Thermodynamics definition

Definition 

• What is the boiling point
• What is the freezing point 
• What is the melting point 
• What is enthalpy
• What is entropy
• What is absolute entropy

What is boiling point

Boiling point :

The temperature at which the liquid boils and turns to vapour is called the boiling point.

In other word, the boiling point of a substance is the temperature at which the vapour pressure of the liquid equals the pressure surrounding the liquid.

When a liquid is heated, it eventually reaches a temperature at which the vapour pressure is large enough that bubbles form inside the body of the liquid. This temperature is called the boiling point.

The boiling point of a liquid varies depending upon the surrounding environmental pressure.

For any liquid boiling point is high in high pressure than its atmospheric pressure.

For a given pressure, different liquids boil at different temperatures. 

For example, water boils at 100 °C (212 °F) at sea level, but at 93.4 °C (200.1 °F) at 2,000 meters (6,600 ft) altitude.

Thermodynamic Equilibrium

Thermodynamic equilibrium, in simple words, is the same temperature. At same temperature bodies do exchange heat but do not gain or lose heat.

A system is said to exist in a state of thermodynamic equilibrium when no change in any macroscopic property used if the system is isolated from its surroundings. an isolated system always reaches in course of time a state of thermodynamic equilibrium and can never depart from it spontaneously.

How can an isolated system experience a change in its macroscopic properties? We know that an isolated system can't exchange matter and energy from its surrounding. How does the change happen without any external interaction?

for example, A cup of tea whose temperature is measured to be 60 degrees Celsius is kept in a room which is at 25 degrees Celsius. So, eventually, the tea's temperature is gonna drop until it reaches 25. And, then we can say that tea is in thermodynamic equilibrium with respect to the room.

But, if we isolate the tea, how can we talk about its equilibrium over the course of time? Since the tea is not interacting with any external environment, how does it's temperature drops? How does it attain thermodynamic equilibrium and with respect to what?

Thermodynamics studies mainly the properties of physical systems that are found in equilibrium states.

A system will be in a state of thermodynamic equilibrium if the conditions for the following three types of equilibrium are satisfied.

1. Mechanical Equilibrium
2. Chemical Equilibrium
3. Thermal Equilibrium

In the absence of an unbalanced force within the system itself and also between the system and the surroundings, the system is said to be in a state of mechanical equilibrium.

If there is no chemical reaction or transfer of matter from one part of the system to another, such as diffusion or solution, the system is said to exist in a state of chemical equilibrium.

When a system existing in mechanical and chemical equilibrium is separated from its surroundings by a diathermic wall and if there is no spontaneous change in any property of the system, the system is said to exist in a state of thermal equilibrium. when this is not satisfied, the system will undergo a charge of state till thermal equilibrium is restored.

When the conditions for any one of three types of equilibrium are not satisfied, a system is said to be in a non equilibrium state.

What is thermodynamic system

The term thermodynamic system is used frequently in the subject of thermodynamics. 
Let us see what the thermodynamic system is and its various types.

Introduction :

The word system is very commonly used in thermodynamics; let us know what it is. A certain quantity of matter or space which is under thermodynamic study or analysis is called a system. Let us say for example we are studying the engine of the vehicle, in this case, the engine is called the system. Similarly, the other examples of the system can be a complete refrigerator, air-conditioner, washing machine, heat exchange, a utensil with hot water etc.

Now, let us suppose that we have to analyze the performance of the engine in different conditions. Here, we will feed the engine with fuels of different grades and load it with different loads to find out its efficiency. We will also find its performance during idling, acceleration, varying speed, slow speed and high speed. A thorough analysis of the engine is carried out; hence it is called a system.

The system is covered by the boundary and the area beyond the boundary is called as universe or surroundings. The boundary of the system can be fixed or it can be movable. Between the system and surrounding the exchange of mass or energy or both can occur.

A thermodynamic system is defined as a quantity of a matter or a region in space upon which attention is concentrated in the analysis of a problem. Everything external to the system is called the surroundings or the environment. The system is separated from the surroundings by the system boundary.

There are three classes of the system : 

• Closed System 
• Open System 
• Isolated System

The closed system is a system of fixed mass. There is no mass transfer across the system boundary. There may be energy transfer into or out of the system. 

The open system is one in which matter crosses the boundary of the system.
There may be mass transfer as well as energy transfer also. Most of the engineering devices are generally open systems.

The isolated system is one in which there is no interaction between the system and the surroundings.

It is of fixed mass and energy, and there is no mass or energy transfer across the system boundary.

If a system is defined as a certain quantity of matter, then the system contains the same matter and there can be no transfer of mass across its boundary. However, if a system is defined as a region of space within a prescribed boundary, then the matter can cross the system boundary. while the former is called a closed system, the latter is an open system.

Gas Laws

The gas laws were developed at the end of the 18th century.

when scientists began to realize that relationships between the pressure, volume and temperature of a sample of gas could be obtained which would hold to a good approximation for all gases. 

Gases behave in a similar way over a wide variety of conditions because they all have molecules which are widely spaced, and the equation of state for an ideal gas is derived from kinetic energy. 

The earlier gas laws are now considered as special cases of the ideal gas equation, with one or more of the variables held constant.

Boyle's Law :

Boyle's Law published in 1662, states that, at a constant temperature, the product of the pressure and volume of a given mass of an ideal gas in a closed system is always constant.

It can be verified experimentally using a pressure gauge and a variable volume container. It can also be derived from the kinetic theory of gases: if a container, with a fixed number of molecules inside, is reduced in volume, more molecules will strike a given area of the sides of the container per unit time, causing a greater pressure.

As a mathematical equation, Boyle's Law is written as either:

The statement of Boyle 's law is as follows:

The volume of a given mass of a gas is inversely related to the pressure exerted on it at a given temperature and given a number of moles.

Charles's Law :

Charles's Law or the law of volumes was found in 1787 by Jacques Charles. 

It states that, for a given mass of an ideal gas at constant pressure, the volume is directly proportional to its absolute temperature, assuming in a closed system.

As a mathematical equation, Charles's Law is written as either:

${\displaystyle V/T=k_{2}}$

${\displaystyle V_{1}/T_{1}=V_{2}/T_{2}}$

Gay-Lussac's Law :

Gay-Lussac's Law or the Pressure Law was found by Joseph Louis Gay-Lussac in 1809. It states that, for a given mass and constant volume of an ideal gas, the pressure exerted on the sides of its container is directly proportional to its absolute temperature. 

As a mathematical equation, Gay-Lussac's Law is written as either:

${\displaystyle P/T=k_{3}}$

${\displaystyle P_{1}/T_{1}=P_{2}/T_{2}}$

where P is the pressure, T is the absolute temperature, and k₃ is another proportionality constant.

Avogadro's Law :

Avogadro's Law states that the volume occupied by an ideal gas is directly proportional to the number of molecules of the gas present in the container. This gives rise to the molar volume of a gas which at STP (273.15 K, 100 kPa) is about 22.7 l/mol. The relation is given by

where n is equal to the number of molecules of gas (or the number of moles of gas).

Combined and Ideal Gas Laws :

With the addition of Avogadro's Law the combined gas Law develops into the Ideal Gas Law:

where

p is pressure

V is volume

n is the number of moles

R is the universal gas constant

T is temperature (K)

where the proportionality constant, now named R, is the universal gas constant with a value of 0.083144598 (kpa∙L)/(mol∙K). An equivalent formulation of this Law is:

${\displaystyle pV=kNT\,}$

where

p is the pressure

V is the volume

N is the number of gas molecules

k is the Boltzman constant (1.381×10⁻²³ J·K⁻¹ in SI units)

T is the absolute temperature

These equations are exact only for an ideal gas which neglects various intermolecular effects.

This law has the following important consequences:

1. If temperature and pressure are kept constant, then the volume of the gas is directly proportional to the number of molecules of gas.
2. If the temperature and volume remain constant, then the pressure of the gas changes is directly proportional to the number of molecules of gas present.
3. If the number of gas molecules and the temperature remain constant, then the pressure is inversely proportional to the volume.
4. If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature.

Other Gas Law :

Graham's Law states that the rate at which gas molecules diffuse is inversely proportional to the square root of its density. 

Combined with Avogadro's law this is the same as being inversely proportional to the root of the molecular weight.

Henry's Law states that at constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

${\displaystyle p=k_{\rm {H}}\,c}$

Dalton's Law of partial pressure states that the pressure of a mixture of gases simply is the sum of the partial pressures of the individual components. Dalton's Law is as follows

${\displaystyle P_{total}=P_{1}+P_{2}+P_{3}+...+P_{n}\equiv \sum _{i=1}^{n}P_{i}\,}$,

or

${\displaystyle P_{\mathrm {total} }=P_{\mathrm {gas} }+P_{\mathrm {H_{2}O} }\,}$

where P_Total is the total pressure of the atmosphere

P_Gas is the pressure of the gas mixture in the atmosphere

and P_H2O is the water pressure at that temperature

Types of thermodynamic process

Introduction of thermodynamic process : 

Before going to study the thermodynamic process and types of thermodynamic processes, let us understand the meaning of the thermodynamic state of the system. The system has a certain temperature, pressure, volume, etc. characteristics. The present values of the system property are called the thermodynamic state of the system. 

Thermodynamic process :

When the system undergoes a change from one thermodynamic state to final state due change in properties such as temperature, pressure, and volume etc the system is said to have undergone the thermodynamic process. Types of the thermodynamic process described below. 

In simple word, a thermodynamic process occurred when the system changes from initial state to the final state.

• Process - Adiabatic 

Properties held constant - Heat energy 

• Process - Isenthalpic 

Properties held constant - Enthalpy

• Process - Isentropic 

Properties held constant - Entropy, Heat energy, Equilibrium 

• Process - Isobaric 

Properties held constant - Pressure 

• Process - Isochoric 

Properties held constant - Volume 

• Process - Isothermal   

Properties held constant - Temperature

• Process - Isotropic 

Properties held constant - Direction 

• Process - Polytropic 

Properties held constant - PVⁿ = C

• Process - Reversible 

Properties held constant - Entropy, Equilibrium 

Adiabatic process:  

An adiabatic process occurs when no heat can flow between a thermodynamic system and its surroundings. 

In this process Q = 0.

Example - Vertical flow of air in the atmosphere, Air expands and cools as it rises, and contracts and grows warmer as it descends. 

Isenthalpic process : 

An isenthalpic process is also called isoenthalpic process. It is a thermodynamic process in which enthalpy is constant. 

In this process H = 0. 

Example - Throttling process, consider the lifting of a relief valve or safety valve on a pressure vessel.

Isentropic process :

An isentropic process is an idealized thermodynamic process in which both adiabatic and reversible. 

It is a process in which entropy remains constant. 

In this process ΔS  = 0.

Example - Some isentropic thermodynamics device such as pumps, gas compressors, turbines, nozzles, diffusers.

Isothermal process :  

An isothermal process is a change of a system, in which the temperature of the system stays constant but heat may flow in or out of the system during an isothermal process. 

In this process ΔT = 0.

Example - Condensation, All the reactions going on in the refrigerator as a constant temperature is maintained in it, Melting of ice at zero degrees, and heat pump. 

Isochoric process :  

An isochoric process as the name suggests iso means same and choric means volume also called constant-volume process or isovolumetric process or isometric process. 

It is a thermodynamic process during which the volume of the closed system is kept constant.

In this process ΔV = 0.

Example - Heating of a gas in a closed cylinder.

Isobaric process : 

An isobaric is a thermodynamic process where the pressure of the system stays constant. 

In this process  ΔP  = 0.

Example: Heating of water in an open vessel and the expansion of a gas in a cylinder with a freely moving piston.

Isotropic process : 

The isotropic process is one that the permittivity ε and permeability μ of the medium is uniform in all directions of the medium. 

Example - Glass and metals are examples of isotropic materials. 

Reversible processes :  

A reversible process is a process whose direction can be reversed by including infinitesimal changes to some property of the system via its surroundings. In thermodynamics, throughout the entire process, the system is in thermodynamic equilibrium with its surroundings.

Example - Frictionless relative motion, and expansion and compression of spring.

Polytropic Process :

A polytropic is a thermodynamic process that obeys the relation where p is the pressure, V is volume, n is the polytropic index and C is a constant. The equation of this process describes multiple expansion and compression processes which include heat transfer. 

PVⁿ = C

From this relationship, we can arrive at relationships for several other types of a thermodynamic process.

• When n = 0 the process is isobaric
• When n = 1 the process is isothermal
• When n = k the process is isentropic
• When n = ∞ the process is isochoric

Example - Expansion of the combustion gasses in the cylinder of a water-cooled reciprocating engine.

Properties of a system in thermodynamics

Introduction of various properties :

Thermodynamic property is a point function and defines the state of a system. It is independent of the path followed. 

Generally, a thermodynamic property is two types one is macroscopic and another one is microscopic property.

The word microscopic means something like so small that it can only be seen with the use of microscope while macroscopic means either to something that can be seen with the naked eye or large in scale. 

If a system contains a large number of chemical species such as atoms, ions, and molecules, called macroscopic system and the properties which are associated with this system are called macroscopic properties.

Examples: pressure, volume, temperature, composition, density, viscosity, surface tension, refractive index, colour etc.

Extensive properties: 

Extensive properties depend upon the quantity of matter which is contained in the system. 

Extensive property is dependent on mass.

Examples: mass, volume, heat capacity, internal energy, enthalpy, entropy, Gibb's free energy. 

Intensive properties:  

Intensive properties depend upon the amount of the substance which is present in the system.

The intensive property is not dependent on mass.

Examples: temperature, refractive index, density, surface tension, specific heat, freezing point, and boiling point.

What is Entropy?

Definition of Entropy :

Entropy is a thermodynamic quantity representing the unavailability of a system's thermal energy for conversion into mechanical work and interpreted as the molecular disorder in the system. 

In other words, entropy is the measure of a system's thermal energy per unit temperature that is unavailable for doing useful work. OR Entropy is also the measure of the number of possible arrangements the atoms in a system can have. 

SI unit for entropy is J / K ( joules/degree Kelvin ).

Example: 

Spraying perfume in the corner of the room and we all know what happens next. The perfume will not just stay in the corner of the room but the perfume molecule eventually fills up the room. The perfume went an ordered state to a state of the disorder so the system gets disorder so is called the higher entropy.

Avogadro's Law | Principle | Formula

Principle of Avogadro's law: 

Avogadro's law is a mole of a substance has a mass numerically equal to the molecular weight of the substance.

1 gm mole of oxygen has a mass of 32 gm.

Avogadro's law state that the volume of a gm mol of all gases at the pressure of 760 mm Hg and the temperature of 0^o C is the same and is equal to 22.4 liters.

For a certain gas, we can say that if m is its mass in kg, and M is its molecular weight, then the number of kg moles of gas n would be given by

n = m kg / M kg/kg mol

n = m / M kg moles

The Moler weight is given by  V / n  m³ / kg mol

V represents the total volume of the gas in m³       

Available Energy | Availability | Irreversibility | Definition | Formula

What is Available Energy?

Energy sources can be divided into two groups:

• High-grade energy
• Low-grade energy

Under the second law of thermodynamics, the complete conversion of low-grade energy, heat, into high-grade energy, shaft-work is impossible, that part of low-grade energy which is available for conversation is called as available energy.

The maximum work output in a cycle obtained from a certain heat input is called available energy.

What is Availability?

Whenever useful work is obtained during a process in which the system undergoes a change of state, the process must be terminated when the pressure and temperature of the system have become equal to the pressure and temperature of the surrounding.

The availability of the given system is defined as the maximum useful work that is obtained in a process in which the system comes to equilibrium with its surroundings.

Availability is, therefore, a composite property depending on the state of both the system and surroundings.

What is Irreversibility?

The actual work done by a system is always less than the idealized reversible work, and irreversibility is called the difference between these two. 

I = W_max - W

This is also sometimes referred to as degradation or dissipation.

Nomenclature of Thermodynamics

NOMENCLATURE :

Symbols	Name	Unit
A	Availability or Energy	KJ
b	Specific Keenan Function	KJ/Kg
c	Velocity of Sound	m/s
Cp	Specific Heat at constant pressure	KJ/Kg K
Cv	Specific Heat at constant volume	KJ/Kg K
COP	Coefficient of Performance	No Unit
D, d	Diameter	Many Units Exist
e	Specific Energy	KJ/Kg
E	Total Energy	KJ
f	Fugacity, Specific Helmholtz Function, u-Ts	KJ/Kg
F	Force	N
F	Helmholtz Function	KJ
g	Gravitational acceleration	m/s2
g	Specific Gibbs Function	KJ/Kg
h	Specific Enthalpy	KJ/Kg
hf	Enthalpy of formation	KJ/Kg mol
HP	Enthalpy of Products	KJ
HR	Enthalpy of Reactants	KJ
hRP	Enthalpy of Combination	KJ/Kg fuel
HHV	Higher heating value	KJ/Kg fuel
i	Specific irreversibility	KJ/Kg
I	Total irreversibility	KJ
K	Boltzmann constant	J/molecule-K
K	Indicator spring constant	N/cm3
k	Thermal conductivity	W/mK
kT	Isothermal compressibility	K-1
kS	Adiabatic compressibility	m2 / N
K	Equilibrium constant
KE	Total kinetic energy	KJ
l	Latent Heat	KJ/Kg
L	Length	m
LHV m	Lower Heating Value Mass	KJ/Kg Kg
ṁ	Mass flow rate	Kg/s
M	Mach Number
M	Molar mass	Kg/Kg mol
mep	Mean effective pressure	N/m2
MF	Mass fraction
n	Polytropic index, Number of moles
N	Number of molecules
p	Pressure	N/m2  (Pa)
p	Partial Pressure	Pa
Pc, Pcr	Critical Pressure	kPa
Po	Atmosoheric Pressure	kPa
Pv	Vapour Pressure	kPa
PE	Total Potential Energy	KJ
Q	Total Heat Transfer	KJ
Q	Heat transfer rate	KJ/s
rC	Cut-off ratio
re	Expansion ratio
Yk	Compression ratio
Yp	Constant volume pressure ratio
R	Characteristics of gas constant	KJ/Kg-K
R,Ru	Universal gas constant	KJ/Kg mol-K
s	Specific entropy	KJ/Kg-K
S	Total entropy	KJ/K
t	Time, Temperature	s, 0C
T	Temperature	K
Tas	Adiabatic saturating temperature	K
Tcr	Critical temperature	K
Tdb	Dry bulb temperature	K,0C
Tdp	Dew point temperature	0C
T0	Temperature of surroundings	K
u	Specific internal energy	KJ/Kg
U	Total internal energy	KJ
v	Specific volume	m3/Kg
vc, vcr	Critical specific volume	m3/Kg
V	Total volume	m3
W	Total Work	KJ
W	Power	KW
Wmax	Maximum Work	KJ
Wrev	Reversible Work	KJ
Wu	Useful Work	KJ
x	Quality, Dryness Fraction
x	Mass fraction, Mole fraction
z	Elevation	m
Z	Compressibility factor