Thermodynamics: It is the field of thermal engineering that studies the properties of systems that have a temperature and involve the laws that govern the conversion of energy from one form to another, the direction in which heat will flow, and the availability of energy to do work.
Mass and Force: Mass is one of the fundamental dimensions, like time; it cannot be defined in terms of other dimensions. Much of our intuition of what mass is follows from its role in Newton’s second law of motion:
F = M · f
In this relationship, the force F required to produce a certain acceleration f of a particular body is proportional to its mass M.
Volume: The familiar property, volume; is formally defined as the amount of space occupied in three-dimensional space. The SI unit of volume is cubic metres (m3).
Pressure: For a fluid system, the pressure is defined as the normal force exerted by the fluid on a solid surface or a neighbouring fluid element, per unit area. From a molecular point of view, the pressure exerted by a gas on the walls of its container is a measure of the rate at which the momentum of the molecules colliding with the wall is changed.
The SI unit for pressure is Pascal,
1 Pa = 1 N/m2
Also commonly used unit is bar, which is defined as
1 bar = 105 Pa = 105 N/m2
As a result of some practical devices measuring pressures relative to the local atmospheric pressure, we distinguish between gauge pressure and absolute pressure. Gage pressure is defined as
Pgauge = Pabs + Patm
System: System is the fixed quantity of matter and/or the region that can be separated from everything else by a well-defined boundary/surface. Thermodynamic system is the system on which thermodynamic investigation is done. The surface separating the system and surroundings is known as the control surface or system boundary. The control surface may be movable or fixed. Everything beyond the system is the surroundings. A system of fixed mass is referred to as a closed system. When there is flow of mass through the control surface, the system is called an open system. An isolated system is a closed system that does not interact in any way with its surroundings.
State: At any instant of time, the condition of a system is called state. The state at a given instant of time is defined by the properties of the system such as pressure, volume, temperature, etc. A property is any quantity whose numerical value depends on the state but not on the history of the system. There are two types of properties—extensive and intensive. Extensive properties depend on the size or extent of the system. Volume, mass, energy, and entropy are examples of extensive properties. An extensive property is additive in the sense that its value for the whole system equals the sum of the values for its molecules. Intensive properties are independent of the size or extent of the system. Pressure and temperature are examples of intensive properties.
Change in State: Thermodynamic system undergoes changes due to flow of mass and energy. The mode in which the changes in the state of a system takes place is known as process such as isobaric (constant pressure) process, isochoric (constant volume) process, isothermal (constant temperature) process, adiabatic (constant entropy) process, etc. The path is loci of series of state changes from initial state to final state during a process. The change in state and path of a process are shown in Figure 1.1. The thermodynamic cycle refers to sequence of processes in which initial and final states of the system are same. For example, Otto cycle, Diesel cycle, Duel cycle, Joule cycle, Rankine cycle, Carnot cycle, etc. have identical initial and final states.
Figure 1.1 Change in State with a Process
Process: Two states are identical if, and only if, the properties of the two states are same. When any property of a system changes in value there is a change in state, and the system is said to undergo a process. When a system from a given initial state goes into a sequence of processes and finally returns to its initial state, it is said to have undergone a cycle.
Phase: Phase refers to a quantity of matter that is homogeneous throughout in its chemical composition and physical structure. A system can contain one or more phases. A mixture of water and water vapour has two phases. A pure substance is one that is uniform and invariable in chemical composition. A pure substance can exist in more than one phase, but its chemical composition must be the same in each phase. For example, if liquid water and water vapour form a system with two phases, the system can be regarded as a pure substance because each phase has the same composition.
Equilibrium: In thermodynamics the concept of equilibrium includes not only a balance of forces, but also a balance of other influencing factors, such as thermal equilibrium, pressure equilibrium, phase equilibrium, etc. To observe a thermodynamic equilibrium in a system, one may test it by isolation of the system from its surroundings and watch for changes in its observable properties. If no change takes place, it may be said that the system is in equilibrium. The system can be at an equilibrium state. When a system is isolated, it cannot interact with its surroundings; however, its state can change as a consequence of spontaneous changes occurring internally as its intensive properties, such as temperature and pressure, tend toward uniform values. When all such changes cease, the system is in equilibrium. At equilibrium, temperature and pressure are uniform throughout. If gravity is significant, a pressure variation with height can exist, as in a vertical column of liquid.
Zeroth law of thermodynamics is law of thermal equilibrium, which states that if a system A is in thermal equilibrium with systems B and C, then systems B and C will be in thermal equilibrium as well.
Quasi-static Process: When a process proceeds in such a way that the system remains infinitesimally close to an equilibrium state at all times, it is called a quasi-static process. A quasi-static process can be understood as sufficiently slow process that allows the system to adjust internally so that properties in one part of the system do not change faster than those at other parts.
Temperature: Temperature is a property of a substance by which it can be differentiated from other substance in terms of degree of hot or cold. A scale of temperature independent of the thermometric substance is called a thermodynamic temperature scale. The Celsius temperature scale (centigrade scale) uses the degree Celsius (°C), which has the same magnitude as the Kelvin. Thus, temperature differences are identical on both scales. However, the zero point on the Celsius scale is shifted to 273.15 K, as shown by the following relationship between the Celsius temperature and the Kelvin temperature:
°C = °K − 273.15
Two other temperature scales which are commonly used are the Rankine and Fahrenheit scale; the various relationships between temperatures scales are shown as follows:
Internal Energy: The Internal Energy (U) of a system is the total energy content of the system. It is the sum of the kinetic, potential, chemical, electrical, and all other forms of energy possessed by the atoms and molecules of the system. The Internal Energy (U) is path independent and depends only on temperature for an ideal gas. Internal energy may be stored in the system in following forms:
- Kinetic energy of molecules
- Molecular vibrations and rotations
- Chemical bonds that can be released during chemical reaction
- Potential energy of the constituents of the system
Work: Work in thermodynamics may be defined as any quantity of energy that flows across the boundary between the system and surroundings which can be used to change the height of a mass in the surroundings.
Heat: Heat is defined as the quantity of energy that flows across the boundary between the system and surroundings because of a temperature difference between system and surroundings. The characteristics of heat are as follows:
- Heat is transitory and appears during a change in state of the system and surroundings. It is not a point function.
- The net effect of heat is to change the internal energy of the system and surroundings in accordance to first law.
- If heat is transferred to the system, it is positive and if it is transferred from the system it is negative.
Enthalpy: Enthalpy, h, of a substance is defined as h = u + PV. It is intensive properties of a substance and measured in terms of kJ/kg.
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