Thermodynamics (Lecture-1)

Thermodynamics is science of energy transfer and its effect on the physical properties of substances.

The applications of Thermodynamic laws and principles are found in all fields of energy
technology , Notably in steam and nuclear power plants, internal combustion engines, gas
turbines, air conditioning and refrigeration etc.

Macroscopic Vs Microscopic viewpoint:

These are two ways of studying the behaviour of matter

Thermodynamic system and Control Volume:

Thermodynamic system is defined as a quantity of matter or a region in space, upon which attention is

paid or focussed in the analysis of the problem.

Everything external to the system is called surroundings.

System is separated from its surroundings by system boundary. The boundary may be well either fixed

or moving.

System + Surrounding = Universe

 System broadly classified into three types: a) closed   b) open c) Isolated

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In closed system, Energy transfer may take place across the system boundary but mass transfer cannot.

Special case of Closed system where along with mass transfer, energy transfer also does not occur and hence has no interactions with surroundings is described as Isolated system.

In open system, both mass and energy transfer can take place across the system boundary.

In Thermodynamic analysis of an open system, (such as air compressor) attention is focussed on a

certain volume in space surrounding the system (say, compressor) called as Control Volume,

bounded by surface called the control surface. Matter as well as energy can cross the control

surface.

Thermodynamic Properties, Processes and Cycles

Thermodynamic properties: Characteristics of a system by which it’s physical condition can be

described. E.g: volume, temperature etc

When all the properties of a system have definite values, the system is said to exist a definite state.

Any operation in which one or more of the properties of a system changes is called change of state.

The succession of states passed through during the change of state is called the path of change of

state.

When the path is completely specified it is called process.

Series of state changes such that the final state is identical with the initial state is called a

Thermodynamic Cycle.

Properties are of two types:

1. Intensive Properties: these are independent of mass in the system. Eg: Pressure, Temperature etc.
   
   
2. Extensive Properties: these are dependent on mass in the system. E.g: mass, volume, energy
   etc.

Specific Extensive properties i.e. extensive properties per unit mass are intensive properties.

E.g specific volume, specific entropy etc.

Thermodynamic Equilibrium:

A system is said to be in thermodynamic equilibrium if following three conditions are satisfied in

regard to the system:

1. Mechanical Equilibrium: Absence of any kind of unbalanced force within system itself and also
   between system and its surroundings
2. Chemical Equilibrium: No chemical reaction or mass transfer from one part of system to another,
   such as diffusion or solution
3. Thermal Equilibrium: satisfied if there is no change in any property of the system existing in
   mechanical and chemical equilibrium when separated from its surroundings by a diathermic wall (wall that allows heat flow)

Thermodynamic properties are macroscopic coordinates  defined for and significant to, only

thermodynamic equilibrium states.

Pressure:

= Force per unit area

Absolute pressure = Gauge pressure + Atmospheric pressure

Pressure units: SI unit - Pascals (Pa)

Other common units: Bar, atm

1 bar = 100 kPa

1 atm = 101.325 kPa = 76 cm of Hg

When pressure in the system is below atmospheric pressure, the gauge pressure becomes negative.

This negative gauge pressure is expressed in terms of positive number and is called as vacuum pressure.

Pressure measuring devices in general measure Gauge pressure.

Examples for pressure measuring devices:

1. Bourdon Gauge
   
   
2. Open U-tube manometer
   
   

Specific volume (𝝂)and density (𝜌): Volume per unit mass and mass per unit volume respectively.

Energy:

Capacity to exert a force through a distance . It manifests itself in various forms.

Units: SI system: joule (J) or Nm.

Specific energy (J/kg) = Energy per unit mass

Power:

Rate of energy transfer or storage.

Units: watt (W)