Railways NTPC (Technical Ability) Thermodynamics


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  • Thermodynamics is a branch of physics concerned with heat and temperature and their relation to energy and work. It defines macroscopic variables, such as internal energy, entropy, and pressure that partly describe a body of matter or radiation.
  • It states that the behavior of those variables is subject to general constraints that are common to all materials, not the peculiar properties of particular materials. These general constraints are expressed in the four laws of thermodynamics.
  • Thermodynamics describes the bulk behavior of the body, not the microscopic behaviors of the very large numbers of its microscopic constituents, such as molecules.
  • Its laws are explained by statistical mechanics, in terms of the microscopic constituents.
  • Thermodynamics applies to a wide variety of topics in science and engineering.
  • Thermo-dynamics is the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency.
  • Initially, thermodynamics, as applied to heat engines, was concerned with the thermal properties of their 'working materials' such as steam, in an effort to increase the efficiency and power output of engines.
  • The plain term 'thermo-dynamics' refers to a macroscopic description of bodies and processes.
  • Thermodynamics arose from the study of two distinct kinds of transfer of energy, as heat and as work, and the relation of those to the system's macroscopic variables of volume, pressure and temperature. Transfers of matter are also studied in thermodynamics.
  • Thermodynamic equilibrium is one of the most important concepts for thermodynamics. The temperature of a thermodynamic system is well defined, and is perhaps the most characteristic quantity of thermodynamics.
  • In engineering practice, thermodynamic calculations deal effectively with such systems provided the equilibrium thermodynamic variables are nearly enough well-defined.
  • Central to thermodynamic analysis are the definitions of the system, which is of interest, and of its surroundings. The surroundings of a thermodynamic system consist of physical devices and of other thermodynamic systems that can interact with it.
  • An example of a thermodynamic surrounding is a heat bath, which is held at a prescribed temperature, regard- less of how much heat might be drawn from it.
  • There are four fundamental kinds of physical entities in thermodynamics, states of a system, walls of a system, thermodynamic processes of a system, and thermodynamic operations.
  • A thermodynamic system can be defined in terms of its states. In this way, a thermodynamic system is a macroscopic physical object, explicitly specified in terms of macroscopic physical and chemical variables that describe its macroscopic properties.
  • A thermodynamic operation is an artificial physical manipulation that changes the definition of a system or its surroundings. Usually it is a change of the permeability or some other feature of a wall of the system that allows energy (as heat or work) or matter (mass) to be exchanged with the environment.
  • A thermodynamic system can also be defined in terms of the cyclic processes that it can undergo.       
  • A cyclic process is a cyclic sequence of thermo-dynamic operations and processes that can be repeated indefinitely often without changing the final state of the system.
  • For thermodynamics and statistical thermodynamics to apply to a physical system, it is necessary that its internal atomic mechanisms fall into one of two classes:
  • those so rapid that, in the time frame of the process interest, the atomic states rapidly bring system to its own state of internal thermodynamic equilibrium; and
  • Those so slow that, in the time frame of the process of interest, they leave the system unchanged.
  • Thermodynamic facts can often be explained by viewing macroscopic objects as assemblies of very many microscopic or atomic objects that obey Hamiltonian dynamics.
  • The microscopic or atomic objects exist in species, the objects of each species being all alike. Because of this likeness, statistical methods can be used to account for the macroscopic properties of the thermodynamic system in terms of the properties of the microscopic species.
  • Thermodynamics states a set of four laws that are valid for all systems that fall within the constraints implied by each. In the various theoretical descriptions of thermodynamics these laws may be expressed in seemingly differing forms, but the most prominent formulations are the following:
  • Zeroth law of thermodynamics: If two systems are each in thermal equilibrium with a third, they are also in thermal equilibrium with each other.          
  • First law of thermodynamics: The increase in internal energy of a closed system is equal to the difference of the heat supplied to the system and the work done by it: \[\Delta \,U\,\,=\,\,Q\,\,-\,\,W\](Note that due to the ambiguity of what constitutes positive work, some sources state that \[\Delta \,U\,\,=\,\,Q\,\,+\,\,W,\] in which case work done on the system is positive.)
  • The first law of thermodynamics asserts the existence of a state variable for a system, the internal energy, and tells how it changes in thermodynamic processes.
  • The law allows a given internal energy of a system to be reached by any combination of heat and work. It is important that internal energy is a variable of state of the system whereas heat and work are variables that describe processes or changes of the state of systems.
  • The first law observes that the internal energy of an isolated system obeys the principle of conservation of energy, which states that energy can be transformed (changed from one form to another), but cannot be created or destroyed.
  • Second law of thermodynamics: Heat cannot spontaneously flow from a colder location to a hotter location.
  • The second law of thermodynamics is an expression of the universal principle of dissipation of kinetic and potential energy observable in nature.
  • The second law is an observation of the fact that over time, differences in temperature, pressure, and chemical potential tend to even out in a physical system that is isolated from the outside world.
  • Entropy is a measure of how much this process has progressed. The entropy of an isolated system that is not in equilibrium tends to increase over time, approaching a maximum value at equilibrium.
  • In classical thermodynamics, the second law is a basic postulate applicable to any system involving heat energy transfer; in statistical thermodynamics, the second law Is a consequence of the assumed randomness of molecular chaos
  • Third law of thermodynamics: As a system approaches absolute zero the entropy of the system approaches a minimum value.
  • The third law of thermodynamics is a statistical law of nature regarding entropy and the impossibility of reaching absolute zero of temperature.
  • This law provides an absolute reference point for the determination of entropy. The entropy determined relative to this point is the absolute entropy.
  • Thermodynamics distinguishes classes of systems by their boundary sectors.
  • AC magnets are made of laminated iron to reduce the heating effect.
  • The purpose of overload protection in a motor circuit is to protect the motor from sustained over currents.
  • If an operator pushes the start-button of the three-phase induction motor starter and the motor start to hum, but does not run, the prop able trouble is one fuse is blown and the motor is of single phasing.
  • When the reset button does not reestablish the control circuit after an overload, the probable cause is the overload trip has not cooled sufficiently.
  • Operation of a thermal overload relay depends on ambient temperature and temperature due to load current.
  • Line voltage of star-connected, 3-phase circuit is 415 V. The phase voltage will be 230 V.
  • The line voltage of a delta-connected, 3-phase circuit is 415 V. The phase voltage will be 415 V.
  • The phase current of a star-connected, 3-phase circuit is 100 amps. The line current will be 100 amps.
  • The appropriate fusing current of 35 SWG copper wire is 5 amps.
  • The approximate full load current of a 3-phase delta connected 3 HP motor is 4.5 amps.
  • An open system has a boundary sector that is permeable to matter; such a sector is usually permeable also to energy, but the energy that passes cannot in general be uniquely sorted into heat and work components. Open system boundaries may be either actually restrictive, or else non-restrictive.
  • A closed system has no boundary sector that is permeable to matter, but in general its boundary is permeable to energy. For closed systems, boundaries are totally prohibitive of matter transfer.
  • An adiabatically isolated system has only adiabatic boundary sectors. Energy can be transferred as work, but transfers of matter and of energy as heat are prohibited.
  • A purely diathermic ally isolated system has only boundary sectors permeable only to heat; it is sometimes said to be a dynamically isolated and closed to matter transfer. A process in which no work is transferred is sometimes called a dynamic.
  • An isolated system has only isolating boundary sectors nothing can be transferred into or out of it.

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Notes - Thermodynamics

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