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Frequently asked basics of Thermodynamics in ESE Objective

By Exergic
May 20, 2017

The first law of thermodynamics is simply an expression of the conservation of energy principle, and it asserts that energy is a thermodynamic property. The second law of thermodynamics asserts that energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy.
The fundamental principles states that during an actual process mass and energy are conserved, entropy is generated, and exergy is destroyed.
Energy can be viewed as the ability to cause changes.
A system is defined as a quantity of matter or a region in space chosen for study.
A closed system (also known as a control mass) consists of a fixed amount of mass, and no mass can cross its boundary.
An open system, or a control volume, as it is often called, is a properly selected region in space. Both mass and energy can cross the boundary of a control volume. A control volume can also involve heat and work interactions just as a closed system, in addition to mass interaction.
The continuum idealization allows us to treat properties as point functions and to assume the properties vary continually in space with no jump discontinuities. The continuum model is applicable as long as the characteristic length of the system (such as its diameter) is much larger than the mean free path of the molecules.
At very high vacuums or very high elevations, the mean free path may become For such cases the rarefied gas flow theory should be used.
A non quasi-equilibrium process is denoted by a dashed line between the initial and final states instead of a solid line.
Steady implies no change with time. The opposite of steady is unsteady, or transient. The term uniform, however, implies no change with location over a specified region.
The equality of temperature is the only requirement for thermal equilibrium.
The zeroth law was first formulated and labeled by R. H. Fowler. As the name suggests, its value as a fundamental physical principle was recognized more than half a century after the formulation of the first and the second laws of thermodynamics. It was named the zeroth law since it should have preceded the first and the second laws of thermodynamics.
Pressure is defined as a normal force exerted by a fluid per unit area. We
speak of pressure only when we deal with a gas or a liquid.
Pressure in a fluid increases with depth because more fluid rests on
deeper layers, and the effect of this “extra weight” on a deeper layer is
balanced by an increase in pressure
The unit mmHg is also called the torr in honor of Torricelli. Therefore, 1 atm 760 torr and 1 torr 3 Pa.
Energy can be transferred to or from a closed system (a fixed mass) in two distinct forms: heat and work. For control volumes, energy can also be transferred by mass flow. An energy transfer to or from a closed system is heat if it is caused by a temperature difference. Otherwise it is work, and it is caused by a force acting through a distance.
The total energy of a system is divided in two groups:
macroscopic and microscopic. The macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame, such as kinetic 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.
The portion of the internal energy of a system associated with the kinetic energies of the molecules is called the sensible energy.
The internal energy associated with the atomic bonds in a molecule is called chemical energy.
Note that pressure itself is not a form of energy. But a pressure force acting on a fluid through a distance produces work, called flow work.
Heat and work are directional quantities, and thus the complete description of a heat or work interaction requires the specification of both the magnitude and direction.
Both are recognized at the boundaries of a system as they cross the
That is, both heat and work are boundary phenomena. Systems possess energy, but not heat or work. Both are associated with a process, not a state. Unlike properties, heat or work has no meaning at a state.
Both are path functions (i.e., their magnitudes depend on the path followed during a process as well as the end states).
A major consequence of the first law is the existence and the definition of the property total energy E.
Energy can be transferred to or from a system in three forms: heat, work, and mass flow.
Most fuels contain hydrogen, which forms water when burned, and the
heating value of a fuel will be different, depending on whether the water in combustion products is in the liquid or vapor form. The heating value is called the lower heating value, or LHV, when the water leaves as a vapor, and the higher heating value, or HHV.