Slide 1

Basic PrinciplesThermodynamics involves the storage, transformation, and transfer energy.It is transformed from one of these forms to another and its transferred across a boundary as either heat or work. The System and Control volumeA thermodynamic system is a fixed quantity of matter upon which attention is focused. The system surface is one like that surrounding the gas in the cylinder of figure. All matter and space external to a system is its surroundings.Thermodynamics concerned with the interactions of a system and its surroundings, or one system interacting with another.If the system does not exchange energy with the surronding, it is an isolated systtem.

An analysis can often be simplifi ed if attention is focused on a particular volume in space into which, and/or from which, a substance fl ows. Such a volume is a control volume. A pump and a defl ating balloon are examples of control volumes. The surface that completely surrounds the control volume is called a control surface.

Macroscopic Description

Such a postulate allows us to describe a system or control volume using only a few measurable properties.Consider the defi nition of density given by

where m is the mass contained in the volume V, shown in Figure. Physically, V cannot be allowed to shrink to zero since, if V became extremely small, m would vary discontinuously, depending on the number of molecules in V.

Properties and State of a System

The matter in a system may exist in several phases: a solid, a liquid, or a gas. A phase is a quantity of matter that has the same chemical composition throughout; that is, it is homogeneous. A property is a ny quantity that serves to describe a system. Using the symbol f to represent a property, the mathematical statement is

Thermodynamic properties are divided into two general types, intensive and extensive. An intensive property is one that does not depend on the mass of the system. An extensive property is one that does depend on the mass of the system; mass, volume, momentum, and kinetic energy are examples.

If we divide an extensive property by the mass, a specific property results . The specific volume is thus defi ned to be

We will generally use an uppercase letter to represent an extensive property (exception: m for mass) and a lowercase letter to denote the associated intensive property.

1.4 EQUILIBRIUM, PROCESSES, AND CYCLESWhen the temperature of a system is referred to, it is assumed that all points of the system have the same, or approximately the same, temperature. When the properties are constant from point to point and when there is no tendency for change with time, a condition of thermodynamic equilibrium exists.In Fig a. If the system, however, goes from one equilibrium state to another through a series of nonequilibrium states (as in combustion) a nonequilibrium process occurs.

In Fig. b the dashed curve represents a nonequilibrium process between (V1, P1) and (V2, P2); properties are not uniform throughout the system and thus the state of the system is not known at each state between the two end states

Whether a particular process may be considered quasi equilibrium or non equilibrium depends on how the process is carried out. Let us add the weight W to the piston of Fig. 1.5 and explain how W can be added in a nonequilibrium manner or in an equilibrium manner. If the weight is added suddenly as one large weight,as in Fig. 1.5a, a non equilibrium process will occur in the gas. If we divide the weight into a large number of small weights and add them one at a time,

as in Fig. 1.5b, a quasi equilibrium process will occur.1.5 UNITSWhile the student is undoubtedly comfortable using SI units , much of the data gathered and available for use in the United States is in English units. Table 1.1 lists units and conversions for many thermodynamic quantities. Observe the use of V for both volume and velocity.

1.6 DENSITY, SPECIFIC VOLUME, AND SPECIFIC WEIGHTBy Eq. (1.1), density is mass per unit volume; by Eq. (1.3), specific volume is volume per unit mass. By comparing their definitions, we see that the two properties are related by

Associated with (mass) density is weight density, or specific weight g :

w ith units N/m3 (lbf/ft3). (Note that g is volume-specific, not mass-specific.) Specific weight is related to density through W = mg:

EXAMPLE 1.3The mass of air in a room 3 m 5 m 20 m is known to be 350 kg. Determine the density, specifi c volume, and specifi c weight of the air.

SolutionEquations (1.1), (1.6), and (1.8) are used:

1.7 PressurePresure is the effect of normal force acting on an area. The SI unit of pressure is the pascal (Pa), where 1 Pa = 1 N/m2. If a force F act s at an angle to an area A, only the normal component Fn enters into the definition of pressure:

In many relations, absolute pressure must be used. Absolute p ressure is gage pressure plus the local atmospher ic pressure:Pabs =Pgage + PatmA negative gage pressure is often called a vacuum, and gages capable of reading negative pressures are vacuum gages. The relationships between absolute and gage pressure at two different points.

1.8 temperatureEQUALITY OF TEMPERATUREIf one is hotter than the other, the hotter body will become cooler and the cooler body will become hotter; both bodies will undergo change until all properties of the bodies cease to change. When this occurs, thermal equilibrium is said to have been establi shed between the two bodies. A rather obvious observation is referred to as the zeroth law of thermodynamics: if two systems are equal in temperature to a third, they are equal i n temperature to each other.ABSOLUTE TEMPERATURE SCALE

The second law of thermodynamics will allow us to defi ne an absolute temperature scale; however, since we have not introduced the second law at this point and we have immediate use for absolute temperature, an empirical absolute temperature scale will be presented. The relations between absolute and relative temperatures are TK = TC + 273.151.9 ENERGY