Chapter Two Basic Concepts of Thermodynamics 1 System

Chapter Two: Basic Concepts of Thermodynamics 1

System • System is defined as a quantity of matter or a region in space chosen for study. • Boundary: The real or imaginary surface that separates the system from its surroundings. • Surroundings: The mass or region outside the system. 2

Closed System • Closed system (also known as a control mass) consists of a fixed amount of mass, and no mass can cross its boundary (no mass can enter or leave the closed system). • Energy may cross the boundary of closed system • The volume of a closed system does not have to be fixed. 3

Isolated System In isolated system, even energy is not allowed to cross the boundary NO 4

Open System § In Open system (also called control volume) Both mass and energy can cross the boundary of a control volume. § The mass of an open system does not have to be fixed. 5

System Properties Any characteristic of a system is called a property. Some familiar properties are – Pressure (P) – Temperature (T) – Volume (V) – Mass (m) – Density (ρ), specific gravity or relative density – Specific volume (ν) 6

Types of Properties Intensive properties are those that are independent of the mass of a system, such as temperature (T), pressure (P), and density (ρ). Extensive properties are those whose values depend on the mass of the system such as : mass (M), volume (V), and total energy (E). Specific properties. Some examples of specific properties are specific volume (v=V/m) and specific total energy (e=E/m). The state of a simple compressible system is completely specified by two independent, intensive properties. 7

How To Differentiate Between Intensive and Extensive Properties? • Divide the system into two equal parts with a partition • If each part has the same property value as the original system, the property is intensive • If each part has only half the property value of the original system, the property is extensive 8

State and Equilibrium State: is the condition of a system at which a set of properties can completely describe it. Equilibrium implies a state of balance. Thermodynamics deals with equilibrium states. 9

Types of Equilibrium • Thermal Equilibrium: The temperature is the same throughout the entire system. • Mechanical Equilibrium: The pressure is the same throughout the entire system. • Phase Equilibrium: The mass of each phase reaches an equilibrium level and remains fixed. • Chemical Equilibrium: Chemical composition does not change with time, i. e. no chemical reactions take place Thermodynamic Equilibrium: A system is said to be in thermodynamic equilibrium if it maintains thermal, mechanical, phase, and chemical equilibrium. 10

Processes, Paths, and Cycles Process: Any change that a system undergoes from one equilibrium state to another. Path: The series of states through which a system passes during a process. In order to describe a process completely, one should specify the initial and final states AND the path the process follows. Cycle: The system returns to its initial state at the end of the process. 11

Types of Processes • In most of the processes we study, one thermodynamic property is held constant. • The prefix iso- is often used to designate a process for which a particular property remains constant. For examples: • Isothermal process: is a process during which the temperature T remains constant; • Isobaric process: is a process during which the pressure P remains constant. • Isochoric (or isometric) process: is a process during which the specific volume v remains constant. • Isentropic process: is a process during which the entropy remains constant. 12

Steady and Unsteady States • Steady state: implies no change with time. • Unsteady state: or transient, the opposite of steady state (there is change with time). • The steady-flow process: a process during which a fluid flows through a control volume steadily (mass, volume and total energy content of the control volume remain constant). However, the fluid properties can change from point to point within the control volume, but at any fixed point they remain the same during the entire process 13

Forms of Energy The total energy of a system is divided into two forms: 1. Macroscopic forms of energy are those which a system possesses as a whole with respect to some outside reference frame, such as kinetic and potential energies. 2. Microscopic forms of energy are those which are related to the molecular structure of a system and the degree of the molecular activity. The sum of all microscopic forms of energy is called the internal energy of a system and is denoted by U. 14

Forms of Energy • Kinetic energy: The energy that a system possesses as a result of its motion relative to some reference: KE = ½ m v 2 (J) • Potential energy: The energy that a system possesses as a result of its elevation in a gravitational field: PE = mgh (J) 15

Forms of Energy The total energy of a system consists of the kinetic, potential, internal energies, etc (i. e. , the sum of all form of energy): E = U + KE + PE = U + ½ m v 2 + mgh • Furthermore, most closed systems remain stationary (no change in their kinetic and potential energies). Therefore, for stationary systems E=U 16

Types of associated energies with internal U 1. Portion of the internal energy of a system associated with the kinetic energies of the molecules is called the sensible energy. 2. The internal energy associated with the phase of a system is called the latent energy. 3. The internal energy associated with the atomic bonds in a molecule is called chemical energy. 4. The energy associated with the strong bonds within the nucleus of the atom itself is called nuclear energy. 17

Energy and Environment Ø The largest source of air pollution is the motor vehicles Ø The pollutants released by the vehicles are usually grouped as hydrocarbons (HC), nitrogen oxides (NOx), and carbon monoxide (CO). Ø There is also sulfur oxides (SOx) emitted to air from some Chemical Industries. Ø Some of the air pollutants cause Acidic Rain and Green House Effect. 18

Temperature and the Zeroth Law of Thermodynamics: when an object is in contact with another object that is at a different temperature, heat is transferred from the object at higher temperature to the one at lower temperature until both objects attain the same temperature (Thermal Equilibrium). The zeroth law can be restated as two objects are in thermal equilibrium if both have the same temperature reading even if they are not in contact. 19

Absolute Temperature P Experimental data Gas (A) Gas (B) extrapolation Gas (C) Gas (D) T(C) -273. 15 20

Temperature • We use four different measures of temperature: Celsius (°C), Fahrenheit (°F), Kelvin (K), Rankine (R). • Kelvin and Rankine are absolute temperature scales. Both are zero at absolute zero. 0 K=0 R • Celsius and Fahrenheit are relative temperature scales, and 0°C=273. 15 K 0°C=32°F 0°F=460 R 100°C=212°F 21

Temperature Conversion Celcius 1 D o. C = 1 D K 1 Do. F = (1. 8) Do. C kelvin Fahrenheit 1 Do. F = 1 DR 1 DK = (1. 8) DR Rankine In ordinary usage the D symbol for the unit temperature difference is omitted. Consequently, you have to infer from the text whether o. F, o. C, o. R, and K mean temperature or unit temperature difference. 22

Temperature Conversion 460 23

Example 1 Solution 24

Example 2 Convert 18 o. C to o. F Convert 550 R to o. C 25

Pressure 26

Pressure • Pressure: is the force exerted by a fluid per unit area. • Atmospheric pressure: is the pressure exerted by the atmosphere at the earth's surface. • Absolute pressure: is the local pressure at a given position (location). • Gage pressure: is the difference between the absolute pressure and the atmospheric pressure when the absolute pressure is greater than the atmospheric pressure. . • Vacuum pressure: is the difference between the atmospheric pressure and the absolute pressure when the absolute pressure is less than the atmospheric pressure. 27

Gage and Vacuum Pressure • Pgage = Pabs - Patm (for pressures above Patm) • Pvac = Patm - Pabs (for pressures below Patm) P gage P atm P vac P atm P abs = 0 Absolute vacuum 28

Pressure Conversion Factors Pressure has units of k. Pa, psi, atm, inches of Hg, mm of Hg, ft of H 2 O, and in of H 2 O, etc. 29. 92 in. of Hg = 1 atm = 760 mm. Hg 29

Example: A vacuum gage connected to a chamber reads 5. 8 psi at a location where the atmospheric pressure is 14. 5 psi. Determine the absolute pressure in the chamber. Solution: Pabs = Patm - Pvac = 14. 5 -5. 8 = 8. 7 psi

Example What is the absolute pressure in psi of a vacuum of 15 inch of Hg? (1 atm=29. 92 in Hg) 31

Pressure in Depth pressure in a fluid does not change in the horizontal direction. Pressure in a fluid increases with depth because more fluid rests on deeper layers, and the effect of this “extra weight” on a deeper layer is balanced by an increase in pressure P=Patm + ρgh, and Pgage =∆P = P 1 – P 2 = ρgh (h = Pressure head) 32

Example • Consider a diver swimming at 50 ft below the surface of the ocean. Determine the gage and absolute pressure on the diver in psi. 33

The Manometer • The manometer is an instrument used to measure the pressure differences of liquids and/or gases. • Small and moderate pressure differences can be measured by the manometer. P 1 = P 2 (horizontally) and P 2 = Patm + ρgh P 1 = P 2 = Patm + ρgh 34

P atm P 1=Patm h 1 Fluid 2 h 2 Fluid 3 h 3 + ρ1 gh 1 + ρ2 gh 2 + ρ3 gh 3 1 Fluid 1 2 a ρ1 A ρ2 h B P 2= P 1+ ρ1 g(a + h) - ρ2 gh – ρ1 ga P 2= P 1+ ρ1 ga + ρ1 gh - ρ2 gh – ρ1 ga P 2= P 1+ ρ1 gh - ρ2 gh P 2= P 1+ gh(ρ1 - ρ2) 35

Example: A manometer is used to measure the pressure in a tank. The fluid used has a specific gravity of 0. 85, and the manometer column height is 55 cm. If the local atmospheric pressure is 96 k. Pa, determine the absolute pressure in the tank. P =96 atm Solution: P ? ? gas h=55 cm • SG = Density of substance / density of water • Density of substance = 0. 85 (1000 kg/m 3) = 850 kg/m 3 • P = Patm + ρgh = 96 k. Pa + 850 kg/m 3 * 9. 81 m 2/s * 0. 55 m (1 k. Pa/1000 N/m 2) * ( 1 N / 1 kg. m/s 2) = 100. 6 k. Pa 36

Barometer and the Atmospheric Pressure The atmospheric pressure (barometric pressure) is measured by a device called a barometer. As Torricelli measured the atmospheric pressure by inverting a mercury-filled tube into a mercury container that is open to the atmosphere. Note that the cross-sectional area of the tube have no effect on the height of the fluid column of a barometer Patm = ρgh 37

• standard atmosphere, which is defined as the pressure produced by a column of mercury 760 mm in height at 0 °C (ρ= 13, 595 kg/m 3) and (g = 9. 807 m/s 2). Called 760 mm. Hg. 1 mm. Hg = 1 torr = 133. 3 Pa • 1 atm = 760 mm Hg = 760 mm torr • If water instead of mercury were used to measure the standard atmospheric pressure, a water column will be about 10. 3 m. How? • As we go up atmospheric pressure drops. Why? 38

39

40

41

Assignment 1 and Quiz 1 Assignment 1 has been posted on the Edu. Wave. The due date is Wednesday (7/3/2011). You should submit your assignment before the lecture. Late submission will not be accepted and student will get zero. There will a Quiz on the same day (7/3/2001)

End of Chapter Two 43