THERMODYNAMICS CONCEPTUAL PROBLEMS Q-01 Is the heat supplied to a system always equal to the increases in its internal energy? Ans Acc. to first law of thermo- dynamics If heat is supplied in such a manner that volume does not change (dv = 0 i.e. isochoric change), then whole of the heat energy supplied to the system will increase internal energy only. Q-02 Is the internal energy of a gas a function of the pressure? Explain. Ans The internal energy of an ideal gas depends only on the temperature of the gas, while that of a real gas depends on the temperature and volume, which in turn is dependent on pressure. Q-03 Give two examples of reversible processes. Discuss their reversibility? Ans (i) Melting as well as vaporization are reversible processes. On reversing the conditions under which they occur, the vapour condense into a liquid and the liquid solidifies. (ii) All isothermal and adiabatic processes, in which no extraneous loss of heat occurs, can be retraced by reversing the boundary conditions and hence they are reversible processes. Q-04 An electric refrigerator transfers heat from the cold cooling coils to the warm surroundings. Is it against the second law of thermodynamics? Justify your answer? Ans No, it is not against the second law. This is because external work is done by the compressor for this transfer of heat. Q-05 Why is conversion of heat into work not possible without a sink at lower temperature? Ans For converting heat energy into work continuously, a part of heat energy absorbed form the source has to be rejected. The heat energy can be rejected only if there is a body, whose temperature is less than that of the source. This body at lower temperature is the sink. Q-06 Can the carnot engine be realized in practice? Ans No, because carnot engine is an ideal heat engine, whose conditions are not realized in practice. Q-07 What is meant by reversible process? Explain why the efficiency of a reversible engine is maximum? Ans The process which can be retraced at any stage of its operation by reversing the boundary conditions is called a reversible process. The efficiency of a reversible engine is maximum, because in such a device, no dissipation of energy takes place. Q-08 No real engine can have an efficiency greater than that of a carnot engine working between the same two temperatures, why? Ans A carnot engine is an ideal heat engine from the following points of view : (a) There is absolutely no friction between the walls of cylinder and the piston. 1
(b) The working substance is an ideal gas. In a real engine, these conditions cannot be fulfilled and hence no heat engine working between the same two temperatures can have efficiency greater than that of carnot engine. VERY SHORT ANSWER QUESTIONS Q-01 Can temperature of a body be raised without heating it? Is the converse true? Ans Yes ; e.g., in adiabatic compression. Converse is also true. For example, when a solid melts, temperature does not rise, although heat is given. Q-02 Can water be made to boil without heating? Ans Yes, by reducing pressure on water, boiling point of water can be brought down to room temperature. Q-03 Does the mass of a body change when it is heated or cooled? Ans Yes in heating or cooling, a body absorbs or loses energy. The mass will, therefore increase or decrease accordingly as per the relation E = mc 2. Q-04 What thermodynamic variable is defined by (a) Zeroth law (b) First law? Ans Zeroth law defines temperature and first law defines internal energy. Q-05 Is it possible to convert internal energy into work? Ans Yes. For example, in explosion of a bomb, chemical energy (which is a form of internal energy) is converted into kinetic energy. Q-06 Which molecules, ice at 0 0 C or water 0 0 C have greater potential energy and why? Ans The potential energy of water molecules at 0 0 C is more, because heat spent in melting is used up in increasing the P.E. Q-07 An ideal gas is compressed at a constant temperature, will its internal energy increase or decrease? Ans No, because internal energy of an ideal gas depends only on temperature of the gas. Q-08 Which type of motion of the molecules is responsible for internal energy of a monoatomic gas? Ans Translational motion of molecules. Q-09 Does the internal energy of an ideal gas change in an isothermal process? In an adiabatic process? Ans In an isothermal process, T = const.. In an adiabatic process, Q = constant Q-10 A sample of an ideal gas in a cylinder is compressed adiabatically to rd of its volume. Will the final Ans pressure be more or less than 3 times the initial pressure? Change in pressure will be more than 3 times the initial pressure. Q-11 A piece of lead is hammered. Does its internal energy increase? Does the heat enter the lead from outside? Ans Yes, internal energy of lead increases. No heat energy from outside enters the lead. 2
Q-12 In a thermodynamical process, 300 joule of heat is supplied to a gas and 200 joule of work is done by the gas. What is the change in internal energy of the system? Ans There will be a net increase in internal energy of the system by 300 200 = 100 joule. Q-13 Can two isothermal curves intersect each other? Ans No. If they intersect, then at two different temperatures (of the isothermals), volume and pressure of gas will be same, which is not possible. Q-14 In summer, when the value of a bicycle tube is removed, the escaping air appears cold. Why? Ans This happens due to adiabatic expansion of the air of the tube of the bicycle. Q-15 When air of the atmosphere rises up, it cools. Why? Ans When air rises up, it expands due to decrease in the atmospheric pressure. Therefore, temperature falls. Q-16 Does the internal energy of a gas change in (i) an isothermal process (ii) an adiabatic process? Ans (i) No, because temperature remains constant (ii) Yes, because temperature changes. Q-17 Why does a gas get heated on compression? Ans Because work done in compressing the gas increases the internal energy of the gas. Q-18 Give one example of a heat pump. Ans A refigeratior is an example of a heat pump. Q-19 Can we decide whether change in internal energy of a system is due to heating or performance of work? Ans No, as internal energy might change due to both-heating or performance of work. Q-20 Can the temperature of a system be increased without heating it? Ans Yes, for example in adiabatic compression. Q-21 Can heat be added to a system without increasing its temperature? Ans Yes, for example, in melting process and boiling process. Q-22 Is it possible to convert internal energy into work or mechanical energy? Ans Yes, for example in an adiabatic expansion and explosion of a bomb (chemical energy is converted into K.E). Q-23 Can whole of work be converted into heat? Ans Yes, through friction. Q-24 Can whole of heat be converted into work? Ans No, in accordance with second law of thermodynamics. Q-25 Is it possible to construct a heat engine, which is free from thermal pollution? Ans No, as in accordance with second law of thermodynamics, whole of heat cannot be converted into work. 3
SHORT ANSWER QUESTIONS Q-01 The temperature of the surface of sun is about 6000 K. Can we produce a temp. of 7000 K by converging sun s rays using a large convex lens? Ans No, as according to 2 nd law of thermodynamics, heat cannot be transferred on its own, from a body at lower temperature to another at higher temperature. The maximum temperature we can produce by this method is 6000 K. Q-02 Can a room be cooled by opening the door of a refrigerator in a closed room? Ans No. When a refrigerator is working in a closed room with its door closed, it is rejecting heat from inside to the air in the room. So temperature of room increase gradually. When the door of refrigerator is kept open, heat rejected by the refrigerator to the room will be more than the heat taken by the refrigerator from the room (by an amount equal to work done by the compressor). Therefore, temp. of room will increase at a slower rate compared to the first case. Note that if heat rejecting portion of refrigerator is outside the closed room, them opening of refrigerator door will cool the room gradually. Q-03 If an electric fan be switched on in a closed room, will the air of the room be cooled? If not, why do we feel cold? Ans No, rather the air will be heated, because due to motion of the fan, the speed of air molecules will increase. Infact, we feel cold due to evaporation of our sweat. Q-04 Which one among a solid, liquid and gas of same mass and at the same temp. has the greatest internal energy? Which one least. Why? Ans A gas has greatest value of internal energy because (negative) potential energy of its molecules is smallest. Internal energy of a solid is the least because (negative) potential energy of its molecules is maximum. Q-05 The volume of an ideal gas is V at a pressure P. On increasing the pressure by P, the change in volume of the gas (V 1) under isothermal conditions and (V 2) under adiabatic conditions. Is V 1 > V 2 or vice-versa and why? Ans Under isothermal conditions, (i) Under adiabatic conditions, (ii) Dividing (ii) by (i), we get (V 1) > (V 2). Q-06 Ice at 0 0 C is converted into steam at 100 0 C. State the isothermal changes in this process? Ans The isothermal changes are : (i) Conversion of ice at 0 0 C to water at 0 0 C. (ii) Conversion of water at 100 0 C to steam at 100 0 C. 4
Q-07 Give briefly the concept of internal energy. Q-08 Define the four thermodynamic processes. What is meant by indicator diagram? Q-09 State the sign conventions used in all thermodynamical processes? Q-10 What do you learn by applying first law of thermodynamics to isothermal and adiabatic processes? Q-11 Show that an adiabatic curve is always steeper than an isothermal curve. Q-12 Discuss work done in an isothermal/adiabatic process in terms of indicator diagram. Q-13 Briefly discuss the limitations of first law of thermodynamics. Q-14 State second law of thermodynamics. LONG ANSWER QUESTIONS Q-01 Explain what is meant by isothermal operations. Give some examples. Q-02 What are adiabatic operations? Enumerate some example. State equations representing these operations. Q-03 Obtain an expression for work done by a gas in an isothermal expansion. Q-04 Derive an expression for work done in an adiabatic process. Q-05 State Zeroth law of thermodynamics. How does it lead to the concept of temperature? Q-06 State and explain first law of thermodynamics. Establish the relation between two principal specific heats of a gas on the basis of this law. Q-07 Discuss briefly any three applications of first law of thermodynamics. Q-08 Establish relation between two principal specific heats of a gas. Q-09 What are cyclic and non cyclic processes? Calculate work done in such processes. Q-10 What are reversible and irreversible processes? Give some examples of each. Q-11 What is a heat engine? Obtain an expression for its efficiency. Mention different types of heat engines. Q-12 Briefly describe a Carnot cycle and derive an expression for efficiency of this cycle. 5
Q-13 Explain briefly the working principle of a refrigerator and obtain an expression for its coefficient of performance. 6