At one point of time, it was believed that heat was a kind of fluid that passed from hot bodies into cold ones. This imaginary fluid was called “caloric.”

Today, we know that heat is the constant motion of atoms and molecules in objects. In the air, for instance, the atoms and molecules move about freely. If they move rapidly, we say that the temperature of the air is high or that the air is hot. If they move slowly, as on a cold day, we feel the air to be cool.

Atoms and molecules can’t move about as freely in liquids and solid objects as they are in gases, but motion is still present.

Even at the temperature of melting ice, the molecules are in constant motion. A hydrogen molecule at this temperature moves with a speed of 1,950 meters per second. Can you imagine? In 16 cubic centimeters of air, a thousand million million collisions per second occur among the molecules every second!

Heat and temperature are not the same thing. A large gas burner may be no “hotter” than a small burner, but it may supply more heat because it burns more gas. Heat is a form of energy, and when we measure heat, we essentially measure nothing, but energy. Quantity of heat is measured in calories. A calorie is the amount of heat energy required to raise the temperature of one gram of water by one degree centigrade. But the temperature of a body only indicates the level to which the heat energy that it contains brings it. Temperature is indicated by a thermometer and is expressed in degrees.

When two bodies are brought together and there is no passage of heat energy from one to the other, we say that they are at the same temperature. But if heat energy is lost by one (its molecules are slowed down), while this same energy is gained by the other (its molecules move faster), we say that heat has passed from the hotter to the colder body, and that the first body was at a higher temperature than the second one.

Heat and Temperature compared

All things are made up of atoms or molecules, which are always moving. The motion gives every object its internal energy. The level of an object’s internal energy depends on how rapidly its atoms or molecules move. If they move slowly, the object has a low level of internal energy. If they move violently, it has a high level. Hot objects have higher internal energy levels than do cold objects. The words hot and cold refer to an object’s temperature.

 Given thus, the Temperature is an indication of an object’s internal energy level. A thermometer is used to measure temperature. Thermometer has a numbered scale so that temperature can be expressed in degrees. The two most common scales are the Celsius, or centigrade and the Fahrenheit scales.

The temperature of an object however, also determines whether that object will take on more internal energy or lose some when it comes into contact with another object. If a hot rock and cold rock touch each other, some of the internal energy in the hot rock will pass into the cold rock as heat. If a thermometer were placed on the hot rock, it would show the rock’s temperature falling steadily. A thermometer on the cold rock would show a steadily rising temperature. Eventually, the thermometers on the two rocks would show the same temperature. Then, no further flow of heat would occur.

Just as water flows only downhill, so heat flows only from an object at a higher temperature to an object at a lower one. The greater the difference in temperature between the two objects, the faster the heat will flow between them. The rapidly moving atoms or molecules in the hot object strike the less energetic atoms or molecules in the cold object and speed them up. In this way, internal energy in the form of heat passes from a hot object to a cold object.

It is important to recognize that temperature and heat are not the same thing. Temperature is simply an indication of the level of internal energy that an object has. Heat, on the other hand, is the energy passed from one object to another. For example, a red-hot spark from a fire is at a higher temperature than that of boiling water in a saucepan but the latter has much more heat and would burn you more severely if you split it over yourself.

Connecting concept: What is a Thermometer & who invented it?

Do you ever find yourself asking: I wonder how hot it is? Or: I wonder how cold this is? If you are interested in heat, just imagine all the questions about heat that the scientists wanted to know! But the first step in developing the science of heat is to have some way of measuring it. And that’s why the thermometer was invented. “Thermo” means “heat,” and “meter” means “measure,” so a thermometer measures heat.

The first condition about having a thermometer must be that it will always give the same indication at the same temperature. With this in mind, an Italian scientist called Galileo began certain experiments around 1592 (100 years after Columbus discovered America). He made a kind of thermometer which is really called “an air thermoscope.” He had a glass tube with a hollow bulb at one end. In this tube there was air. The tube and bulb were heated to expand the air inside, and then the open end was placed in a fluid, such as water.

As the air in the tube cooled, its volume contracted or shrank, and the liquid rose in the tube to take its place. Changes in temperature could then be noted by the rising or falling level of the liquid in the tube. So here we have the first “thermometer” because it measures heat. But remember, it measures heat by measuring the expansion and contraction of air in a tube. So it was discovered that one of the problems with this thermometer was that it was affected by variations of atmospheric pressure, and therefore, wasn’t completely accurate.

The type of thermometer we use today uses the expansion and contraction of a liquid to measure temperature. This liquid is hermetically sealed in a glass bulb with a fine tube attached. Higher temperature makes the liquid expand and go up the tube; lower temperature makes the liquid contract and drop in the tube. A scale on the tube tells us the temperature.

This kind of thermometer was first used about 1654 by the Grand Duke Ferdinand II of Tuscany.   

A Clinical thermometer: This is a special kind of thermometer used by doctors and nurses. Its scale covers only a few degrees on either side of the normal human body temperature of 37⁰C. The tube has a constriction (that is, a narrower part) just beyond the bulb. When the thermometer is placed under the patient’s tongue the mercury expands, forcing its way past the constriction. When the thermometer is removed (after 1 minute or so) from the mouth, the mercury in the bulb cools and contracts, breaking the mercury thread at the constriction. The mercury beyond the constriction stays in the tube and allows the body temperature to be read. After the reading has been taken, the mercury is returned to the bulb by shaking the thermometer.

Putting Heat to Work: The Physics behind the working of ‘Refrigerators,’ ‘Thermos flasks’ & ‘Air conditioners’:

Refrigeration:

The temperature of an object can be lowered by bringing it in contact with a colder object. The temperature difference causes heat to flow from the warmer object into the colder one. For example, ice put in an insulated chest keeps food cold by removing heat from it. Another way to remove heat from an object without using a colder object is mechanical refrigeration. Mechanical refrigeration works by changing a substance called a refrigerant which is commonly called as Freon in refrigerators and air-conditioners, from a gas to a liquid and back to a gas again. In a refrigerator, for example, a compressor squeezes a gaseous refrigerant (Freon) into a small space. The compression reduces the refrigerant’s disorder so much that it becomes a liquid as we already know that a gas on being compressed can be converted into a liquid state. The compressed liquid refrigerant then expands at a value leading to pipes in the simulated part of the refrigerator. As the pressure falls, so does the temperature and the refrigerant absorbs heat from the foods in the refrigerator. As heat flows out of the foods, their temperature falls with the heat from the foods is passed on to the liquid refrigerant & raises its temperature. The warmed refrigerant thus, becomes a gas again and then, flows through pipes back to the compressor. There, the refrigeration cycle begins again.

♥Thermos Flask:

The thermos flask is a container that keeps liquids hot or cold for many hours. It is also called a vacuum flask or Dewar flask. Thermos bottles vary widely in size, ranging in capacity from 60 ml to 60 l, and have many uses. They are commonly used to carry coffee, juice, milk or soup. Vacuum flasks are also used in scientific and medical work to store chemicals and drugs, to transport tissues and organs, and to reserve blood plasma. Sir James Dewar, a British chemist, invented the vacuum flask in 1892. He developed it for storing liquefied gases. Although his flask was designed to prevent the entry of heat from outside the container, it worked equally well in keeping liquids hot by reducing the loss of heat from the inside. The modern thermos flask has the same basic design as Dewar’s flask. It blocks the three processes through which heat is transferred – conduction, convection and radiation. A typical thermos flask has an inner container that consists of two glass flasks, one within the other. Glass does not transmit heat well, and so reduces heat transfer by conduction. The flasks are sealed together at their lips by melting the glass. Most of the air between the flasks is removed to create a partial vacuum. This vacuum hinders heat transfer by convection because it has so few air molecules to carry heat between the flasks. The facing surfaces of the flasks are coated with a silver solution. These coated surfaces act like mirrors and reflect much of the heat coming from either the inside or outside of the container back to its source. In doing so, they prevent heat transfer by radiation. Other features of thermos flasks help minimize both the loss and entry of heat. Most thermos flasks have a small mouth, which reduces heat exchange. The flasks are closed with a stopper made of cork, plastic or some other material that conducts heat poorly. The fragile inner container of a thermos flask is encased in metal or plastic for protection. A rubber collar around the mouth holds the inner container in lace, and a spring at the base serves as a shock absorber.

Air-conditioning

Cooling by evaporation is also used in an air-conditioning unit exactly in the same manner as it happens in the working and operation of a refrigerator. In an air conditioner, the warm air is pulled in through a dust filter by a fan and cooled by supplying latent heat to the liquid evaporating in the coiled pipe.

THERMAL EXPANSION: Affect of heat on three states of the Matter: When heat flows into a substance, the motion of the atoms or molecules in the substance increases. As a result of their increased motion, the atoms or molecules take up more space and thus, the substance expands. The opposite occurs when heat flows out of a substance that is the atoms or molecules move more slowly. They therefore, take up less space and the substances contracts.

All gases and most liquids and solids expand when heated. But they do not expand equally. If a gas, a liquid, and a solid receive enough heat to raise their temperatures by the same amount, the gas will expand the most, the liquid much less, and the solids, the least of all.

Affect of heat on Solids: Did you ever imagine why do railway lines are built with gaps in between the railings?  Well! The plausible explanation for the same is to provide space to the iron railings as they tend to expand during extreme summers. In fact, most solids expand when heated and contract when cooled, but only by a very small amount. Nevertheless, large forces are created, which are under some circumstances useful and in others a nuisance. The kinetic theory’s explanation is that the molecules of a solid vibrate more vigorously when heated, forcing each other a little farther apart thus, expansion in all directions results. Cooling on the other hand reduces the vibrations and the forces of attraction between molecules bring them closer together and the solid contracts slightly.

Connecting concepts: How does a thermostat work? A thermostat makes use of a bimetal strip that is made of two different metals noted for exhibiting a remarkable property of thermal expansion; an effect of heat on solids. A bimetal strip as is being made of equal lengths of two different metals called as brass and invar (an iron-nickel alloy with a very small expansion on the application of heat – the name is taken from the word ‘invariable’). The two metals are fixed together so that they cannot move separately. When heated, brass expands more than invar and to allow this, the strip bends with brass on the outside. This property & behaviour of metals with respect to heat makes the use of bimetal strips in devices called thermostats an optimal choice. However, functionally, the thermostats are meant to keep the temperature of a room or an appliance about constant. When the iron of the invar reaches the required temperature, the strip bends down, breaks the circuit at the contacts and switches off the heater. After cooling a little, the strip straightens, remakes contact and turns the heater on again. A near steady temperature results. If the control knob is screwed down, the strip has to bend more to break the heating circuit and must reach a higher temperature to bend sufficiently.

Connecting concepts: Why does a glass container break when hot liquid is poured into it? When hot water is poured into a thick glass tumbler the inside expands rapidly. The outside of the glass, however, does not expand at the same rate, because the heat takes some time to pass through the glass. (Glass is a poor conductor of heat). The result is that a strain is produced in the glass which makes it to crack. Nowadays, a type of glass known as Pyrex glass is used for making glass vessels. This material expands only slightly on heating, and so vessels made of such glass materials can hold hot liquids without cracking. Vitreosil and quartz are other substances having the same property as Pyrex. They are used in making cooking utensils.

The filaments inside electric-light bulbs must be connected to the outside wiring for the lamp to be used, and this means that the wires must pass through the glass. For this purpose a substance must be found for the filament which would expand to the same extent as glass to prevent cracking from unequal expansion. Platinum may be used, but it is expensive. An alloy of nickel and steel (45% nickel) has been found suitable which expands at the same rate as glass. It is known as platinite.

How do watches keep a constant time? Watches have a special kind of ring which continually executes periodic motion under the action of a hair spring what is called as balance wheel. The period of the oscillations of the balance wheel depends on its diameter. The seasonal change in temperature will cause the diameter to change and hence, its periodic time. The watch will lose time when the diameter increases and will gain time when the diameter decreases. The balance wheel is specially designed so that the diameter does not change with the change in temperature. The rim of such a wheel is made of a bimetallic strip and is divided in two parts: the more expanding metal on the outwards so that the wheel rim bends inwards. This compensates the outward expansion of the spokes. The decrease in temperature tends to decrease the diameter, the rim bends outwards and the diameter remains unchanged. Thus, any change in temperature does not change the period of oscillation and hence, watches show accurate timing…

Affect of heat on Liquids & Unusual behaviour of water: When water is cooled to 4⁰C it contracts, as we could expect, but as it cools further down from 4⁰C to 0⁰C, it expands, surprisingly. Water therefore has a maximum density at 4⁰C. At 0⁰C where water freezes, a considerable expansion occurs and every 100 cm³ of water becomes 109 cm³ of ice; this accounts for the bursting of water pipes in very cold weather. Further cooling of the ice causes it to contract. The unusual behaviour of water is represented by the volume temperature graph in Fig (a) below.

The expansion of water between 4⁰C and 0⁰C is due to the fact that above 4⁰C water molecules form into groups which break up when the temperature drops below 4⁰C. The new arrangement occupies a larger volume and this more than cancels out the contraction due to the fall in temperature.

Freezing of ponds (Question asked in GS-pre-2011): The behaviour of water between 4⁰C and 0⁰C explains why fish survive in a frozen pond. The water at the top of the pond cools first, contract and being denser sinks to the bottom. Warmer, less dense water rises to the surface to be cooled. When all the water is at 4⁰C the circulation stops. If the temperature of the surface water falls below 4⁰C, it becomes less dense as a result of expansion behaviour of water and thus, remains at the top, eventually forming a layer of ice at 0⁰C. Temperatures in the pond are then as shown in Figure (b above).

TRANSMISSION OF HEAT

Heat passes from one object or place to another by three methods: (1) Conduction, (2) Convection, and (3) Radiation.

I) CONDUCTION: Conduction is the flow of heat through matter from places of higher to places of lower temperature without movement of the matter as a whole. When heat travels by conduction, it moves through a material without carrying any of the material with it. For example, the end of a copper rod placed in a fire quickly becomes hot. The atoms in the hot end begin to vibrate faster and strike neighbouring atoms. These atoms then vibrate faster and strike adjoining atoms. In this way, the heat travels from atom to atom, until it reaches the other end of the rod. But during the process, the atoms themselves do not move from one end to the other.

The substances through which heat can be easily transmitted are called good conductors of heat, for example, silver, copper, mercury, etc. Silver and copper are exceptionally good. All the metals are usually good conductors. The conductivity of a metal depends directly on the number of free electrons. The larger the concentration of free electrons the more is the conductivity. The substances which do not conduct heat easily are called poor conductors such as glass, wood, cloth, air, distilled water, wax, paper, clay, plastics, corks; stone is moderately good.

Liquids and gases also conduct heat, but very slowly. Water is a poor conductor. Example: A refrigerator needs to be defrosted every now and then, as the ice which forms on the freezer being a poor conductor, slows down cooling.

Ebonite and asbestos are the worst conductors of heat and are called insulators.

Air is one of the worst conductors (that is, one of the best insulators). This is why houses keep warmer in winter and cooler in summer if they have cavity walls, which consist of two walls separated by an air space, and double glazed windows.

Materials which trap air, such as wool, felt, fur, feathers, polystyrene foam and glass fibers are also very bad conductors. Some are used as ‘lagging’ to insulate water pipes, hot water cylinders, ovens, refrigerators and the (roofs and walls) of houses. Others make warm winter clothes.

Connecting concept: A stone floor feels cold to the bare feet, but a carpet on the same floor feels warm. Stone, being the better conductor as compared to the carpet and thus, conveys heat away from the feet more rapidly than the carpet.

II)  CONVECTION:  Convection is the flow of heat through a fluid from places of higher to places of lower temperature by the movement of the fluid itself. Example: A hot stove in the room heats the air around it by conduction. This heated air expands and so is lighter than the colder air surrounding it. The heated air rises, and cool air replaces it. Then the cooler air near the stove becomes warm and rises. This movement of heated air away from a hot object and the flow of cooler air toward that object is called convection current. The current of air carries heat to all parts of the room.

Convection occurs in liquids as well as in gases. For example, convection currents will form in a pan of cold water on a hot stove. As the water near the bottom of the pan warms up and expands, it becomes lighter than the cold water near the top of the pan. This cold water sinks and forces the heated water to the top. The convection current continues until all the water reaches the same temperature.

Connecting concepts: Examples of ‘Convection’ in Live Action: If some liquid in a vessel is heated at the top, the liquid there expands and remains floating on the denser liquid beneath. No convection current is set up in this case and hence, the only way in which heat can travel downwards under these conditions is by conduction only. In order to ensure that convection should play its role rather than the conduction that the heating element in electrical appliances such as geyser or an oven is fitted near the bottom. So is true of the refrigerators in which the freezer is always fitted at the top such that the cooled air would descend down to give place to warmer air and hence, ensure the cooling of the entire unit. The room heaters warm up our homes in winters through convection and thus, are called as convection heaters.

What is the origin of coastal breezes? Coastal breezes occur due to convection. As during the day time, the temperature of the land increases more quickly than that of the sea (because, the specific heat capacity of the land is much smaller). The hot air above the land thus, rises and is replaced by the colder air from the sea. A breeze from the sea thus, results. At night, the opposite happens. Since, the sea has more heat to lose and cools down more slowly. The air above the sea is warmer than that over the land and a breeze thus blows away from the land.

III) RADIATION: Radiation is the flow of heat from one place to another by means of electromagnetic waves. As we note that in conduction and convection, the motion of particles i.e. moving atoms or molecules actually transmit the heat, in case of radiations however, heat can travel and be transmitted even through a vacuum which of course, has no particles, atoms or molecules as such.

 In any object, the moving atoms or molecules create waves of radiant energy. When this radiant energy strikes an object, it speeds up the atoms or molecules of that very object. Energy in the form of sun rays or radiations from the sun travels through space down to the earth and hence warm the earth’ surface.

 Radiations of any kind and from any source possess all the properties of electromagnetic waves say, for example, they travel at the speed of radio waves and give interference effects. When they fall on any object, they are partly reflected, partly transmitted and partly absorbed. It is the absorbed part of such radiations that is responsible for raising the temperature of an object on which they fall.

What is Green-house effect? To be noted that the Radiations are emitted by almost all the bodies above absolute zero which consist mostly of infrared radiations consisting of pretty longer wavelengths but, light and ultraviolet radiations are also emitted by very hot bodies such as the sun. In short, it can be said that the very hot objects like the sun are a source of predominantly short wavelength IRs that eons ago carried the warmth of the sun down to the earth and became responsible for the phenomenon of natural green house effect on that primitive earth otherwise; the planet earth could have been a freezingly cold and a lifeless habitat at-18 degree centigrade.

Over the years unfortunately, due to wanton human activities, leading to environment pollution, the average temperature of the earth has been increasing abnormally leading to artificial or man made green house effect. As the short wavelength IR coming down from the sun when fall on any object on the earth, a part of them are being reflected back by the earth objects in the form of longer wavelength IR which are trapped up there in the atmosphere by the gaseous cloud of CO2 instead of allowing their escape back into the space and hence, an abnormal heating up of the atmosphere results. The term green house effect is thus given that the light and short-wavelength infrared radiations from the sun penetrate the glass of a greenhouse and are absorbed by the soil and plants inside it, raising their temperature. They in turn emit infrared radiation but, because of their (soil/plants) being relatively at low temperature as compared to the sun, the reflected or emitted IRs have a longer wavelength (less energetic) and thus, cannot pass out through the glass. The greenhouse thus, acts as a ‘heat-trap’ and its temperature rises. This is exactly what that is happening to our planet earth today as a consequence of what we call as “thermal pollution”.

Since, water vapors in the lower layers of the atmosphere exhibit the same ‘selective absorption’ effect as a greenhouse window actually does and prevent long wavelength infrared radiations emitted by the earth from escaping.

It has been estimated that if the combustion of fossil fuels and the resulting increase of carbon dioxide in the atmosphere should lead to a rise in the earth’s average temperature of only up to 3.5⁰C, both geographical features and climates worldwide could alter dramatically as well as drastically putting every life on the planet earth virtually at the brink of disaster.

COMBUSTION & OPERATION OF HEAT ENGINES: What is combustion?

A substance combining with oxygen is always oxidized. All such combinations result in the evolution of heat energy. If the rate of reaction is slow, only heat energy & no light is given off and the process is called slow oxidation. But if oxygen combines with the other substance so rapidly that both light energy as well as heat is evolved, the process is known as combustion. In short, combustion is the burning of a substance in the presence of oxygen to produce both heat and light energy. Say for example, the rusting of iron is a slow oxidation whereas; the burning of wood however, is an example of combustion. Total amount of energy released by the oxidation of a substance is the same regardless of the rate of the combustion or oxidation process.

Before a substance can burst into flames it must be heated to a definite temperature. This minimum temperature is known as the kindling temperature of a solid or the flash point of a liquid.

Connecting concepts: “Fire Extinguishers” & principle of their Action:

As we know that there are three essential conditions for fire to occur i.e. heat which is required to let a substance reach a definite critical temperature before it bursts into a flame, Oxygen without which no fire can be possible and Fuel, a substance that actually burns. Given thus, a fire can be controlled say, even in a conventional manner by blocking any one of the above three conditions say, by controlling the heat (by cooling), fuel blocking (by cutting the fuel supply) or by de-oxygenation (by excluding oxygen). A fire extinguisher thus, actually works by removing or blocking at least, one of the above three conditions necessary for a fire to continue and invariably, makes use of the third option to quell the fire i.e. by blocking flow of oxygen to the fire.  

The simplest fire extinguishers however, contain water, which when propelled onto the fire, cools it down. Unfortunately, water extinguishers cannot be used on electrical fires, as there is a danger of electrocution, or on burning oil, as the oil will float on the water and spread the blaze. Given this limitation of the simplest water using fire extinguishers,

Many of our domestic extinguishers thus, contain liquid carbon dioxide under pressure and usually work by cutting the supply of oxygen to the fire. When the handle of the extinguishing device is pressed down, carbon dioxide is released as a gas that blankets the burning material and prevents oxygen from reaching it. Dry extinguishers spray powder, which then releases carbon dioxide gas. Wet extinguishers on the other hand, are often of the soda-acid type; when activated, sulphuric acid mixes with sodium bicarbonate, producing carbon dioxide. The gas pressure forces the solution out of a nozzle, and a foaming agent may be added to produce foam.

Some extinguishers contain halons (hydrocarbons with one or more of their hydrogens substituted by a halogen such as chlorine, bromine or fluorine). These are very effective at smothering fires, but cause damage to the ozone layer in the atmosphere.  

HEAT ENGINES

A heat engine is a device which changes the heat energy, obtained by burning of a fuel into kinetic energy. In an internal combustion engine, such as petrol, diesel or jet engines, the fuel is burnt in the cylinder or chamber where the energy changes occur i.e. heat energy is converted into the kinetic energy. This is not so in other engines, such as the steam turbine.

Petrol Engines: Petrol engines make use of the rapid expansion of heated gases to force a piston to move inside the cylinder or chamber.

In a four-stroke engine, the action takes place in the following manner:

On the intake stroke, the piston is moved down (by the starter motor in a car or the kick start in a motor cycle turning the crankshaft), so reducing the pressure inside the cylinder. The inlet valve of the cylinder opens and the petrol-air mixture from the carburetor is forced into the cylinder by atmospheric pressure.

On the compression stroke, both valves are closed and the piston move up, compressing the mixture.

On the power stroke, a spark jumps across the points of the sparking plug and explodes the mixture, forcing the piston down.

On the exhaust stroke, the outlet valve opens and the piston rises, pushing the exhaust gases out of the cylinder.

The crankshaft turns a flywheel (a heavy wheel), whose momentum keeps the piston moving between one power stroke and the next.

Most cars have at least four cylinders on the same crankshaft; each cylinder ‘fires’ in turn, in the order 1-3-4-2 giving a power stroke every half-revolution of the crankshaft. As a result the running is smooth.

Two-stroke engines work in mopeds and small boats. The cycle of operations is completed in two strokes. Valves are replaced by ports on the side of the cylinder which are opened and closed by the piston as it moves.

The efficiency of petrol engines is just about 30 per cent, which means that only 30 per cent of the heat energy supplied becomes kinetic energy: much of the rest is lost with the exhaust gases.

Diesel Engines: The operation of two and four-stroke diesel engines is similar to that of the corresponding, petrol-driven engines. Diesel (fuel oil) is used instead of petrol; however there is no sparking plug and the carburetor is replaced by a fuel injector.

Air is drawn into the cylinder on the down stroke of the piston and on the upstroke it is compressed to about one-sixteenth of its original volume (which is twice the compression in a petrol engine). This very high compression increases temperature of the air considerably (mechanical energy is changed to heat – just as the air in a bicycle pump gets hot when it is squeezed). Thus, when at the end of the compression stroke, fuel is pumped into the cylinder by the fuel injector, it ignites automatically. The resulting explosion drives the piston down on its power stroke.

Diesel engines, sometimes, called compression ignition engines, though heavier than petrol engines are reliable and economical. Their efficiency of about 40 per cent is higher than that of any other heat engine.

Jet Engines and rockets: Jet engines (or gas turbines) may be of different kinds. In a turbo-jet, an electric motor sets the compressor rotating to start the engine. The compressor is a kind of fan; it blades draw in and compress air at the front of the engine. Compression raises the temperature of the air before it reaches the combustion chamber. Here kerosene (paraffin) fuel is injected and burns to produce a high-speed stream of hot gas which escapes from the rear of the engine, so thrusting it forward. The exhaust gas also drives a turbine (another fan) which is on the same shaft as the compressor and which keeps it turning once the engine is started.

Turbo-jet engines have a high power-to-weight ratio, that is, they produce large power for their weight, and are ideal for use in aircraft.

Rockets like jet engines, obtain their thrust from the hot gases they eject when they burn a fuel. They can travel where there is no air, however, since they carry the oxygen needed for burning the fuel instead of taking it from the atmosphere as a jet engine does.

Steam Turbines: Steam turbines are used in power stations and on ships. Steam produced in a separate boiler enters the turbine and is directed by the stator (sets of fixed blades) on to the rotor (sets of blades on a shaft that can rotate). The rotor revolves and drives whatever is connected to it whether it is an electrical generator or a ship’s propeller. The steam expands as it passes through the turbine and the size of the blades increases along the turbine to allow for this. Rotary engines like the steam turbine run more smoothly than piston (reciprocating) engines do.