers. Machines change energy from one form to another, or move it from one place to another. If the energy we get out of a machine is much less than the energy we put in, then we say the machine is not efficient. It is wasting valuable energy. We can measure the efficiency of any machine as the percentage of the energy output as compared to the energy input. The chart on the right represents a machine with an efficiency of 70%. You will find out more about the efficiency of particular machines later in this chapter. Dramatic energy changes! The photo below was taken inside a famous crater in Arizona, USA. The crater is 1200 metres wide and 170 m deep. It was formed about 50 thousand years ago when a meteorite 50m wide, with a mass of tons, crashed into the Earth at a speed of more than 10 km per second. Try to imagine what happened. Because of its mass and speed, the meteorite has an enormous amount of kinetic energy, so what happens to all that energy? The meteorite takes only a second or two to pass through the atmosphere but the air slows it down slightly and some of the kinetic energy is converted into heat. The air in front of the meteorite cannot get out of the way. As it compresses its temperature rises to several thousand degrees Celsius. On the ground in front of the meteorite, everything flashes into flame and rocks start to melt even before the impact. When the meteorite hits the ground, it makes a huge crater and its remaining kinetic energy (which is still enormous) converts instantly into heat. The meteorite vaporises and so do some of the surrounding rocks. The explosive blast transfers kinetic energy to the superheated atmosphere and to the huge mass of debris displaced from the crater. The hot air and the rocky debris rush out at supersonic speeds for many kilometres in all directions, destroying everything in their path. The sound of the impact travels at only 1190 kilometres per second and arrives only after the destruction! Mass and nuclear energy: Early last century, Albert Einstein showed that mass can be thought of as a very concentrated form of energy. Nuclear fission and fusion produce so much energy because a tiny amount of mass is lost when the protons and neutrons are rearranged. This lost mass is changed into a huge amount of energy as shown by Einstein s famous equation E = mc 2 (where E is the energy released, m is the mass converted and c is the speed of light!). If you want to understand why the speed of light is involved, first you will have to study a lot more physics! energy in 100% MACHINE energy out 70% energy wasted 30% 1. What do we mean by (i) the conservation of energy, and (ii) the degradation of energy? 2. Explain how the first and second laws of thermodynamics apply to the formation of the Arizona crater.

7 14.5 HEAT EXCHANGERS AND HEAT PUMPS Heat exchangers transfer heat from one fluid (liquid or gas) to another without mixing or contaminating either of them. A heat exchanger is often used in solar hot water systems. The diagram shows a solar panel on the roof of a house. The panel contains a network of pipes, painted black to absorb as much heat as possible from the sun. In cold countries, the pipes are filled with a solution of glycol that will not freeze in winter. A pump circulates this solution through the solar panel, and the hot solution then goes to a simple heat exchanger where heat, but not the glycol, is transferred to the domestic water supply. In industry, heat exchangers help to avoid wasting heat energy. There are many different designs and one example is shown on the left. One fluid (green arrows) passes through many narrow, parallel tubes. Partitions called baffles force the other fluid (blue arrows) to flow back and forth around the parallel tubes so that as much heat as possible is transferred between the two fluids. baffles Heat exchangers are used when iron ore is reduced in a blast furnace. The diagram in Module shows two heat exchangers (10) where the very hot and dirty mixture of waste gases from the top of the furnace are used to heat the clean air that is blown into the bottom of the furnace. In cold climates, heat exchangers are often used to extract waste heat from dirty industrial fluids and transfer it to clean water for heating homes and offices. Heat pumps. In heat exchangers, heat flows from a hot fluid to a colder one. A heat pump is a machine that can move heat in either direction; it can even take heat from a cold place and deliver it to a hotter one! The diagram below shows the principle of the evaporative type of heat pump. A liquid called a refrigerant, that easily evaporates, circulates through the pipes. Useful refrigerants include liquid ammonia, propane, butane and tetrafluoroethane. The heat pump works because, when a liquid evaporates and expands, the internal kinetic energy of its particles is spread out and its temperature falls; when it is compressed and condensed, the internal kinetic energy of its particles is concentrated and its temperature rises. The refrigerant is compressed by a pump (1) and passes through a series of coils (2) where it delivers heat to area A. A fan may be used to blow air over the heating coils. After this, the refrigerant evaporates through a valve (3) and the cold vapour expands through another coil (4) where it cools area B. A fan may be used to blow air over the cooling coils. Finally, the refrigerant returns to the compressor (1) and the process starts again. The table summarises how the heat pump works in various machines. In a refrigerator, the inside (area B) is cooled and heat escapes at the back (area A). In an air conditioner, a fan behind the cooling coil blows cold air into the room (area B) and a fan behind the heating coil blows the heat away outside (area A). In a room heater, areas A and B are reversed so the room is heated and the outside is cooled. In a reverse cycle air conditioner the compressor can be switched to pump in either direction. When it pumps one way the room is cooled, and when it pumps the other way the room is heated! Evaporative heat pumps are very efficient. The amount of heat they transfer depends on changes in the internal energy of the refrigerant and not on the electrical energy used by the compressor. The amount of heat transferred may be 3 or 4 times the amount of electrical energy consumed! baffles A 1. Give two examples where it is important to transfer heat between two fluids without them mixing solar panel cold glycol solution pump hot glycol solution hot water to taps Machine Area A Area B heat exchanger cold water Refrigerator Back of the fridge Inside the fridge Air conditioner Outside the house In the house 3 Room heater Inside the house Outside the house B 2. What is a heat pump? Explain briefly how a reverse cycle air conditioner works. 4

8 14.6 THE INTERNAL COMBUSTION ENGINE 1 BASICS The internal combustion engine (ICE) is an engine that burns a mixture of fuel and air in a closed cylinder. Internal combustion engines convert the chemical energy stored in fuels, such as petrol and diesel oil, into the kinetic energy that powers our motor intake valve cycles, cars and other vehicles. The petrol engine in a typical car has four cylinders. The diagram (right) shows the main parts associated with each cylinder. Let us see what happens when the engine is running. As the piston moves down, a mixture of petrol and air enters the cylinder through a valve called the intake valve. As the piston moves up again, the mixture is compressed then ignited by the spark plug. The mixture explodes and creates a rapidly expanding mixture of hot gases. For example, one of the compounds in petrol is pentane. This may burn to produce hot carbon monoxide and steam as shown in this equation. 2C 5 H O 2 10CO + 12H 2 O Because the supply of oxygen is limited, a lot of the carbon in the fuel burns to carbon monoxide instead of carbon dioxide. Carbon monoxide is poisonous so an ICE must not be used in a confined space where the exhaust gases can poison people. The expanding gases from the explosion force the piston down fast! As it moves down, the piston pushes on a connecting rod (con rod) that pivots on bearings at both ends (see Module 14.7). The con rod pushes down on a crank pin on a crankshaft. The crank pin acts like the pedal of a bicycle. The up and down movement of the con rod makes the crankshaft rotate in the same way that the up and down movement of a person s leg makes the pedals of a bicycle rotate. As the crankshaft continues rotating, it pushes the con rod up and the piston slides back up the cylinder again. As it does so, a second valve, called the exhaust valve, opens and allows the hot gases to escape into an exhaust pipe. Four distinct piston movements or strokes occur in the ICE as discussed above. These strokes are called intake, compression, power and exhaust (or suck, squeeze, bang, blow!). Intake (suck) intake valve opens as piston moves down; fuel and air sucked into cylinder. Compression (squeeze) both valves shut; piston moves up and compresses fuel and air. Power (bang) both valves shut; spark plug sparks and explosion drives piston down. Exhaust (blow) exhaust valve opens as piston rises; exhaust gases pushed out. intake valve open 190 exhaust valve open Crank shaft for 4 cylinder engine intake compression power exhaust spark plug exhaust valve cylinder piston connecting rod crank pin crank shaft 1. What are (i) an internal combustion engine, (ii) a cylinder, (iii) a piston, (iv) a con rod, (v) a crankshaft, and (vi) an exhaust pipe? 2. List, in order, the energy changes that occur in an ICE. 3 Why is it dangerous to run an ICE in a confined space? 4. How is the up and down motion of the piston converted into rotation? 5. Find out what the groves round the top of pistons are for.

9 14.7 THE INTERNAL COMBUSTION ENGINE 2 MORE DETAILS A 4-cylinder 4-stroke petrol engine is the kind of engine found in many cars. In the photo (right), the top of the engine has been removed and the four open cylinders can be seen. For the crankshaft to rotate smoothly, the pistons in these cylinders must deliver power in a well coordinated way. This is controlled by the position of the crank pins on the crankshaft and the timing of the spark in each cylinder. If cylinders are numbered from the front of the car, they are often set to fire in the order 1,3,4,2. As the piston in cylinder 1 goes down on its power stroke, piston 2 terminal connected to distributor insulator spark gap comes up on its exhaust stroke, piston 3 comes up on its compression stroke (ready for the next power stroke), and piston 4 goes down on its intake stroke. The rotation of the crankshaft is transmitted to the wheels of the car and also to one or more drive belts (or chains). These drive auxiliary machines such as the fan that helps to cool the radiator, the camshaft that opens each valve at the right time, and the distributor that coordinates the firing of the spark plugs. A terminal at the top of each spark plug (see diagram above) is connected to the distributor by a wire which delivers a high voltage at the right time. This makes a spark jump across the spark gap and explodes the mixture of air and fuel. The engine is cooled by water that circulates around the cylinders and through the radiator. The radiator is a heat exchanger at the front of the car where the water looses heat to the air. 2-stroke engines are used in motor cycles, outboard motors, chain saws and some large diesel engines. The diagram (right) shows a basic 2-stroke engine. It has no valves and the four strokes are replaced by two! The valves are replaced by openings called ports in the sides of the cylinder. As the piston rises in the cylinder, fuel and air enter through the intake port (blue arrows) and are compressed; intake and compression happen on the same stroke. When the piston reaches the top, the plug sparks and the mixture explodes. As the piston moves down, the burnt gases escape through the exhaust port (red arrow); power and exhaust happen on the same stroke. Every down stroke is a power stroke, so the 2-stroke engine delivers more power than a 4-stroke engine of the same size. It also makes more noise! Diesel engines burn diesel oil instead of petrol. They can be 4- stroke or 2-stroke engines but they have no spark plug. In a diesel engine, the explosion is caused by the heat generated when the fuel and air in the cylinder are compressed by the piston. In a diesel engine, the fuel and air are forced into the cylinders under pressure Bearings. The pictures below show the little end and big end bearings on a con rod. These allow the con rod to pivot and rotate where it is attached to the piston and the crankshaft. Large bearings support the crankshaft and allow it to rotate. Bearings must be well lubricated. little end bearing big end bearing through an injector so there is no intake valve or intake port. A diesel engine is heavier than a petrol engine of the same capacity but it is simpler and more reliable and it generates more power. It uses a safer fuel too. All ICEs waste a lot of their energy as heat. The efficiency of a typical petrol engine is volume of 400 cm only about 20% but that 3 of a diesel may be 30-40%. Diesels are used mainly in heavy road vehicles, ships and small electricity generators. 1. What are (i) drive belt, (ii) camshaft, (iii) distributor, (iv) spark plug, (v) radiator, (vi) injector, (vii) a bearing, (viii) engine capacity? 2. Compare some advantages and disadvantages of (i) 2-stroke and 4-stroke engines, and (ii) petrol and diesel engines cylinders water circulates around here top of piston spark plug exhaust port piston con rod crank shaft drive chain cooling fins intake port The capacity of an ICE is the total volume of all its cylinders. If an engine has 4 cylinders with a (or cc) each, the engine has a capacity of 1600 cc or 1.6 litres. The bigger the engine capacity, the more power!

10 14.8 THE ELECTRIC MOTOR The electric motor converts electrical energy into kinetic energy, usually in the form of rotation. Electric motors depend on the magnetic effect of an electric current. You may study this effect later in a physics course, but for now we will focus on the diagram (left) which shows the principle of the iron core brush stator magnet electric motor. The stationary, outer part of the motor is called the stator. The most important part of this is a large magnet whose north and south poles are shown in the diagram. The rotating, inner part of the motor is called the rotor. The rotor consists of a coil of wire wrapped around an iron core. The core, and a device called a commutator, are attached to an axel that is free to rotate. The two ends of the wire in the coil are connected to the two sides of the commutator. These two sides are conductors and they are separated by strips of insulator. The commutator is connected to two electric terminals through two brushes (usually made of graphite) that brush against it as it rotates. terminals When the terminals are connected to a suitable battery, electricity flows through the coil on the rotor and the rotor becomes a magnet. In the diagram, the north pole of the rotor magnet is at the top and the south pole is at the bottom. It is well known that like magnetic poles repel, and unlike poles attract. The top of the rotor is repelled by the north pole of the stator (on its left) and attracted to the south pole of the stator (on its right). The bottom of the rotor is repelled by the south pole of the stator and attracted to the north pole. These magnetic forces all combine to make the rotor rotate clockwise. Without the commutator, the rotor would stop as soon as it was horizontal. However at that point the brushes make contact with the opposite side of the commutator! Now electricity flows through the coil in the opposite direction so the poles on the rotor are reversed. They are now attracted to the opposite sides of the stator so the rotor continues to rotate clockwise. The picture (right) is the rotor from a small electric motor. This one has three coils but the principle is the same. A large electric motor may have many coils. and trucks. Electric motors are much more efficient than internal combustion engines with efficiencies of about 80 90%. A little energy is wasted by friction between the brushes and the commutator and brushes have to be replaced when they wear out. axel coil S N commutator brush stator magnet 1. In an electric motor what are (i) a stator magnet, (ii) a rotor, (iii) a commutator, (iv) a brush, (v) a terminal? 2. List all the things you can think of, that you have ever used, that have electric motors in them. 3. For a motor, what does an efficiency of 85% mean? In the modern world, electric motors are used in many ways; in fans, hair dryers, food mixers, vacuum cleaners and the compressor pumps in refrigerators and air conditioners. They are also used in DVD and CD players, the hard drives of computers, sewing machines, power tools, and the starter motors in cars Dynamos and alternators are like electric motors used backwards; they convert kinetic energy into electricity. If you use another machine to spin the axel of a suitable motor, it will generate an electric current. A dynamo (which has a commutator) produces ordinary direct current (DC). An alternator (which has no commutator) produces alternating current (AC). AC reverses direction many times a second. AC electricity is used for many purposes, and it can easily be converted to DC if required. 192

11 14.9 THE POWER STATION One of the biggest differences between our own lives and those of our ancestors is that we have electricity! The first power station to supply electricity to homes and businesses started work in London in Now every city and town all over the world, and most villages too, have wires that carry electricity from a power station, often far away, into every building. The picture on the right shows a typical coal-fired power station in Britain. How a power station works. Every power station has one or more generators like the one shown on the left. A generator is like a huge alternator (Module 14.8) and works like an electric motor in reverse. When the axel is rotated, it delivers an alternating electric current to the terminals. Thermal power stations all start by making heat, either by burning fuels or by controlled nuclear fission (Module 13.4). In some small, local power stations, diesel oil is burnt in an ordinary diesel engine and this turns the axel of a generator. In big Generator power stations, water is heated in special vessels to make high pressure steam. The steam turns huge turbines and these drive the generators. A turbine works like a fan in reverse; a moving stream of gas or liquid forces it to rotate. In big thermal power stations, steam turbines are forced to rotate by jets of high pressure steam. Steam turbine Hydro power stations use no heat, and have water turbines instead of steam turbines. The turbines are rotated by water flowing through huge pipes from specially constructed dams. The picture below shows a hydro power station in China. Power stations and energy efficiency. All the power stations we have discussed start with potential energy and convert it by stages into electrical energy. In thermal power stations, the chemical energy stored in fossil fuels, or the nuclear energy stored in atomic nuclei, is converted first to heat energy, then to kinetic energy, then to electrical energy. In hydro power stations, the potential energy of the huge mass of water that is at a higher level than the turbines, is converted first to kinetic energy and then to electrical energy. For thermal power stations, the efficiency of converting the original potential energy into electrical energy is low: about 30% for nuclear Hydro power station fuel and coal, up to a maximum of about 50% for natural gas. Evidence of the lost energy can be seen in the photo of the coal-fired power station (top right). Heat energy is lost (i) through the tall chimney as hot gases from the combustion of coal, and (ii) through the many cooling towers where steam escapes after rotating the turbines. By comparison, a large hydroelectric power station, with modern turbines, may be up to 90% efficient. In addition to the energy losses at power stations, about 5 to 10% of the electrical energy produced is lost as heat in the wires that carry it from the power station to the places where it is used. 1. What is (i) a turbine (ii) a generator (iii) alternating current? 2. List the energy conversions in a big thermal power station. 3. Try to explain why hydro power stations are much more efficient than thermal power stations. 193 Coal-fired power station

12 14.10 ENERGY SOURCES As the world population increases, and as more countries develop industrially, the demand for energy and electricity is growing fast! This raises issues of efficiency, sustainability and pollution. Fossil-fuel-fired power stations waste up to 70% of the energy they use. Fossil fuels are not renewable; one day they will run out and they are already rising in cost. Burning fossil fuels pollutes the atmosphere with oxides of sulphur and nitrogen that cause acid rain, and carbon dioxide that causes global warming. Present technology can remove the first two oxides before they get into the air but this adds to the cost of electricity. New technology is being developed to capture and store carbon dioxide too, but even if this succeeds it will increase costs. Cogeneration is a system used in some urban areas, especially in cooler climates, to improve the efficiency of power generation. Some or all of the waste heat from power stations is used to heat nearby buildings. Trigeneration is a recent development in which waste heat is processed by heat pumps to produce cooling (for refrigeration and air conditioning) as well as heating. Trigeneration plants claim overall efficiencies of 80%. Solar water heater in Australia Nuclear power stations are thermal power stations and 60 70% of the energy they use is wasted as heat. Uranium is a non-renewable fuel but it will probably not run out for a long time. No greenhouse gases are produced and radioactive materials are normally contained within the reactor buildings so there ought to be no pollution problems. Unfortunately it is impossible to avoid accidents and these can lead to widespread and dangerous radioactive pollution as in the notorious disasters at Chernobyl and Fukushima. Safe disposal of radioactive waste materials is also a difficult issue. Hydro power stations have many advantages as compared to thermal power stations; they have high efficiencies (about 80%), are cheaper to operate and have a longer life; there are no serious pollution issues, and they rely only on regular rainfall. Where the geography is suitable, hydroelectric power is almost always the best option. However the value of the power generated has to be balanced against that of the land, and sometimes communities, that are flooded when the dam is built. Dams also interfere with the flow of rivers and this may be bad for communities downstream. Finally, the lakes behind dams may fill with slit. This is often a problem where there has been deforestation and may shorten the life of the power station and cause flooding. Renewable energy sources are sources that are being generated all the time by natural processes or that we can replace as fast as we use (for example by growing crops). All the examples below are already used on a small scale and are likely to be developed further in the future. Solar photoelectric panels in Malaysian village charge batteries that power lights and fans at night Solar energy can be used on a small scale to Wind turbines generate electricity in Austria make hot water and electricity for homes. In solar furnaces, mirrors focus heat from the sun onto boilers to make steam to drive turbines. Wind, waves and tides can also be used as energy sources to drive suitable turbines. Biofuels like ethanol and biodiesel can be made from certain crops and used in IC engines. Geothermal energy is useful energy that we can get from hot rocks underground. 1. What are renewable and non-renewable energy sources? List as many examples of each as you can. Which energy sources come ultimately from the sun? List the main advantages of hydro power stations. 3. Look at the photo of the Malaysian village; explain all the energy changes that go into using a fan at night.