lec 2

System, Boundary and Surroundings
System.
STATEMENTS OF SECOND LAW OF THERMODYNAMICS
The second law of thermodynamics has been enunciated meticulously by Clausius, Kelvin and Planck in slightly different words although both statements are basically identical. Each statement is based on an irreversible process. The first considers transformation of heat between two thermal reservoirs while the second considers the transformation of heat into work.
Clausius Statement
“It is impossible for a self acting machine working in a cyclic process unaided by any external agency, to convey heat from a body at a lower temperature to a body at a higher
temperature”.
In other words, heat of, itself, cannot flow from a colder to a hotter body.
Kelvin-Planck Statement
“It is impossible to construct an engine, which while operating in a cycle produces no other effect except to extract heat from a single reservoir and do equivalent amount of work”.
Although the Clausius and Kelvin-Planck statements appear to be different, they are really equivalent in the sense that a violation of either statement implies violation of other.

Closed System
If the boundary of the system is impervious to the flow of matter, it is called a closed system. An example of this system is mass of gas or vapour contained in an engine cylinder, the boundary of which is drawn by the cylinder walls, the cylinder head and piston crown. Here the boundary is continuous and no matter may enter or leave.

Open System
An open system is one in which matter flows into or out of the system. Most of the engineering systems are open.
Isolated System
An isolated system is that system which exchanges neither energy nor matter with any other system or with environment.
Adiabatic System
An adiabatic system is one which is thermally insulated from its surroundings. It can, however, exchange work with its surroundings. If it does not, it becomes an isolated system. Phase. A phase is a quantity of matter which is homogeneous throughout in chemical composition and physical structure.
Homogeneous System
A system which consists of a single phase is termed as homogeneous system. Examples : Mixture of air and water vapour, water plus nitric acid and octane plus heptane. Heterogeneous System
A system which consists of two or more phases is called a heterogeneous system. Examples:Water plus steam, ice plus water and water plus oil.

MACROSCOPIC AND MICROSCOPIC POINTS OF VIEW
Thermodynamic studies are undertaken by the following two different approaches.
1. Macroscopic approach—(Macro mean big or total)
2. Microscopic approach—(Micro means small)
THERMODYNAMIC EQUILIBRIUM
A system is in thermodynamic equilibrium if the temperature and pressure at all points are same ; there should be no velocity gradient ; the chemical equilibrium is also necessary.
Systems under temperature and pressure equilibrium but not under chemical equilibrium are sometimes said to be in metastable equilibrium conditions. It is only under thermodynamic equilibrium conditions that the properties of a system can be fixed.
Thus for attaining a state of thermodynamic equilibrium the following three types of equilibrium states must be achieved

1.Thermal equilibrium. The temperature of the system does not change with time and has same value at all points of the system.
2. Mechanical equilibrium. There are no unbalanced forces within the system or between the surroundings. The pressure in the system is same at all points and does not change with respect to time.
3. Chemical equilibrium. No chemical reaction takes place in the system and the chemical composition which is same throughout the system does not vary with time.
PROPERTIES OF SYSTEMS
A property of a system is a characteristic of the system which depends upon its state, but not upon how the state is reached. There are two sorts of property :
1. Intensive properties. These properties do not depend on the mass of the system. Examples : Temperature and pressure.
2. Extensive properties. These properties depend on the mass of the system. Example : Volume. Extensive properties are often divided by mass associated with them to obtain the intensive properties. For example, if the volume of a system of mass m is V, then the specific volume of matter within the system is V/m = v which is an intensive property
CYCLE
Any process or series of processes whose end states are identical is termed a cycle. The processes through which the system has passed can be shown on a state diagram, but a complete section of the path requires in addition a statement of the heat and work crossing the boundary of the system.
Ideal Gas
From experimental observations it has been established that an ideal gas (to a good approximation) behaves according to the simple equation
pV = mRT
where p, V and T are the pressure, volume and temperature of gas having mass m and R is a constant for the gas known as its gas constant. Eqn. can be written as
pv = RT (where v = V/m)
In reality there is no gas which can be qualified as an ideal or perfect gas. However all gases tend to ideal or perfect gas behaviour at all temperatures as their pressure approaches zero
pressure.
For two states of the gas, Eq. Above can be written as
P1V1/T1=P2V2/T2
T2/T1=P2/P1*V2/V1
A fluid at a pressure of 3 bar, and with specific volume of 0.18 m3/kg, contained in a cylinder behind a piston exapnds reversibly to a pressure of 0.6 bar according to a law, p =C/v2 where C is a constant. Calculate the work done by the fluid on the piston






The piston moving in a cylinder does not develop any friction during motion.
(ii) The walls of piston and cylinder are considered as perfect insulators of heat.
(iii) The cylinder head is so arranged that it can be a perfect heat conductor or perfect heat
insulator.
(iv) The transfer of heat does not affect the temperature of source or sink.
(v) Working medium is a perfect gas and has constant specific heat.
(vi) Compression and expansion are reversible.
Following are the four stages of Carnot cycle :
Stage 1. (Process 1-2). Hot energy source is applied. Heat Q1 is taken in whilst the fluid expands isothermally and reversibly at constant high temperature T1.
Stage 2. (Process 2-3). The cylinder becomes a perfect insulator so that no heat flow takes place. The fluid expands adiabatically and reversibly whilst temperature falls from T1 toT2.
Stage 3. (Process 3-4). Cold energy source is applied. Heat Q2 flows from the fluid whilst it is compressed isothermally and reversibly at constant lower temperature T2.
Stage 4. (Process 4-1). Cylinder head becomes a perfect insulator so that no heat flow occurs. The compression is continued adiabatically and reversibly during which temperature is raised from T2 to T1.
The work delivered from the system during the cycle is represented by the enclosed area of the cycle. Again for a closed cycle, according to first law of the thermodynamics the work obtained is equal to the difference between the heat supplied by the source (Q1) and the heat
rejected to the sink (Q2).
? W = Q1 – Q2