Thermodynamics is defined as the science of energy and its effect on physical properties of substances. The term Thermodynamics stems from Greek.

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1 Thermodynamics Thermodynamics is defined as the science of energy and its effect on physical properties of substances. The term Thermodynamics stems from Greek. Therme ( heat) Dynamis ( power ). Today Thermodynamics include all aspects of energy and energy transformations including power generation, refrigeration and relationships among the properties of substance.

2 Thermodynamics can be studied in two ways. Macroscopic or Classical: Attention is made on a certain quantity of matter without consideration of events occurring at molecular level Microscopic or Statistical: Attention is made on a certain quantity of matter with consideration of events occurring at molecular level

3 A substance consists of a large number of particles called molecules. The properties of the substance naturally depend on the particles to determine the pressure in the container. It would be sufficient to attach a pressure gage to the container. This macroscopic approach to the study of thermodynamics that does not require a knowledge of the behavior of individual particles behavior of these particles.

4 Surrounding System Boundary A system is defined as a quantity of matter or a region in space chosen for study. The mass or region outside the system is called the surroundings. The real or imaginary surface that separates the system from its surroundings is called the boundary.

5 A cross-sectional view of a hot water flask, which is used to store hot or cold substances.

6 System Open or control volume Closed or control mass Isolated Both mass and energy can cross the boundary of a control volume. Ex: Nozzle, turbine, compressor,etc It consists of a fixed amount of mass, and no mass can cross its boundary. But energy, in the form of heat or work, can cross the boundary Ex: Piston cylinder arrangement No mass and energy transfer cross the boundary. Ex:Truly isolated systems cannot exist in nature and they are thus hypothetical concepts only

7 Open system Closed system Energy in System Energy out Mass out Energy in System Energy out Mass in Surrounding Surrounding No mass Transfer Isolated system System Surrounding No mass and energy transfer

8 Property of a system. Properties are characteristics of a system by which its physical condition may be described. Eg: volume, temperature, pressure, etc. Thermodynamic Properties are considered to be either intensive or extensive. Extensive properties are dependent on mass. Intensive properties are independent of mass. Specific extensive properties i.e., extensive properties per unit mass are intensive properties i.e., specific volume, specific energy, etc. M(mass) V(volume) T(Temp) P(pressure) ρ(density) M/2 V/2 T P ρ M/2 V/2 T P ρ Extensive Intensive

9 State: Consider a system not undergoing any change. At this point, all the properties can be measured or calculated throughout the entire system, which gives us a set of properties that completely describes the condition, or the state, of the system. At a given state, all the properties of a system have fixed values. If the value of even one property changes, the state will change to a different one. In Fig. below, system is shown at two different states.

10 Process Any change from one equilibrium state to another equilibrium state is called a process. Path The series of states through which a system passes during a process is called Path Pressure Process Path State 2 State 1 Temperature A process between state 1 and state 2. P1,T1 P2,T2 State 1 State 2

11 Cyclic Process When a system in a given initial state goes through a number of different states (going through various processes) and finally returns to its initial values, the system has undergone a cyclic process or cycle. Therefore, at the conclusion of a cycle, all the properties have the same value they had at the beginning. Water at room temperature. Heated upto 50 Deg C Let it to cool down to room temperature. State 1 A Closed container containing water. HEAT State 2 State 1

12 Pressure Process Path State 2 State 1 Temperature Cyclic process

13 Properties are either point function or path function. Pressure Process Path State 2 State 1 Volume Ex of path function : Heat and work as the change in magnitudes depends on the path followed during the process. Ex of point function : Pressure and temperaure as the change in magnitudes depends only on the final states during the process.

14 Isothermal process: a process in which the temperature remains constant. Isobaric process: a process in which the pressurer emains constant. Isochoric (or isometric) process: a process during which the specific volume remains constant. Steady flow process: A process in which the properties of the system do not change with time. Illustration : Open system Time: 10:30 AM Open system 11:00 AM

15 Equilibrium Thermal There is no temperature differential in the system which is the driving force for heat flow. Mechanical There is no pressure change at any point at any time. Chemical There is no change in chemical composition at any point at any time.

16 Quasi Equilibrium / Static process When a process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times, it is called a quasistatic or quasi equilibrium process.

17 To consider how a gas (or liquid) might be expanded or compressed in a quasie-quilibrium fashion, refer to Figure, which shows a system consisting of a gas initially at an equilibrium state. As shown in the figure, the gas pressure is maintained uniform throughout by a number of small masses resting on the freely moving piston. Imagine that one of the masses is removed, allowing the piston to move upward as the gas expands slightly. During such an expansion the state of the gas would depart only slightly from equilibrium. The system would eventually come to a new equilibrium state, where the pressure and all other intensive properties would again be uniform in value. If several of the masses were removed one after another, the gas would pass through a sequence of equilibrium states without ever being far from equilibrium. In the limit as the increments of mass are made vanishingly small, the gas would undergo a quasiequilibrium expansion process. A quasiequilibrium compression can be visualized with similar considerations.

18 Engineers are interested in quasiequilibrium processes for two reasons. First, they are easy to analyze; Second work-producing devices deliver the most work when they operate on quasi-equilibrium processes. Therefore, quasi-equilibrium processes serve as standards to which actual processes can be compared.

19 Reversible and Irreversible process. A reversible process is a process which can be reversed without leaving any trace on the surroundings. Reversible process does not occur in nature. It is merely an idealization of actual process. Reversible process can be approximated by actual device. Processes that are not reversible are called irreversible. Irreversible processes normally include one or more of the following irreversibilities: Heat transfer through a finite temperature difference Unrestrained expansion of a gas or liquid to a lower pressure Spontaneous chemical reaction Spontaneous mixing of matter at different compositions or states Friction sliding friction as well as friction in the flow of fluids Electric current flow through a resistance Magnetization or polarization with hysteresis Inelastic deformation

20 Causes of irreversibility: Lack of thermodynamic equilibrium Dissipative effect. Irreversible processes normally include one or more of the following irreversibilities: Heat transfer through a finite temperature difference Unrestrained expansion of a gas or liquid to a lower pressure Spontaneous chemical reaction Spontaneous mixing of matter at different compositions or states Friction sliding friction as well as friction in the flow of fluids Electric current flow through a resistance Magnetization or polarization with hysteresis Inelastic deformation

21 Adiabatic wall ( Insulator) Zeroth Law of Thermodynamics. Zeroth law states that if two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. Body A Body B Diathermic wall Body C

22 Replacing Body C with a mercury Thermometer T2 T1 Thermometer Body A Body B Adiabatic wall

23 Zeroth law serves as a basis for the validity of temperature measurement. It is necessary only to see if they are individually in thermal equilibrium with a third body. The third body is usually a thermometer. Temperature is a thermodynamic property that determines whether or not a system is in thermal equilibrium with other systems. Thermometers Any body with at least one measurable property that changes as its temperature changes can be used as a thermometer. Such a property is called a thermometric property. The particular substance that exhibits changes in the thermometric property is known as a thermometric substance. A familiar device for temperature measurement is the liquid-in-glass thermometer. As temperature increases, the liquid expands in volume and rises in the capillary. The length L of the liquid in the capillary depends on the temperature. Accordingly, the liquid is the thermometric substance and L is the thermometric property.

24 Temperature Scales All temperature scales are based on some easily reproducible states such as the freezing and boiling points of water, which are also called the ice point and the steam point, respectively. A mixture of ice and water that is in equilibrium with air saturated with vapor at 1 atm pressure is said to be at the ice point, and a mixture of liquid water and water vapor (with no air) in equilibrium at 1 atm pressure is said to be at the steam point. Temperature scale in SI system is Celcius scale Temperature scale in English system is Fahrenheit scale In thermodynamics, it is very desirable to have a temperature scale that is independent of the properties of any substance or substances. Such a temperature scale is called a thermodynamic temperature scale, The thermodynamic temperature scale in the SI is the Kelvin scale, The lowest temperature on the Kelvin scale is absolute zero, or 0 K. A temperature scale that turns out to be nearly identical to the Kelvin scale is the idealgas temperature scale. This thermometer is based on the principle that at low pressures, the temperature of a gas is proportional to its pressure at constant volume.

25 Constant volume gas thermometer R The constant-volume gas thermometer shown in Fig. above is so exceptional in terms of precision and accuracy that it has been adopted internationally as the standard instrument for calibrating other thermometers. The thermometric substance is the gas (normally hydrogen or helium), and the thermometric property is the pressure exerted by the gas. As shown in the figure, the gas is contained in a bulb, and the pressure exerted by it is measured by an opentube mercury manometer. As temperature increases, the gas expands, forcing mercury up in the open tube. The gas is kept at constant volume by raising or lowering the reservoir. The gas thermometer is used as a standard worldwide by bureaus of standards and research laboratories.

26 PRESSURE: Pressure is defined as a normal force exerted by a fluid per unit area. Relationships among the absolute, atmospheric, gage, and vacuum pressures. ##Absolute pressure is measured relative to absolute vacuum (i.e., absolute zero pressure).

27 Ex: Absolute Pressure of a Vacuum Chamber A vacuum gage connected to a chamber reads 0.8 x 10 5 bar a location where the atmospheric pressure is x 10 5 bar. Determine the absolute pressure in the chamber. Hint: P absolute = P atm P vaccuum Variation of Pressure with Depth: The pressure of a fluid at rest increases with depth (as a result of added weight). Note: pressure in a fluid does not vary in the horizontal direction within a fluid

28 PRESSURE MEASUREMENT Two commonly used devices for measuring pressure are the manometer and the Bourdon tube. Manometers measure pressure differences in terms of the length of a column of liquid such as water, mercury, or oil.

29 P 1 =P atm + rgl 0 H 1 2

30 Problem: The water in a tank is pressurized by air, and the pressure is measured by a multifluid manometer as shown in Fig. The tank is located on a mountain at an altitude of 1400 m where the atmospheric pressure is 85.6 kpa. Determine the air pressure in the tank if h1 = 0.1 m, h2= 0.2 m, and h3 = 0.35 m. Take the densities of water, oil, and mercury to be 1000 kg/m3, 850 kg/m3, and 13,600 kg/m3, respectively. Hint:

31 BourdonTube A Bourdon tube gage is shown in Fig. The figure shows a curved tube having an elliptical cross section with one end attached to the pressure to be measured and the other end connected to a pointer by a mechanism. When fluid under pressure fills the tube, the elliptical section tends to become circular, and the tube straightens. This motion is transmitted by the mechanism to the pointer. By calibrating the deflection of the pointer for known pressures, a graduated scale can be determined from which any applied pressure can be read in suitable units. Because of its construction, the Bourdon tube measures the pressure relative to the pressure of the surroundings existing at the instrument. Accordingly, the dial reads zero when the inside and outside of the tube are at the same pressure. Pressure measurement by a Bourdon tube gage.

32 Problems: 1.Can you carry 1 m3 of liquid water? 2. You dive 5 m down in the ocean. What is the absolute pressure there? 3.What pressure difference does a 10 m column of atmospheric air show? ρ = 1.2 kg/m3.