Basic Concepts of Thermodynamics Objective Definition of Thermodynamic System Concepts of State and its properties Pressure, Temperature and Specific Volume Quasi-static or Quasi-equilibrium Process and Reversible Process Thermodynamic Cycle Objective

Thermodynamic System System—A quantity of matter or a region in space bounded by an arbitrary surfaces for study. Surrounding—The mass or region outside the system. Boundary—The surfaces separate the system from its surroundings. The boundary can be real or imaginary, fixed or variable. –It is critically important to define your system before attempting to solve a thermodynamic problem.

Thermodynamic System Closed system—No mass can cross its boundary. It consists of a fixed amount of mass. (also known as control mass) Open system —A selected region in space. Both mass and energy can cross its boundary. (also known as control volume)

Thermodynamic System –Simple Thermodynamic System Surroundings Heat One Type Work Surroundings Adiabatic System —No heat cross its boundary. Isolated System— Nether mass nor energy can cross its boundary.

Thermodynamic System Single Composition System —Consists of only one kind of substance. Multi-Composition System —Consists of tow or more kinds of substances. Single Phase System —Consists of only one phase. Multi-phases System —Consists of tow or more phases. Homogeneous System —The composition and property are identical all over the system.

Thermodynamic State Thermodynamic state of a system –the condition of the system as characterized by the values of its properties. Property –any characteristic of a system Basic Property –can be directly measured Derived Property –indirectly calculated from basic ones

Pressure Common Use Units: English SI Other

Pressure Measurement

Gage and Absolute Pressure Scales We commonly use three different kinds pressure scales (not units). Absolute — is relative to full vacuum (absolute zero). Gage (Gauge) — is relative to the ambient pressure. Used for pressure above the ambient pressure. Vacuum — is relative to the ambient pressure. Used for pressure below the ambient pressure.

Gage and Absolute Pressure Scales Vacuum Vacuum

Temperature Two objects in thermal equilibrium are at the same Zeroth law — if two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. Thermometers — measure the temperature

Temperature Scale Celsius (Centigrade) scale and Fahrenheit scale —Two points scale — Dependent of physical properties of substances steam point 100℃ Celsius Fahrenheit ice point 0℃

Temperature Scale Thermodynamic temperature scale — based on the principle that at low pressure, the temperature of gas is proportional to its pressure at constant volume. — One point scale — independent of property of substance Hydrogen Helium

Temperature Scale so that: T = a + b P For gases at low pressures (ideal gasses) For Celcius scale, a=-273.15 For Fahrenheit scale, a=-459.67

Temperature Scale Four temperature scales are in common use: –Rankine –Fahrenheit –Kelvin –Celsius Unit Absolute Relative SI Kelvin Celsius English Rankine Fahrenheit differ in: - dependent / independent of property of substance location of the zero points size of the degree unit

Temperature Scale Comparisons

Specific Volume Specific Volume Density

Characteristic of State Property Mathematical integral 1 2 A C B differential

Characteristic of State Property intensive and extensive –Intensive property: a property that is independent of the mass of the system. Example –temperature, pressure, density, … –Extensive property: a property whose value is proportional to the mass of the system. Example – volume, total enthalpy, energy ,… –Specific property: An extensive property per unit mass. Specific properties are intensive. Example –specific volume (= volume/mass)

Equilibrium Stable equilibrium state —a state in which the system is not capable of spontaneous change to another state without a finite change in the surroundings. —There are no driving forces to carry out a change. •Sufficient and Necessary Conditions for Equilibrium: –Thermal: the temperature is the same throughout the system. –Mechanical: the pressure is the same. –Phase: there is no driving force for the total mass in each phase to change. –Chemical: there is no driving force for chemical composition to change.

State Equation We need only specify a certain number of properties to fix the state of system. The exact number depends on compositions and phases of system? State postulate(相律） number of independent properties number of different compositions of system number of different phases of system State equation

Property Diagram The state must be Equilibrium. Any point illustrates the state of system. The state must be Equilibrium. 2 Final state 1 Initial state

Thermodynamic Process Thermodynamic Processes —transformations from one equilibrium state to another. Path —The series of states through which the system passes. 1 Initial state 2 Final state path

Quasi-equilibrium Process System remains infinitesimally close to an equilibrium state at all times. 1 Initial state 2 Final state Quasi-equilibrium Process Process occurs slow enough to keep the properties identical throughout the system. The interval of process change takes much longer time than system spontaneously adjusts to a new state after the previous equilibrium state was destroyed. To connect states with line must be QE process.

Reversible Process Reversible process: the reverse process could be performed so that the system and surroundings can be restored to their initial states with no change in the system or surroundings. Sufficient and necessary conditions: Quasi-equilibrium process no dissipation in process -friction, non-elastic deformation, resistance…

Thermodynamic Cycle Cycle –a collection of processes that ultimately lead to the system being returned to the original state. clockwise heat work Anti-clockwise work heat