By Rasikh Tariq

An Easy Approach-Thermodynamics

Thermodynamics is the branch of natural science concerned with heat and its relation to other forms of energy and work.

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What is Thermodynamics?

Basis of Thermodynamics

Thermodynamics is entirely based upon following items : Zeroth Law of Thermodynamics First law of Thermodynamics Second Law of Thermodynamics Entropy

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Basics of Thermodynamics

Surroundings:-Any mass or region outside the

system is called the surroundings.

System:-A system is defined as a quantity of matter or a

region in space chosen for study.

Boundary:-The Real or Imaginary surface that separates the system from its surroundings is called the boundary. Mathematically, the boundary has zero thickness.

System

Closed System

Open System

Isolated System

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Types of System

A closed system consists of a fixed amount Of mass, and no mass can cross its boundary.

In open systems, matter may flow in and out of the system boundaries.

Human Body is a Open System

Characteristic of a system is called a Property.Common are:-

Pressure PTemperature TVolume VMass m

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Properties of a System

Property

Extensive Intensive

Depends upon Size-or extent-of the system. E.g. Total mass, total

volume and momentum

Properties Independent of the mass of the system.

E.g. Temperature, pressure and density

Note This Specially

Density is defined as Mass per unit Volume.

The Reciprocal of Density is called Specific Volume.

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Density and Specific Volume

Equilibrium

Thermal Equilibrium Mechanical Equilibrium Phase Equilibrium Chemical Equilibrium

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Equilibrium

Thermodynamics deals with Equilibrium States. The Word “Equilibrium” implies a state of balance

If the Temperature is

the same throughout the entire system

If there is no change in

pressure at any point of the system with

time

When the mass of each phase reaches an equilibrium level & stays

there

If the Chemical

Composition does not

changes with time

A System will not be in Equilibrium unless all the relevant equilibrium criteria are satisfied

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The State PostulateAny two independent intensive thermodynamic properties are sufficient to describe

the state of a system containing a single pure substance.

Processes and CyclesAny Change that a System undergoes from one

equilibrium state to another is called a Process.And the series of states through which a system passes is called the Path of the Process

Quasi-Equilibrium StateA Quasi-equilibrium process can be viewed as a sufficiently slow process that allows the system to

adjust itself internally so that properties in one part of the system do no change any faster than those at

the Other parts.

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Zeroth Law of Thermodynamics

By Replacing the third body with a thermometer, The Zeroth Law can be

restated as “Two Bodies are in Thermal equilibrium

if both have the same temperature reading even if they are not in contact.”

The Zeroth Law of Thermodynamics states that if two bodies are in Thermal Equilibrium with a third body, they are also in Thermal Equilibrium

with each other

Zeroth Law serves as the basis of

validity of TEMPERATURE

measurement.

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Temperature ScalesWe had 4 temperature Scales:1. Celsius Scale2. Kelvin Scale3. Fahrenheit Scale4. Rankine ScaleThe Kelvin Scale is related to Celsius scale by

The Fahrenheit Scale is related to Celsius scale by

The Rankine Scale is related to scale by

Ice-Point:- Mixture of ice and liquid water that is in equilibrium with air saturated with vapor at1atm pressure

Steam-Point:- Mixture of Liquid water and water vapor that is in equilibrium at1atm pressure

Pressure is defined as normal force exerted by a fluid per unit area.

Unit is Pascal Some different units of Pressure are

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Pressure

Absolute Pressure:- The actual pressure at a given position is called Absolute Pressure and is measured relative to absolute Vacuum.Gage Pressure:-The difference between the absolute pressure and the local atm. Pressure are called the Gage Pressure.Vacuum Pressure:-Pressure below the atm. Pressure are called Vacuum pressure measured by vacuum gauges.

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Forms of EnergyThermal Mechanica

l Kinetic Potential Electric Chemical Nuclear

The Total Energy of a system on a unit mass basis is denoted by “e” and is expressed as

Microscopic and Macroscopic Forms of EnergyThe Macroscopic forms of energy are those a system possesses as a whole with respect

to some outside reference frame such as Kinetic Energy and Potential Energies.

The Microscopic forms of Energy are those related to the molecular structure of a system and the degree of the molecular activity, and they are independent of outside

Reference Frames.The sum of all the microscopic forms of energy is called the Internal Energy of a system and is denoted by

“U”.

On Unit Basis

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Total EnergyThe Magnetic, electric and surface tension effects are significant in some

specialized cases only and are usually ignored. Hence, The total Energy is expressed as

And on Unit basis can be expressed as

Mass & Volume Flow RateAmount of mass flowing through as cross section

per unit time is called mass flow rate It is related to the volume flow rate.

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Internal EnergyThe sum of all the microscopic forms of energy is called the Internal Energy of a system.

Energy Transfer by HeatHeat is defined as

“The Form of Energy that is transferred between two systems (or a system and its surroundings) by

virtue of a temperature difference.”

Adiabatic Process:-A process in which there is no heat transfer is called an adiabatic process. i.e dQ=0

Method of Heat Transfer

Conduction Convection Radiation

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Conduction is the transfer of energy from the more energies particle of a substance to the adjacent less energetic ones as a result of interaction between the particles.Convection is the transfer of heat between a solid surface and the adjacent fluid that is in motion.Radiation is the transfer of energy due to emission of electromagnetic waves.

Energy Transfer by WorkWork is energy transfer associated with a force acting through a distance

And on Unit basis can be expressed as:

It is also called Power.

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Path Functions have Inexact differentials designated by a symbolDifferential amount of heat or work is represented by or respectively. However, properties are point functions have exact differentials.

Forms of Work

Mechanical forms of Work

Shaft Work

Spring Work

Non-Mechanical

forms of Work

Electric Work

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The First Law of ThermodynamicsEnergy can not be created nor destroyed but can change its form from one

to another.

Energy BalanceThe net change in the total energy of a system during a process is equal to the difference between the total energy entering and the total energy leaving the

system during that process

Where

A Practical Application

Δz

Here,PE = 10 kJ

KE = 0

Now,PE = 7 kJKE = 3 kJ

Mechanism of Heat Transfer

Mass Flow Work Transfer Heat Transfer

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For a Cycle

And in Ratio Form:

Energy balance for any system

undergoing any kind of process expressed more compactly as

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EfficiencySimply, Desired output divided by desired input is called

Efficiency

Efficiency of Generator

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Pure SubstanceA substance that had a fixed chemical composition

throughout is called a Pure SubstanceAre composed of a single chemical species (e.g., either O2 or CO2

but not a mixture of O2 and CO2).May exist in more than one phase (e.g., solid and liquid)

A mixture of snow, ice, liquid water and water vapor is a pure substance.

Properties Extensive

Intensive

Mass m -

Temperature - T

Pressure - P

Volume V v = V/m (specific volume)

Internal Energy U u = U/m (specific internal energy)

Enthalpy H h = H/m (specific enthalpy)

Any extensive property can be made intensive (specific) by dividing by mass.

Compressed LiquidExistance of pure Substance in Liquid Phase Or the Liquid not about to Vaporize.Saturated LiquidA Liquid about to Vaporize.Satutrated Vapour:-A vapour that is about to condense.Super-Heated Vapour:-A vapor that is not about to condenseThe temprature at which pure Substance changes phase is called a Saturation Temprature.The Pressure at which pure Substance changes phase is called a Saturation Pressure.The point at which saturated liquid and saturated vapour states are identical.

P-v-T Relations

Constant Pressure Heating in Piston-Cylinder at P = 1 atm

Constant Pressure Heating in Piston-Cylinder at P = 2 atm

Temperature-Volume (T-v)Relation

Note directions of Isobars

Pressure-Volume (P-v)Relation

Note directions of Isotherms

P-v-T Surface