elements of mechanical engineering

05/02/2023 Ravi Vishwakarma

UNIT-4


Contents

1. Thermodynamics2. Macroscopic & Microscopic point of view3. Concept of Continuum4. Thermodynamic Equilibrium5. Thermodynamic Properties6. Quasi Static Process7. Energy & Its Form8. Zeroth Law of Thermodynamics9. First Law of Thermodynamics


Thermodynamics

Thermodynamics is a branch of science which deals with energy. Engineering thermodynamics is modified name of this science when applied to design and analysis of various energy conversion system. Thermodynamics is core to engineering and allows understanding of the mechanism of energy conversion.


Macroscopic view point of thermodynamics

• In this approach, a certain quantity of matter is considered, without taking into account the events occurring at the molecular level.

• This macroscopic approach to the study of thermodynamics that does not require knowledge of the behavior of individuals particles.

• Macroscopic thermodynamics is only concerned with the effect of the action of many molecules ,and these effect can be perceived by human senses.

• The macroscopic observations are completely independent of the assumptions regarding the nature of matter.


Microscopic view point of thermodynamics

• From the microscopic view point ,matter is composed of a large number of small molecules and atoms.

• This microscopic approach to the study of thermodynamics that require knowledge of the behavior of individuals particles.

• Microscopic thermodynamics is concerned with the effect of the action of many molecules, and these effect cannot be perceived by human senses.

• The microscopic observations are completely dependent on the assumptions regarding the nature of matter.


Concept of continuum

In engineering thermodynamics where focus lies upon the gross behavior of the system and substances in it, the statistical approach is to be kept aside and classical thermodynamics approach be followed.in classical thermodynamics ,for analysis the atomic structure of substance is considered to be continuous. For facilitating the analysis this concept of continuum is used in which the substance is treated free from any kind of discontinuity.


Thermodynamic Equilibrium

A system is considered to be in a thermodynamic equilibrium when it is in thermal equilibrium ,mechanical equilibrium and chemical equilibrium. A system is said to be in thermal equilibrium when the temperature of the system is uniform. A system is said to be in mechanical equilibrium having uniform pressure as well as the effect of gravity is not significant. A system is said to be in chemical equilibrium ,there is no chemical reactions take place or no chemical changes occur.


Thermodynamic Properties

Thermodynamic properties are the observable characteristics of the Thermodynamic system. Pressure ,temperature volume, viscosity ,modulus of elasticity etc. are the examples of property.Intensive Properties –the intensive properties are those properties which have same value for any part of the system or the properties that are independent of the mass of system are called intensive properties. Example-Pressure ,temperatureExtensive Properties-Extensive Properties on the other hand are those which depend upon the mass of system and do not maintain the same value for any part of the system. Example-Mass, volume, energy, enthalpy etc.


Quasi Static ProcessBasically the thermodynamics systems are concerned with the system of equilibrium state. Whenever a system undergoes a change from one state to another state is called process.Consider a piston cylinder assembly with a gas enclosed within the cylinder .piston carries many small weights on the piston. The system is initially in equilibrium with pressure P1,tempertaure T1 and volume V1.when these small weights are removed one by one very slowly. The unbalanced potential is infinitesimally small. the piston will move upward very slowly so that each state will consider as an equilibrium state. The path followed by these equilibrium state on removal of each masses called a quasi-static or quasi-equilibrium process. It may represent as a continuous line or graph.


1

2

Equilibrium State

P

V

Reversible & Quasi static Process


Energy & Its Form

“Energy is usually defined as the ability to do mechanical work”.It is indeed quite difficult to precisely define the energy. Energy can be classified in the following general categories:Potential energy: this type of energy is based an relative position of bodies in a system i.e. elevation in a gravitational field.Potential energy=mgzHere g is the gravitational acceleration and elevation z is measured from some reference point.Kinetic energy: It is based on the relative movement of bodies. For a mass m moving with certain velocity c the kinetic energy could be mathematically expressed as:

2.21 cmKE


Internal Energy: Internal energy of a system is the energy associated with the molecular structure at the molecular level. Internal energy is sum of energy associated with molecules which may have translational ,vibrational and rotational motion etc. and respective energies causing these motions.

Total Energy=KE+PE+IEWhereKE=Kinetic energyPE=Potential energyIE=Internal energy


Zeroth Law of thermodynamics

Zeroth law of thermodynamics states that if the bodies A and B are in thermal equilibrium with a third body C separately then the two bodies A and B shall also be in thermal equilibrium with each other. This is the principle of temperature measurement.

A B

C

Thermal EquilibriumThermal Equilibrium

Thermal Equilibrium


First Law of Thermodynamics

First law of thermodynamics states that “in closed system undergoing a cyclic process the net work done is proportional to the net heat taken from the surrounding’’ or “ for any cycle of a closed system the net heat transfer equals the net work”Mathematically expression for the first law of thermodynamics can be rearranged and it shall be ,

Which shows that the quantity is a thermodynamics property.

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elements of mechanical engineering

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