• Unit 11: Applications of laws of thermodynamics

    HEAT AND THERMO-DYNAMICS    Thermal effects



    Key unit Competence

    By the of this unit, the learner should be able to evaluate applications of first and second laws of thermodynamics in real life.

    Unit goals

    By the end of this unit, I will be able to:

    differentiate between Internal energy and total energy of a system.

    * explain the work done by the expanding gas.

    * state the first law of thermodynamics.

    * state the second law of thermodynamics.

    * explain thermodynamic processes in heat engines.

    Introduction

    Before, you learnt that:

    • Heat is a form of energy.

    • Heat can be changed / transformed from one form to another.So, if in a system heat changes from one form to another, its called thermal dynamic system.

    The systems to discuss in this unit include refrigerators, heat pumps, car engines. Remember that heat is the measure of total internal energy of a body. This means that particles of a body vibrate because of energy they have.

    Thermal energy and internal energy

    Activity 1

    Have you ever boiled water on a sauce pan with a cover?

    Describe what happens to the cover when water boils?

    When water boils, the vapour pushes the cover off the sauce pan. You have already seen in your early secondary that heat is a form of energy. Therefore, when this saucepan is heated, the heat gained is used to boil off the water and extra work is done to push the sauce pan cover. This total heat energy supplied is called thermal energy.

    Science in action! Discover

    • In groups of five, explain why an inflated bicycle tube bursts when it is left on sunshine for a very long time.

    • Similarly explain why a balloon full of air bursts as it rises in the atmosphere.

    • Note down your observation in your exercise books.You already know the characteristics of the three states of matter that is; solids, liquids and gases. In this unit, we shall be interested in studying the behaviour of molecules in matter.

    When the bicycle tube is left exposed to sunshine, it gets heated and the molecules in the gas gain energy and hence its kinetic energy increases. As a result, they collide frequently with the walls of the tube and therefore exert high pressure on the walls and the tube bursts.

    The same thing happens with the balloon in air.

    The energy possessed by the molecules of the gas is called internal energy of the gas. This energy depends on the temperature of the gas. When a gas is heated its temperature increases and hence the average speed of molecules also increases increasing the internal energy of the gas. Further increase of heat supplied means that extra energy is absorbed by the molecules of the gas, hence expanding and pushing the tyre. As a result the tyre bursts.

    Activity 2

    List down three utensils used for cooking food in your homes.Describe how these utensils are used to cook the food. Are they always left open while cooking?

    In all the above, there exists energy exchanges and such things are called systems. Systems can either be closed or open. When water is being boiled in an open sauce pan, vapour is allowed to escape. It is an example of an open system. When someone cooks meat using a closed container, no gas is allowed to escape. Its an example of a closed system.

    Whenever heat flows to or from a system, or work is done on or by a system, there is a change in the energy of this system. The study of the processes that cause these energy changes is termed thermodynamics.

    Thermodynamic systems

    Activity 3: Discover

    In groups of five, discuss how heat is transferred in the three states of matter. Do you think heat can be transferred from one state of matter to another? For example from gas to a solid or from a solid to a liquid or from a liquid to a gas and vice versa?

    Heat is the energy that flows by conduction, convection or radiation from one body to another because of a temperature difference between them. These bodies where exchange of heat to other forms of energy occurs are called thermodynamic systems.

    A thermodynamic system consists of a fixed mass of matter, often a gas, separated from its surroundings, perhaps by a cylinder and a piston. For example heat engines such as a petrol engine, a steam turbine and jet engine all contain thermodynamic systems designed to convert heat into mechanical work. Head pumps and refrigerators are thermodynamic devices for transferring heat from a cold body to a hotter one.

    In such devices, energy is transferred from one system to another by a force moving its point of application in its own direction.

    The energy of a system, whether transferred to it as heat or work is termed as the internal energy of the system.

    When there is no heat transfer between two systems, that is, the two are at the same temperature, they are said to be in thermal equilibrium.

    Activity 4

    Have you ever observed smoke moving in the atmosphere.Move outside class and go towards the kitchen and observe how smoke is moving. Describe briefly how it moves.Why does it move like that?

    You have already seen in your early secondary that molecules in a gas are more further apart and are always in constant random motion while moving at high speed colliding with one another and the walls of the container, and when the gas is heated their speed increases.

    Smoke particles are always in random motion and when they are moving in air, they collide with air molecules and a zigzag pattern is seen.

    Similarly, when smoke is put in a container and then closed, the particles are seen to be in a random motion. Smoke is an example of a real gas.

    In thermodynamics, we are mainly interested in ideal gas. At higher temperatures, a real gas behaves like an ideal gas.

    Activity 5

    Have you ever heard of an ideal gas?What are the differences between a real gas and an ideal gas?

    When a gas is heated, molecules move further apart and the forces of attractions between them become negligible and the gas becomes ideal.

    When the molecules become further apart, the gas expands and the volume of the individual molecule becomes so small compared to the entire volume of the gas. It therefore becomes negligible compared to the volume of the gas and the gas becomes ideal.

    When the molecules are colliding with one another, collisions are assumed to be perfectly elastic. In this case, the gas becomes ideal because for a real gas we expect to have time between approach and separation during collision.

    Exercise

    What is a perfectly elastic collision?

    Work done by an expanding gas

    Activity 6: Discover

    Explain why a pump gets hot when one pumps air into a tyre.

    When you compress air in a bicycle pump, your muscles transfer energy to the handle, which in turn transfers energy to the molecules of air in the pump. This additional energy makes the molecules move faster. As they are compressed into a smaller space, they also collide more often with the wall of the pump, so they transfer more energy to the metal wall and it becomes hot.

    We have already seen how heat can be transferred, so you probably have a good idea what Q means. Work is simply a force Multiplied by distance in the direction of force.

    A gas can be heated by compressing it, for example with a bicycle pump. Hence the temperature of the gas can be raised either by doing work in compressing it or by heating it. Likewise the temperature can be lowered by either making the gas do work in expanding or by extracting heat from it.

    Consider a mass of gas enclosed in a cylinder by a frictionless piston of cross-section area A which is in equilibrium under the action of an external force, F, acting downwards (i.e pressing the gas). Let this force be infinitely reduced so that the piston is pushed up by the gas a distance Δx, which is so small that the pressure of the gas is virtually unchanged by the expansion

    If the gas is heated, it will expand and push the piston thereby doing work on the piston. If the piston is pushed down, on the other hand, it does work on the gas. This is an example on how work is done by a thermodynamic system.

    The piston must be held in position by force PA (by definition of pressure).

    The external work done by the gas,

    ΔW against F will be;ΔW = FΔx = PAΔx= PΔV,

    ΔV = FΔx is the increase in volume of the gas.

    Suppose that the pressure is kept constant during the expansion, and the gas expands from V1 to V2, then the total work done by the gas is given by calculus as; W = ʃdW= ʃPdW

    It follows that; W = P (V2 – V1)

    Specific heat capacities of gases

    (i) Weigh a given quantity of a gas.

    (ii) Confine the gas in a closed container.

    (iii) Place a thermometer in the container.

    (iv) Using a given source of heat, supply heat to a gas by keeping its volume constant.

    (v) Record the change in temperature using the thermometer.

    (vi) You can do this by closing the container.(vii) Calculate the quantity of heat supplied.

    (viii) Repeat the above procedures by keeping the pressure constant.

    (ix) Calculate the heat supplied for the same temperature change as in above.

    (x) Compare the two quantities of heat supplied.

    Do you notice that the heat needed at constant pressure is higher than that at constant volume?

    Why do you think it is so?

    Gases are considered to have a number of specific heat capacities. A change in temperature of a gas is likely to cause large changes in pressure and volume of the gas but for solids or liquids, the change in pressure is neglected.

    In your early secondary, you have already seen that heat energy is calculated by measuring the mass of liquids and solids. However, in gases, we replace the mass with the number of moles of a gas.

    When the specific heat capacity of a gas is measured in terms of its moles, it is known as principal specific heat capacity. There are two important heat capacities: the molar heat capacity and constant volume (Cv ) and molar heat capacity at constant pressure (CP).

    The principal molar heat capacity at constant volume (Cv) is defined as the heat required to increase the temperature of one mole of a gas at constant volume by one Kelvin.

    The principal molar heat capacity at constant pressure (CP) is the amount of heat required to increase the temperature of one mole of a gas at constant pressure by one Kelvin.

    The molar heat capacities have units Jmol—1K-1

    Since at constant volume, the work done by a gas is zero from (W=P∆V, ∆V, =0), then it is evident that the principal K molar heat capacity at constant pressure, CP is greater than that at constant volume, Cv. Why?

    The reason as to why this is so can be done by considering cylinders in (a) and (b) each initially containing one mole of gas at temperature T and pressure, P. The piston in (a) is fixed and that in (b) is frictionless and can move freely but has a constant force applied to it. If heat is supplied to each until thetemperature has risen by one Kelvin, the increase of internal energy must be the same in each case (Since the temperature rise is the same).

    All the heat supplied in case (a) is used to increase the internal energy of the gas. In (b), however, the gas expands and work is done by it on the piston; the heat supplied in this case equals the increase of internal energy plus the work done in the expansion of the gas.

    The first law of the thermodynamics

    We have already seen that when a quantity of heat ∆Q is supplied to a gas, two things happen: (i) the heat supplied may increase the internal energy, U of the gas and (ii) the gas may expand and do some work, W in moving the piston.

    The magnitude of internal energy depends on; the temperature of the gas i.e. the internal energy is high at a high temperature and low at low temperature.

    The amount of heat supplied is equal to the change in internal energy of the gas plus the work done by the gas.i.e. ∆Q = ∆U + ∆W (i)

    The above is the statement of the first law of thermodynamics.

    since ∆W = P∆V,

    It follows that ∆Q = ∆U + P∆W

    When n moles of a gas are considered, the amount of heat supplied at constant pressure is nCPΔT, whereas the amount of heat supplied at constant volume would be nCvΔT.

    Relationship between Cp and Cv

    .Consider one mole of a gas at a pressure P, temperature T and volume V, heated to cause the same temperature change, ∆T first at constant volume and secondly, at constant pressure.

    Applications of first law of thermodynamics in particular gas changes

    Isovolumetric process (Isochoric process)

    Activity 7

    (i) Have you ever heard of an isovolumetric or isochoric process?Study the Figure and answer questions that follow;

    (ii) What substance is likely to be getting cooked using the can on the left? Give a reason for your answer.

    (iii) Why is the can covered and not open?

    (iv) If one tried to open it while its on fire, what do you think would happen?

    The above can be used to boil liquids for example milk effectively. As the liquid inside the can heats up, its pressure increases, but its volume stays the same (unless, of course, the can explodes). It therefore means that all the processes inside the can take place at constant volume. They are called isovolumetric or isochoric processes.

    When the pressure in a system changes but the volume is constant, you have what is called an isochoric process. An example of this would be a simple closed container, which can’t change its volume as seen above.

    Activity 8

    How much work does the fire do on the can? In this case, the volume is constant, from the law of thermodynamics, no

    work is being done since ∆V = 0

    This process takes place at constant volume and since

    ∆V = 0, ∆W = sP∆W = 0

    ∆Q = ∆U + CV ∆T

    In this process, the energy supplied is used to increase the internal energy since the internal energy is independent of the volume.

    Isobaric process

    Activity 9Study the Figure here showing a woman preparing sauce.

    (i) What name of the utensil is she using?

    (ii) Why is the utensil open?

    (iii) Do you think it is good to use an open utensil to boil liquids?

    Boiling liquids in open containers is very safe for example if the container is closed, pressure may build up in the container and force it to burst. Boiling in open containers imply that the pressure of the substance is kept constant. This process is called an isobaric process. An isobaric process is the one that occurs at constant pressure.

    Heating of water in an open vessel and the expansion of a gas in a cylinder with a freely moving piston are typical examples of isobaric processes. In both cases, the pressure is equal to atmospheric pressure. For example when water

    is being heated, its volume increases and the pressure inside the container is constant since the number of collisions between water molecules and the walls of the container is constant.

    The same process occurs when a gas enclosed in a cylinder with a frictionless piston is heated such that at any time, the gas pressure equals the external pressure.

    Work done by the gas in the isobaric process

    When the gas expands from volume V1 to V2,

    ΔW = PΔV = P(V2 – V1)

    From a PV graph, the work equals the area under the graph.

    In this process, the energy supplied is used to increase the internal energy since the internal energy is independent of the volume.

    ∆Q = ∆U + ∆W

    ∆Q = CV∆T + P(V2- V1)

    Isothermal change (constant temperature)

    Activity 10

    (i) Get a polythene bag and fill it with air.

    (ii) Insert a thermometre in the bag and place it in the ice-water mixture.

    (iii) Note what happens.Do you notice that the gas condenses and the volume decreases?What happens to the temperature recorded by the thermometer?

    You can notice that the temperature remains constant. This change is called Condensation and is an example of isothermal process.

    Do you think this process is reversible?

    An isothermal change can be reversible. An isothermal change is the change that occurs at constant temperature. It is either a compression or expansion of a gas at a constant temperature.

    If the volume increases, the pressure must decrease and if the volume decreases, the pressure must increase


    Conditions necessary for an isothermal process to occur

    Activity 11: Discover

    (i) On a cold day, how do you keep yourself warm?

    (ii) In groups of five, describe how you can keep the temperature of the system constant.

    For an isothermal process to take place, the gas must be contained in a thin –walled heat conducting vessel/container in good thermal contact with a constant temperature.

    The process must be carried out slowly to allow time for heat exchange to take place

    Work done in Isothermal Change

    Activity 12: Science at work

    (i) Have you ever tried to boil water in a closed sauce pan?

    (ii) What happens to the cover when the vapour starts to come off the water?

    (iii) Notice that this vapour pushes the cover off the pan

    We say that the vapour does work on the cover.

    From the first law of thermodynamics, ∆Q = ∆U + ∆W.

    When the volume of gas changes by ∆V at constant temperature then the

    pressure has also to change so that the ideal gas equation is satisfied.

    From the above equation, the following can be drawn;

    (i) When the gas expands (i.e V2 > V1), then W is positive.

    (ii) When the gas is compressed (i.e V1 >V2), thus W is negative, meaning that work is done on the gas in compressing it.

    Adiabatic change

    Activity 13

    (i) Pump a bicycle tyre using a pump until it is full.

    (ii) Open the tube slowly while placing your other hand in its path.

    (iii) Do you notice that the the air coming out of the tyre is hotter than the surrounding air?

    As one pumps, the air molecules are compressed into a smaller space. They also collide more often with the wall of the pump, so they transfer more energy to one another and become hot. No heat has been supplied to the system. It is called an adiabatic compression.

    Activity:14

    (i) Now pump the tyre and leave it standing for sometime.

    (ii) Make sure you don’t expose it to sun shine.

    (iii) Open the valve after two hours while your hand is placed in the path of air from it.

    Do you notice that the air is colder than its surrounding?

    Heat has been lost but not to the surroundings. When the air is left standing, expansion occurs. This is associated with a decrease in temperature. It is called an adiabatic expansion.

    An adiabatic change is process in which no heat enters or leaves the gas system. It is either an expansion or a compression.

    Since ∆Q = ∆U + P∆V and ∆Q = 0

    ∆Q = CV∆T + P∆V Or ∆U = P∆V

    If the gas expands, it does work, its internal energy is reduced and hence the temperature is lowered.

    If the gas is compressed, work is done on the gas, its internal energy will increase and therefore its temperature rises.

    Conditions that are necessary for an adiabatic change to occur

    Activity 15

    How do you always protect yourself from a bad weather?

    On a cold day, we always wear woolen jackets to protect ourselves from coldness. Therefore no heat is either lost to the surrounding and or gained. In this case, an adiabatic process is achieved.

    For an adiabatic process to be achieved, the gas must be contained in a thick –walled and perfectly insulated isolated container.

    The process must be carried out rapidly to avoid any possible heat exchanges between the gas system and the surroundings

    Equations for an adiabatic change

    From the first law of thermodynamics, ∆Q = ∆U + P∆V

    For an adiabatic process, ∆Q = 0, and for 1 mole, ∆U = CV∆T1, CV∆T = 0 (i)

    For infinitesimal small changes, but from the ideal gas equation, for one mole,

    Activity 16

    Derive the expression for temperature and pressure for adiabatic change i.e T γ P1– γ = a constant

    Example I

    A gas has a volume of 0.02 m3 at a pressure of 2 x 105 Pa and a temperature of 27oC. It is heated at constant pressure until its volume increases to 0.03 m3.

    Calculate the:

    (i) External work done.

    (ii) New temperature of the gas.

    (iii) Increase in internal energy of the gas if its mass is 16g, its molar heat capacity at constant volume is 0.8Jmol-1K-1 and the molar mass is 32g.

    Example 2

    An ideal gas at 17o C has a pressure of 760mm Hg is compressed (i) isothermally (ii) a diabatically, until its volume is halved.

    Calculate in each case the final temperature and pressure of the gas. Assume that Cp= 2100Jmol-1K-1 and CV = 1500Jmol-1K-1

    Exercise

    1. Distinguish between isothermal and adiabatic changes, clearly stating the conditions under which they occur in practice.

    2. Define the two principal molar heat capacities of a gas and derive an expression relating the two. Explain the difference between these two principal molar heat capacities.

    3. A quantity of oxygen is compressed isothermally until its pressure is doubled, it is then allowed to expand adiabatically until its original volume is restored. Find the final pressure in terms of its original pressure. Draw a PV diagram for the above processes.

    4. 0.45m3 of a gas at a temperature of 15o C expands adiabatically and its temperature falls to 4o C.a) What is the new volume if γ = 1.40b) The gas is then compressed isothermally until the pressure returns to its original value. Calculate the final volume of the gas.

    5. A vessel containing 2m3 of air initially at a temperature 25o C and pressure 760mmHg, is heated at constant pressure until its volume is doubled. Find

    (a) the final temperature

    (b) the external work done by the air in expanding,

    (c) the quantity of heat supplied.

    6. (Assume that the density of air at s.t.p is 1.293kgm-3 and that the principal molar heat capacity of air at constant volume is 20.4Jmol-1K-1.An ideal gas at a temperature 45o C and pressure 1.0 x 105Nm-2occupies a volume of 2.0 x 10-3 m3. It expands adiabatically to twice its volume. Find the final temperature and pressure. Represent this process on PV- diagram.(Take γ = 1.40)

    Second Law of thermodynamics

    The second law of thermodynamics can be stated in many equivalent ways, each expressing a different facet of its meaning. There are so many different forms of it because it is of such significance. William Thomson (Lord Kelvin) in 1851 stated it in this form;

    “no heat engine can perform a cyclic operation whose only result is to convert internal energy into mechanical energy”

    The second law was stated by Rodolph Clausus in 1850 in this form;

    “no refrigerator (or heat pump) can transfer internal energy from a cold reservoir to a hot reservoir without some external agent doing work.”

    Applications of the second law of thermodynamics

    Heat engines

    Activity 17

    * Have you ever heard of an engine?

    * Where exactly do we find engines?

    * What do you think an engine is?

    * How do you think the engine operates?

    Any device which will convert heat cyclically into mechanical work is called a heat engine.The material which, on being supplied with heat, performs mechanical work is called the working substance. It is a machine, which changes heat energy, obtained by burning a fuel, to kinetic energy. In an internal combustion engine, e.g petrol. Diesel, jet engine, the fuel is burnt in the cylinder chamber where the energy change occurs. This is not so in other engines e.g steam turbines. All practical engines use one of the two working substances either water (in the reciprocating steam engine and the turbine) or air (in the internal combustion engine). The working of an engine is a thermodynamic process in which at one point no heat enters or leaves a system during expansion or compression of the fluid composing the system.

    The Carnot Cycle

    The cycle of operations through which the working substance has been taken is calledcarnot’s cycle

    There are two main ways in which the carnot cycle differs from that of any practical engine.

    First, the heat absorbed is all taken in at one constant temperature and all the heat rejected to the sink is given out at another constant temperature. In this manner, it is very much simpler than any practical engine.

    Secondly, as no work is done at any stage in overcoming frictions, and no heat is lost to the surrounding, the cycle is completely reversible. This means that if we had carried out the whole sequency of changes in the reverse order, every operation would have been exactly reversed. This is called an ideal heat engine because in all practical engines work, is done in overcoming friction and heat is lost to the surroundings.

    Otto Cycle and Diesel Cycle

    Otto Cycle

    An Otto cycle is an idealized thermodynamic cycle which describes the functioning of a typical spark ignition reciprocating piston engine, the thermodynamic cycle most commonly found in automobile engine

    The Pressure Volume diagram above represents the Otto cycle which has the following strokes; the intake (A) stroke is performed by an isobaric expansion, followed by an adiabatic compression (B) stroke (along 1-2). Through the combustion of fuel, heat is added in an isovolumetric process (2-3), followed by an adiabatic expansion process ( 3-4), characterising the power (C) stroke. The cycle is closed by the exhaust (D) stroke, characterized by isovolumetric cooling and isobaric compression processes.

    The processes are described by:

    Process 1-2 is an isentropic compression of the air as the piston moves from bottom dead centre (BDC) to top dead centre (TDC)

    .Process 2-3 is a constant –volume heat transfer to the air from an external source while the piston is at top dead centre. This process is intended to represent the ignition of the fuel –air mixture and the subsequent rapid burning.

    Process 3-4 is an isentropic expansion (power stroke).

    Process 4-1 completes the cycle by a constant-volume process in which heat is rejected from the air while the piston is a bottom dead centre.

    The Otto cycle consists of adiabatic compression, heat addition at constant volume, adiabatic expansion, and rejection of heat at constant volume. In the case of a four-stroke Otto cycle, technically there are two additional processes; one for the exhaust of waste heat and combustion products (by isobaric compression), and one for the intake of cool oxygen –rich) air (by isobaric expansion); however, these are often omitted in a simplified analysis. Even though these two processes are critical to the functioning of a real engine, wherein the details of heat transfer and combustion chemistry are relevant, for the simplified analysis of the thermodynamic cycle, it is simpler and more convenient to assume that all of the waste-heat is removed during a single volume change.

    Diesel Cycle

    The diesel cycle is the thermodynamic cycle, which approximates the pressure and volume of the combustion chamber of the Diesel engine, invented by Rudolph Diesel in 1897. It is assumed to have constant pressure during the first part of the “combustion” phase V2to V2 in the diagram, below). This is an idealised mathematical model; real physical diesels do have an increase in pressure during this period, but it is less pronounced than in the Otto cycle. The idealized Otto cycle of a gasoline engine approximates constant volume during that phase, generating more of a spike in a P-V diagram.

    The Idealised Diesel Cycle


    From P-V diagram for the Ideal Diesel cycle, the cycle follows the numbers 1-4 in clockwise direction.

    The image on the top shows a P-V diagram for the ideal Diesel cycle; where P is pressure and V is specific volume. The ideal Diesel cycle follows the following four distinct processes (the colour references refers to the colour of the line on the diagram):
    Process 1-2 is isentropic (adiabatic) compression of the fluid (blue colour).
    Process 2-3 is reversible (isobaric constant pressure heating (red)
    .Process 3-4 is isentropic (adiabatic) expansion (yellow).
    Process 4-1 is reversible constant volume cooling (green).

    The Diesel is a heat engine; it converts heat into work. The isentropic processes are impermeable to heat; heat flows into the loop through the left expanding isobaric process and some of it flows back out through the right depressurising process, and the heat that remains does the work.

    Work in (Win) is done by the piston compressing the working fluid.
    Heat in (Qin) is done by the combustion of the fuel.

    Work out (Wout) is done by the working fluid expanding on to the piston (this produces usable torque).
    Heat out (Vout) is done by venting the air.

    A heat engine is a machine, which changes heat energy, obtained by burning a fuel, to kinetic energy. In an internal combustion engine, e.g petrol, diesel, jet engine, the fuel is burnt in the cylinder of chamber where the energy change occurs. This is not so in other engines e.g steam turbine.

    Petrol engine

    Activity 18

    (i) How many types of fuels do vehicles use to operate?
    (ii) Have you ever heard of vehicles which use petrol in order to operate?
    (iii) List down four vehicles which use petrol.(iv) What type of engine do they have?

    Many vehicles use petrol in order to move. Such vehicles are small cars and motorcycles. The engine they have is called a petrol engine since it uses petrol to operate. It operates by moving the piston. The upward and downward movement of the piston is called a stroke.

    a) Four – stroke engine: On the intake stroke, the piston moves down (due to 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 value 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 moves 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 power strokes.

    Most cars have atleast 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. Smoother running results.

    b) Two-stroke engine: This is used in mopeds, lawnmovers and small boats. Valves are replaced by ports on the side of the cylinder which are opened and closed by the piston as it moves.

    Diesel engine

    Activity 19

    (i) Have you ever heard of vehicles which use diesel in order to move?
    (ii) What kind of vehicles are they?
    (iii) What is the name of the engine in such vehicles?

    The engine which uses diesel is called a diesel engine. A diesel engine can operate by making two or more strokes.

    The operation of two and four stroke diesel engines is similar to that of the petrol varieties. However, fuel oils is used instead of petrol, 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 the temperature of the air considerably and 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. (You may have noticed that the air in a bicycle pump gets hot when it is squeezed.The same applies here.)

    Activity: 20

    State the advantages of a diesel engine over a petrol engine.

    Diesel engines, sometimes called compression ignition (C.I) engines, though heavier than petrol engines, are reliable and economical. Their efficiency of about 40% is higher than that of any other heat engine. A disadvantage of the diesel engine is that its higher compression ratio means that it needs to be more robust, and is therefore more massive.

    The Refrigerator

    Activity: 21

    * How many of you have seen a refrigerator?
    * With the help of a teacher visit any place where there is a refrigeration and observe it carefully.
    * How useful is it to our daily lives?
    * Who can describe how it works?
    * Write your suggestions in the notebook.


    You should know that

    A refrigerator is used to cool substances. It cools things by evaporation of a volatile liquid called Freon. The coiled pipe around the freezer at the top contains Freon which evaporates and takes latent heat from the surroundings so causing cooling. The electrically driven pump removes the vapour and forces it into the heat exchanger (pipes with cooling fins outside the rear of the refrigerator). Here the vapour is compressed and liquefies giving out latent heat of vapourization to the surrounding air. The liquid returns to the coils around the freezer and the cycle is repeated. An adjustable thermostat switches the pump on and off, controlling the rate of evaporation and so the temperature of the refrigerator.

    Review exercise

    1. (a) (i) What is meant by a reversible isothermal change?

    (ii) State the conditions for achieving a reversible isothermal change.

    (b) (i) What is meant by adiabatic change?

    (ii) An ideal gas at 27oC and a pressure of 1.01x105 Pa is compressed reversibly and isothermally until its volume is heated. It is then expanded reversibly and adiabatically to twice its original volume. Calculate the final pressure and temperature of the gas if Mba= = 1.4.

    2. (a) Explain why the specific heat of a gas at constant pressure is higher than that at constant volume.

    (b) The density of an ideal gas is 1.6kgm-3 at 27oC and 1.00 × 10Nm2 pressure and specific heat capacity at constant volume is 0.312KJkg-1. Find the ratio of the specific heat capacity at constant pressure to that at constant volume. Point out any significance attached to the result.

    3. (a) Explain why the cooling compartment of a refrigerator is always on top.

    (b) The refrigerator cools substances by evaporation of a volatile liquid. Explain how evaporation causes cooling.

    (c) State the reason why water is used in the cooling system of a car engine.

    4. (a) With the aid of a labelled diagram, describe how a refrigerator works.

    (b) The cooling system of a refrigerator extracts 0.7 Kw of heat. How long will it convert 500g of water at 20oC to ice?

    (c) Explain how evaporation takes place in the refrigerator.

    (d) Explain why water in a porous pot keeps at a lower temperature than that of the surrounding.

    Heat engine and climatic change

    Activity 22(i)

    In groups of five, discuss the causes of air pollution and water pollution.

    (ii) Explain how water and air pollution affect the environment and the climate.

    (iii) Note down your findings as a group.

    (iv) Present your findings to the whole class.

    Most of air pollution is caused by the burning of fuels such as oil, natural gas etc. The air pollution has an adverse effect on the climate. Climate change is the greatest environmental threat of our time endangering our health.

    When a heat engine is running, several different types of gases and particles are emitted that can have detrimental effects on the environment. Of particular concern to the environment are carbon dioxide, a greenhouse gas; hydrocarbons -- any of more than a dozen volatile organic compounds, nitrogen oxides; sulfur oxides; and particulate matter, tiny particles of solids, such as metal and soot.

    Engines emit greenhouse gases, such as carbondioxide, which contribute to global warming. Fuels used in heat engines contain carbon. The carbon burns in air to form carbon dioxide. The Carbondioxide and other global warming pollutants collect in the atmosphere and act like a thickening blanket, and destroy the ozone layer. Therefore the sun’s heat from the sun is received direct on the earth surface and causes the planet to warm up. As a result of global warming, the vegetation is destroyed, ice melts and water tables are reduced.

    Heat engines especially diesel engines produce Soot which contributes to global warming and its influence on climate.The findings show that soot, also called black carbon, has a warming effect. It contains black carbon particles which affect atmospheric temperatures in a variety of ways. The dark particles absorb incoming and scattered heat from the sun; they can promote the formation of clouds that can have either cooling or warming impact; and black carbon can fall on the surface of snow and ice, promoting warming and increasing melting. Therefore soot emissions have significant impact on climate change .

    Similarly, some engines leak, for example, old car engines and oil spills all over. When it rains, this oil is transported by rain water to lakes and rivers. The oils then create a layer on top of the water and prevent free evaporation of the water.

    Critical thinking exercise

    In which ways can the dangers resulting from heat engines be minimised?
    Unit 10: Effects of electric and potential fieldsUnit 12: General Structure of the Solar System