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The lecture deals with

First Law of Thermodynamics Heat is a Path Function Energy is a Property of the System A Perpetual Motion Machine of First Kind Analysis of Closed Systems

Energy is a Property of the System

Refer to Figure 6.2 again and consider Eq. (6.5)

(6.8)

and depend on path function followed by the system. The

quantity is the same for both the processes and connecting

the states 2 and 1. The quantity does not depend on path followed by

the system, depends on the initial and final states. Hence is an exactdifferential.

Differential of property of the system

This property is the internal energy of system, E

(6.9)

Energy of an Isolated System

An isolated system is one in which there is no interaction of the system with the

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surroundings. For an isolated system

(6.10)

A Perpetual Motion Machine of First Kind

Thermodynamics originated as a result of man's endeavour to convert the disorganized form ofenergy (internal energy) into organized form of energy (work).

(6.11)

An imaginary device which would produce work continuously without absorbing any energyfrom its surroundings is called a Perpetual Motion Machine of the First kind , (PMMFK). APMMFK is a device which violates the first law of thermodynamics. It is impossible to devise aPMMFK (Figure 6.3)

Figure 6.3

The converse of the above statement is also true, i.e., there can be no machine which wouldcontinuously consume work without some other form of energy appearing simultaneously.

Analysis of Closed System

Let us consider a system that refers to a definite quantity of matter which remains constant while thesystem undergoes a change of state. We shall discuss the following elementary processes involving theclosed systems.

Constant Volume Process

Our system is a gas confined in a rigid container of volume V (Refer to Figure 6.4)

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Figure 6.4

Let the system be brought into contact with a heat source.

The energy is exchanged reversibly. The expansion work done (PdV) by the system is zero.

Applying the first law of thermodynamics, we get

(6.12)

or,

(6.13)

Hence the heat interaction is equal to the change in the internal energy of the system.

Constant Volume Adiabatic Process

Refer to Figure 6.5 where a change in the state of the system is brought about by performing paddlewheel work on the system.

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Figure 6.5

The process is irreversible. However, the first law gives

(6.14)

or

(6.14)

Interaction of heat and irreversible work with the system is same in nature. (-W) represents the work doneon the system by the surroundings

Specific Heat at Constant Volume

By definition it is the amount of energy required to change the temperature of a unit mass ofthe substance by one degree.

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(6.16)

While the volume is held constant. For a constant volume process, first law of thermodynamicsgives

(6.17)

Therefore,

(6.18)

Where is the specific internal energy of the system. If varies with temperature, one canuse mean specific heat at constant volume

(6.19)

The total quantity of energy transferred during a constant volume process when the system

temperature changes from

(6.20)

The unit for is kJ/kgK. The unit of molar specific heat is kJ/kmolK.