.E. = 0. Thus E = U i.e. the total energy of the system is equal to the internal energy of the system.
1.30 LAW OF CONSERVATION OF ENERGY This law states that energy can neither be created nor destroyed, though it can be transformed from one form to another form, in which the energy can exist. In other words, the total energy possessed by a body remains constant.
1.31 SPECIFIC VOLUME AND DENSITY The space occupied by a substance is called its volume. It is measured in m3 or litre, such that 1 m3 = 1000 litres = 1 kilo litre The volume per unit mass of a substance is termed as specific volume. It is usually expressed in m3 / kg. The mass per unit volume of a substance is called density. It is generally stated in kg / m3. It is usually denoted by a Greek letter rho (r). From the above, we find that the density of any substance is the reciprocal of the specific volume of the substance. Similarly, specific volume is the reciprocal of density of a substance.
\ Kinetic energy imparted by the fan to the air, 1 1 = × mV 2 = × 4.72 (10)2 = 236 N-m/s = 236 J/s...(Q 1 N-m = 1 J) 2 2 \ Minimum power that must be supplied to the fan = 236 J/s = 236 W Ans.
1.32 PRESSURE The pressure is defined as the force per unit area. In S.I. system of units, the unit of pressure is Pascal (briefly written as Pa), such that 1 Pa = 1 N/m2 The pressure, depending upon its magnitude, may be expressed in bar, kilo-pascal (kPa) or Megapascal (MPa), such that 1 bar = 1 × 105 N/m2 = 100 × 103 N/m2 = 100 kPa = 0.1 MN/m2 Sometimes, the pressure is also expressed in terms of the height of a liquid column e.g. mm of Hg or mm of water.
Fig. 1.10. Relation between absolute pressure, gauge pressure and vacuum pressure.
Fig. 1.11. U-tube manometer.
1.35 NORMAL TEMPERATURE AND PRESSURE The conditions for normal temperature and pressure (N.T.P.) are 0°C and 760 mm of Hg respectively.
1.36 STANDARD TEMPERATURE AND PRESSURE The conditions for standard temperature and pressure (S.T.P.) are 15°C and 760 mm of Hg respectively. Example 1.2. If a mercury barometer reads 720 mm of Hg, find the atmospheric pressure in kPa. Solution. Given: Barometer reading = 720 mm of Hg We know that 1 mm of Hg = 133.3 N/m2 \ Atmospheric pressure = 720 × 133.3 = 95 976 N/m2 or Pa = 95 976/1000 = 95.976 kPa Ans. Example 1.3. The steam leaving a turbine is at a pressure of 70 kPa vacuum. Find the absolute pressure in bar, if the atmospheric pressure is 101 kPa. Solution. Given: Vacuum pressure = 70 kPa Atmospheric pressure = 101 kPa We know that Absolute pressure = Atmospheric pressure – Vacuum pressure = 101 – 70 = 31 kPa = 31/100 = 0.31 bar... (Q 1 bar = 100 kPa) Example 1.4. The pressure of the gas supplied to an engine is measured as 76.2 mm of water gauge. If the barometer reads 730 mm of Hg, what is the absolute pressure of the gas in mm of Hg and kPa. Solution. Given: Gauge pressure of gas = 76.2 mm of water 76.2 = = 5.6 mm of Hg...(Q 1 mm of Hg = 13.6 mm of water) 13.6 Barometer reading or Atmospheric pressure = 730 mm of Hg We know that absolute pressure of the gas = Gauge pressure + Atmospheric pressure = 5.6 + 730 = 735.6 mm of Hg Ans. = 735.6 × 133.3 = 98 055 N/m2 or Pa...(Q 1 mm of Hg = 133.3 N/m2) = 98.055 kPa Ans.
Following are the three laws of thermodynamics : 1. Zeroth law of thermodynamics ; 2. First law of thermodynamics ; and 3. Second law of thermodynamics. These laws are discussed, in detail, in the subsequent chapters.
HIGHLIGHTS 1. The science which deals with the conversion of heat into mechanical energy is called Thermodynamics. 2. A substance through which the conversion of heat into work and vice versa takes place, is known as working substance. 3. A substance whose chemical composition is both homogeneous and constant, is known as pure substance. 4. In thermodynamics, macroscopic approach is concerned with gross or overall behaviour of matter. This is called classical thermodynamics. 5. In the microscopic approach, the matter is considered to be composed of particles and each particle having a certain position, velocity and energy, at a given constant. Such a study which is concerned directly with the structure of the matter is known as statistical thermodynamics. 6. The mass (m) of a system is a measure of quantity of matter as well as property of matter called inertia. The weight (W) refers to the force exerted by gravity on mass. Mathematically, m = W/g or W = m  g where m is in kg and W is in newton and g is acceleration due to gravity (9.81 m /s2). 7. The force is proportional to the product of mass (m) and acceleration (a). Its unit is newton. 8. Any region of space or finite quantity that occupies a volume and has a boundary, is known as thermodynamic system. Anything external to the system is called surroundings. The system is separated from the surroundings by an envelope, known as boundary. 9. A thermodynamic system is of four types, i.e. closed system (or non-flow system); open system (or flow system); isolated system and adiabatic system. 10. In a closed system, mass within the boundary remains constant. In other words, there is no mass transfer across the boundary. 11. In an open system, both mass and energy cross the boundary. 12. In an isolated system, there is no interaction between the system and the surroundings. 13. In an adiabatic system, only work can be exchanged and not heat with the surroundings. 14. A quantity of matter which is homogeneous throughout in physical structure and chemical composition is called a phase. 15. A system which consists of a single phase is known as homogeneous system and a system which consists more than one phase is called hetrogeneous system. 16. The condition of physical existence of a system at any instant of time is called a state and a system passing through a series of states constitutes a path. 17. Every system has some characteristic by which its physical condition may be described such as pressure, volume, temperature etc. Such characteristics are called properties of the system. 18. The properties which are independent of the mass of the system, are known as intensive properties. 19. The properties which are dependent upon the mass of the system, are known as extensive properties. 20. Any extensive property per unit mass becomes intensive property.
Basic Concepts  15 21. When any property of a system changes, there is a change of state and the system is then said to have undergone a thermodynamic process. 22. When a process or processes are performed on a system is such a way that the final state is identical with the initial state, it is then known as a thermodynamic cycle or cyclic process. 23. A system is said to be in thermodynamic equilibrium when it does not tend to undergo any further change of its own accord. 24. A quasi-static process is one in which the system deviates from the thermodynamic equilibrium state by only infinitesimal amount throughout the entire process. 25. A process which can be reversed in direction and the system retraces the same continuous series of equilibrium states, is said to be a reversible process. 26. A process in which the system passes through a sequence of non-equilibrium states, is known as irreversible process. 27. The heat may be defined as the energy transferred, without the transfer of mass, across the boundary of a system due to the temperature difference between the system and the surrounding. The unit of heat is joules. 28. The amount of heat required to raise the temperature of a unit mass of a substance through one degree is known as specific heat. The unit of specific heat is kJ/kg K. 29. The thermodynamic work may be defined as the energy transferred (without the transfer of mass) across the boundary of a system because of an intensive property difference other than temperature that exists between the system and the surroundings. 30. The mechanical work is defined as the product of the force (F ) and the distance moved (x) in the direction of the force. The unit of work depends upon the unit of force and distance moved. The practical unit of work is N-m. 31. When the thermodynamic property has a definite value for a given state, it is called a point function. For example, pressure, volume, temperature etc. are point functions. 32. The quantities which are not thermodynamic properties, are known as path functions. For example, heat and work are path functions. 33. The rate of doing work (or workdone per unit time) is termed as power. The unit of power is watt (W) or kilowatt (kW). 34. The capacity to do work