Chapter 6 Introduction and Basic Concepts Thermodynamics

What is Thermodynamics? Greek: therme = heat and dynamis = power (The science of energy) Loard Kelvin  used for the first time the word thermodynamics in 1849 William Rankine  the first thermodynamic textbook was written in 1859 Thermodynamic’s Law a) Conservation of Energy (Energy can change from one form to another form but cannot be created OR destroyed) b) 1st Law Thermodynamics (expression of the conservation of energy/energy is a thermodynamics property)

c) 2nd Law Thermodynamics (energy has quality as well as quantity, and actual processes occur in direction of decreasing quality of energy) Study of Thermodynamics a) Macroscopic Approach = Classical Thermodynamics (does not require knowledge of the behavior of individual particles) b) Microscopical Approach = Statistical thermodynamics

Defining Systems System: whatever we want to study; (Quantity of mass or region in space) Surroundings: mass or region outside the system. Boundary: real/imaginary surface that separates system from its surroundings. Can be fixed/movable, shared by both(system & surrounding), no thickness, no mass/volume Boundary Surroundings System

Closed System (Control mass) A system that always contains the same matter (same mass). No transfer of mass across its boundary can occur. Heat/work can cross the boudary. Isolated system: special type of closed system that does not interact in any way with its surroundings (no energy crosses its boundary)

Control Volume (Open System) A given region of space through which mass flows. Mass may cross the boundary of a CV Boundary of CV is called Control Surface (real or imaginary)

Properties of System Any measureable or observable characteristics of the substance when the system remains in equilibrium is called a PROPERTY. e.g. Pressure (P), Volume (V), Temperature (T) and mass (m), etc. also Viscosity (μ), Electric Resistance (R), Thermal Conductivity (k), etc. Intensive : Independent on mass of system. - e.g. Pressure (P), Elevation (h), etc. Extensive : Dependent on mass of system. - e.g. total mass, total volume, etc. Specific : Extensive properties per unit mass. - e.g. Sp. Vol (v=V/m), Sp.Enthalpy (h=H/m), etc-Intensive Property

State Consider a system that is not undergoing any change. A set of properties that completely describe the condition of the system is known as its STATE. At a given state all of the properties are known; changing one property changes the state. A system at two different state

Equilibrium EQUILIBRIUM : State of Balance Thermal Equilibrium : NO Temperature Gradient throughout the system (uniform temperature). Mechanical Equilibrium : NO Pressure Gradient throughout the system(uniform pressure). Phase Equilibrium : - System having more than 1 phase. - Mass of each phase is in equilibrium. Chemical Equilibrium : - Chemical composition is constant - NO reaction occurs.

The State Postulate The number of properties required to fix the state of a system is given by the state postulate: The state of a simple compressible system is completely specified by two independent, intensive properties. Simple compressible system: If a system involves no electrical, magnetic, gravitational, motion, and surface tension effects.(the only energy transfer by work is by volume change). State of nitrogen is fixed by two independent, intensive properties.

Example The density and the specific volume of a simple compressible system are known. The number of additional intensive, independent properties needed to fix the state of this system is (a) 0 (b) 1 (c) 2 (d) 3 (e) 4 Answer (b) n = 0 "The state of a simple compressible substance is fixed by two intensive, independent properties. Specific volume and density are independent, so they count as two properties. Therefore, we no need more property."

Path & Process Any change a system undergoes from one equilibrium state to another is known PROCESS. Series of states through which system passes during the process is known as its PATH Property A State 1 State 2 Property B Path State 1 State 2

Quasi means ‘almost’. During a quasi-equilibrium or quasi-static process the system remains practically in equilibrium at all times. We study quasi-equilibrium processes because they are easy to analyze (equations of state apply) and work-producing devices deliver the most work when they operate on the quasi-equilibrium process.

In most of the processes that we will study, one thermodynamic property is held constant. Some of these processes are Temperature (T) Enthalpy (h)/ Entropy (s) T=Const Isothermal h=Const Isenthalpic s=Const Isentropic Pressure (P) Volume (V) V=Const Isochoric P=Const Isobaric

Cycle CYCLE : A system is said to have undergone a cycle if it returns to its ORIGINAL state at the end of the process. Hence, for a CYCLE, the INITIAL and the FINAL states are identical. Property A State 1 State 2 Property B

Temperature (T) If two blocks (one warmer than the other) are brought into contact and isolated from their surroundings, they would interact thermally with changes in observable properties. When all changes in observable properties cease, the two blocks are in thermal equilibrium. Temperature is a physical property that determines whether the two objects are in thermal equilibrium.

Temperature (T) The zeroth law of thermodynamics: If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. By replacing the third body with a thermometer, the zeroth law can be restated as two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact. Two bodies reaching thermal equilibrium in isolated enclosure.

Temperature Scales(1) Kelvin scale: An absolute thermodynamic temperature scale whose unit of temperature is the kelvin (K); the SI base unit for temperature. Rankine scale: An absolute thermodynamic temperature scale with absolute zero that coincides with the absolute zero of the Kelvin scale; the English base unit for temperature. T(oR) = 1.8T(K) Celsius scale (oC): T(oC) = T(K) – 273.15 Fahrenheit scale (oF): T(oF) = T(oR) – 459.67