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Showing posts with the label Thermodynamics


Basic concepts of thermodynamics course with all fundamentals including introduction to laws of thermodynamics, thermodynamic system and properties of system and thermodynamic cycles. Below is complete outline of the subject as taught in mechanical engineering undergraduate course. It is compulsory for all mechanical engineers to revise basics of thermodynamics and other courses to stay updated with knowledge of the subjects and for a better performance during interviews. It has been learnt that many interviewers like to ask basic questions from thermodynamics during interview. Once you have gone through all the short topics given below, you will be in a position to answer all interview questions related to thermodynamics.  Thermodynamics - IntroductionThermodynamic SystemProperties of a SystemThermal EquilibriumLaws of ThermodynamicsLaws of Perfect GasesGeneral Gas EquationCharacteristic Equation of a GasJoule's LawAvogadro's LawUniversal Gas ConstantSpecific Heats of a GasRe…

Thermodynamic System

The thermodynamic system may be defined as a definite area or a space where some thermodynamic process takes place. It may be noted that a thermodynamic system has its boundaries, and anything outside the boundaries is called its surroundings. The thermodynamic system may be classified into the following three groups :

1. Closed system. This is a system of fixed mass whose boundaries are determined by the space of the working substance occupied in it. In a closed system, heat and work cross the boundary of the system, but there is no addition or loss of the original mass of the working substance. Thus the mass of the working substance which comprises the system, is fixed.

2. Open system. In this system, the working substance crosses the boundary of the system. The heat and work may also cross the boundary.

3. Isolated system. It is a system of fixed mass and no heat or work cross its boundary.

Properties of a System

The state of a system may be identified by certain observable quantities such as volume, temperature, pressure and density etc. All the quantities which identify the state of a system are called properties. The thermodynamic properties are divided into the following two classes:

1.Extensive properties. The properties of the system, whose value for the entire system is equal to the sum of their values for the individual parts of the system, are called extensive properties. For example, total volume, total mass and total energy of a system are extensive properties.

2. Intensive properties. The properties of the system, whose value for the entire system is not equal to the sum of their values for the individual parts of the system, are called intensive properties. For example, temperature, pressure and density of a system are intensive properties.

Thermal Equilibrium

When there are variations in temperature from point to point of an isolated system. the temperature at every point first changes with time. This rate of change decreases and eventually stops. When no further changes are observed, the system is said to be in thermal equilibrium.

Laws of Thermodynamics

Following are the three laws of thermodynamics : (Zeroth law of thermodynamics, First law of thermodynamics & Second law of thermodynamics).

1. Zeroth law of thermodynamics. This law states that when two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other

2. First law of thermodynamics. This law states that the heat and mechanical work are mutually convertible. According to this law, a definite amount of mechanical work is needed to produce a definite amount of heat and vice versa.

This law also states that the energy can neither be created nor destroyed, though it can be transformed from one form to another. According to this law, the energy due to heat supplied (Q) must be balanced by the external workdone (W) plus the gain in internal energy (E) due to rise in temperature. In other words,

Q= W + E

3. Second law of thermodynamics.
This law states that there is a definite limit to the amount of mechanical energy, which can be obta…

Laws of Perfect Gases

A perfect gas (or an ideal gas) may be defined as a state of a substance, whose evaporation from its liquid state is complete. It may be noted that if its evaporation is partial, the substance is called vapor. A vapor contains some particles of liquid in suspension. The behavior of super-heated vapors is similar to that of a perfect gas.

The physical properties of a gas are controlled by the following three variables :

1. Pressure exerted by the gas,
2. Volume occupied by the gas, and
3. Temperature of the gas.

The behavior of a perfect gas, undergoing any change in these three variables, is governed by the following laws :

1. Boyles law. This law was formulated by Robert Boyle in 1662. It states, "The absolute pressure of a given mass of a perfect gas varies inversely as its volume, when the temperature remains constant." Mathematically,

The more useful form of the above equation is :

p1v1 = p2v2 = p3v3 = .. = Constant

where suffixes 1, 2 and 3 refer to different sets of condition…

General Gas Equation

The gas laws give us the relation between the two variables when the third variable is constant. But in actual practice, all the three variables i.e.. pressure, volume and temperature, change simultaneously. In order to deal with all practical cases, the Boyle's law and Charles' law are combined together, which gives us a general gas equation.

According to Boyle's law ... (Keeping T constant)

and according to Charles' law ...(Keeping p constant)

It is obvious that