Thermodynamics

Important terminologies of Thermodynamics

1. Thermodynamics 

Thermodynamics is the science relating heat and work transfers and the related changes in the properties of the working substance. The working substance is isolated from its surroundings in order to determine its properties.

2. System

A thermodynamic system may be defined as any specified portion of matter in the universe which is under study. A system may consist of one or more substances. Or if we defined it in terms of physics then In physics, the term “system” usually refers to any set of physical interactions isolated from the rest of the universe. Anything outside of the system, including all factors and forces irrelevant to a discussion of that system, is known as the environment.

e.g., portion of test tube where reaction is taking place, is called system.

2.1 Homogeneous System

A system is said to be homogeneous when it is completely uniform throughout, for example, a pure solid or liquid or a   solution or a mixture of gases. In other words, a homogeneous system consists of only one phase.

2.2 Heterogeneous system

A system is heterogeneous when it is not uniform throughout, i.e. it consists of two or more phases, e.g., a mixture of two solids or two or more immiscible liquids, a solid in contact with a liquid, a liquid in contact with its vapours.

2.3 Isolated System

It can neither exchange heat nor matter with the surroundings When the boundary is both sealed and insulated, no interaction is possible with the surroundings. Therefore,

An isolated system is one that can transfer neither matter nor energy to and from, its surroundings. Consider an example of a system consisting of a liquid in contact with its vapour in a closed vessel. Since the vessel is closed, no matter (liquid vapours) can go out or enter in the vessel. If the vessel is insulated also (as shown into figure), it can neither lose nor gain heat from the surroundings. Thus, this system is an isolated system. A substance, say boiling water, contained in a thermos flask, is another example of an isolated system.

2.4 Open system

The system which can exchange matter and energy with the surrounding is called open system, e.g., a cup of tea is an open system because it will become cold as well as its taste will also change, Le., it is exchanging energy and matter with the surrounding. Hot water contained in a beaker on laboratory table is an open system. The water vapour (matter) and also heat (energy) is transferred to the surroundings through the imaginary boundary.

2.5 Closed system

It can exchange energy but not matter with the surroundings, e.g., tea placed in a closed kettle. Here the boundary is sealed but not insulated. Therefore, A closed system is one which cannot transfer matter but can transfer energy in the form of heat, work and radiation to and from its surroundings.

A specific quantity of hot water contained in a sealed tube is an example of a closed system. While no water vapour can escape from this system, it can transfer heat through the walls of the tube to the surroundings.

A phase may be defined as a homogeneous and physically distinct part of a system which is bound by a surface and is mechanically separable from other parts of the system.

A gas contained in a cylinder with a piston constitutes a closed system. As the piston is raised, the gas expands and transfers heat (energy) in the form of to the surroundings.

3. Surrounding

Everything else in the universe except system is called surroundings, e.g., except the portion of the test where reaction is taking place is surrounding, i.e. above and around, the test tube.

4. Boundary

A physical or imaginary surface, enveloping the system and separating it from the surroundings.

Above concepts are cleared very well by understanding below examples…

In experimental work, a specific amount of one or more substances constitutes the system. Thus 200 g of water contained in a beaker constitutes the thermodynamic system. The beaker and the air in contact are the surroundings.

Similarly, 1 mole of oxygen confined in a cylinder fitted with a piston, is a thermodynamic system. The cylinder and the piston and all other objects outside the cylinder, form the surroundings. Here the boundary between the system (oxygen) and the surroundings (cylinder and the piston) are clearly defined.

5. Property

Property is any quantity whose changes are defined only by the end states and by the process. Examples of thermodynamics properties are the pressure, Volume and Temperature of the working fluid in the system above.

5.1 Macroscopic Properties

Such properties which can measured by simple instrument and sensed by human eye are known as macroscopic properties e.g., pressure, colour, Temperature and volume

5.2 Microscopic Properties

Such properties which cannot be sensed by human eye and are observed at molecular level are known as microscopic properties e.g., velocity, energy.

5.3 Extensive property of a system is that which depends upon the amount of the substance or substances present in the system. The examples are mass, volume, energy, heat capacity, enthalpy, entropy, free change etc.

5.4 Intensive property of a system is that which is independent of the amount of the substance present in the system. The examples are temperature, pressure, density, viscosity, refractive index, surface tension and specific heat.

The extensive properties are additive, while intensive properties are not. Sometimes an extensive property e becomes an intensive property by specifying unit amount of the substance concerned. Mass and volume are extensive properties but mass per unit volume i.e. density becomes an intensive property of the substance.

6. Pressure

The normal force exerted per unit area of the surface within the system. For engineering work, pressures are often measured with respect to atmospheric pressure rather than with respect to absolute vacuum.

P(abs) = P(atm) + P(gauge)

In SI units the derived unit for pressure is the pascal (pa), where 1 Pa = 1N/m^2. This is very small for engineering purposes, so usually pressures are given in terms of kilopascals ( 1kPa = 10^3 Pa ), Megapascals ( 1 MPa = 10^6 ), or bars ( 1 bar = 10^5 ). The imperial unit for pressure are the pounds per square inch (Psi) where 1 Psi = 6894.8 Pa

7. Temperature

Temperature is the degree of hotness or coldness of the system. The absolute temperature of a body is defined relative to the temperature of ice; for SI units, the Kelvin scale. Another scale is Celsius scale. Where the ice temperature under standard ambient pressure at sea level is: 0˚C,

273.15 K and the boiling point for water (steam) is 100˚C, 373.15 K

The imperial units of Temperature is the Fahrenheit where

T˚F = 1.8 x T˚F +32

8. Energy

Energy is defined as the ability to do work.

8.1 Kinetic Energy: It is the energy that an object possesses by virtue of its motion or it is the energy of motion and is proportional to the square of the velocity as well to the mass of the moving body.

8.2 Potential Energy: It is the energy that an object possesses due to its position. Or it is the energy of location of mass in a force field.

8.3 Internal Energy: It is the property of a system covering all forms of energy arising from the internal structure of the substance.

8.4 Mechanical Energy: It is the sum of Kinetic energy and potential energy.

8.5 Thermal Energy: It is the form of kinetic energy produced by the movement of atomic or molecular particles. The greater the movement of the particles, greater the thermal energy.

8.6 Chemical Energy: It related to the relationship between molecules in chemical compounds. When chemicals react with each other, they may give off heat (exothermic reaction) or require heat (endothermic reaction)

8.7 Nuclear Energy: It is related to the energy of atomic relationships between the fundamental particles. Nuclear fission and fusion are reactions which release nuclear energy.

9. Enthalpy

 A property of the system conveniently defined as h = u + PV where u is the internal energy.

10. Entropy

The microscopic disorder of the system. It is an extensive equilibrium property. Or in another terms it is the rendensy of natural systems toward breakdown, and specifically, the tendency for the energy in a system to be dissipated. Entropy is closely related to the second law of thermodynamics. Where it is a property of matter that measures the degree of randomization or disorder.

11. Heat

Internal energy that flows from one body of matter to another body. Heat is transferred by three modes conduction, convection and radiation.

11.1 Conduction: In this mode heat is transferred by successive molecular collisions. Conduction is the type of transfer from solid to solid.

11.2  Convection: In this mode of heat transfer heat is transfer by either fluid or gas. It is the type of transfer from gas to gas or liquid to liquid or gas to liquid.

11.3   Radiation: In this mode of heat transfer, heat is transferred by means of electromagnetic waves.

11.4    Heat Capacity or specific heat

The heat capacity of a substance is the amount of heat required to change its temperature by one degree, and has units of energy per degree.

12. State of System and State Variables

When macroscopic properties of a system have definite values, the system is said to be in a definite state. Whenever there is a change in any one of the macroscopic properties, the system is said to change into a different state. Thus the state of a system is fixed by its macroscopic properties.

Since the state of a system changes with the change in any of the macroscopic properties, these are called state variables. It also follows that when a system changes from one state (called initial state) to another state (called final state), there is invariably a change in one or more of the macroscopic properties.

Pressure, temperature, volume, mass and composition are the most important variables. In actual practice it is not necessary to specify all the variables because some of them are interdependent. In the case of a single gas, composition is not one of the variables because it remains always 100%.

Further, if the gas is ideal and one mole of the gas is under examination, it obeys the gas equation, PV = RT, where R is the universal gas constant. Evidently, if only two of the three variables (P, V and T) are known, the third can be easily calculated. Let the two variables be temperature and pressure. These are called independent variables. The third variable, generally volume, is said to be a dependent variable as its value depends upon the values of P and T. Thus, the thermodynamic state of a system consisting of a single gaseous substance may be completely defined by specifying any two of the three variables e.g. temperature, pressure and volume.

13. Thermodynamic Equilibrium

A system will be in thermodynamic equilibrium, if it is incapable of any spontaneous change of its macroscopic properties and it is in complete balance with its surrounding. If a system is in thermodynamic equilibrium, then it will satisfy the conditions of mechanical, thermal and chemical equilibrium.

13.1 Mechanical Equilibrium: A system is said to be in equilibrium, if there are no unbalanced forces in the system with its surrounding.

13.2 Thermal Equilibrium: If the temperature of a system remains same as its surrounding, then its known as thermal equilibrium. Hence, temperature gradient doesn’t exist.

13.3 Chemical Equilibrium: It represents the absence of any phase change or chemical reaction of any system.

 14. Quality of the working Substance

A pure substance is one, which is homogeneous and chemically stable. Thus it can be a single substance which is present in more than one phase, for example liquid water and water vapour contained in a boiler in the absence of any air or dissolved gases.

14.1 Phase: Is the state of the substance such as solid, liquid or gas.

14.2 Mixed phase: It is possible that phases may be mixed, eg ice + water, water + vapour etc.

14.3 Quality of a mixed phase or dryness fraction: The dryness fraction is defined as the ratio of the mass of pure vapour present to the total mass of the mixture ( Liquid and vapour; say 0.9 dry for example ). The quality of the mixture may be defined as the percentage dryness of the mixture (ie. 90%dry)

14.4 Saturated State: A saturated liquid is a vapour whose dryness fraction is equal to zero. A saturated vapour has a quality of 100% or a dryness fraction of one.

14.5 Superheated vapour: A gas is described as superheated when its temperature at a given pressure is greater than the saturated temperature at that pressure, ie the gas has been heated beyond its saturation temperature.

14.6 Degree of superheat: The difference between the actual temperature of a given vapour and the saturation temperature of the vapour at a given pressure.

14.7 Subcooled liquid: A Liquid is described as undercooled when its temperature at a given pressure is lower than the saturated temperature at that pressure, ie the liquid has been cooled below its saturation temperature.

14.8 Degree of subcool: The difference between the saturation temperature and the actual temperature of the liquid is a given pressure.

14.9 Triple point: A state point in which all solid, liquid and vapour phases coexist in equilibrium.

14.10 Critical Point: A state point at which transitions between liquid and vapour phases are not clear H2O (water).

15. Thermodynamic Processes

A process is a path in which the state of the system change and some properties vary from their original values. There are six type of processes associated with thermodynamics:

          Adiabatic:  No heat transfer from or to the fluid

          Isothermal: No change in temperature of the fluid

          Isobaric: No change in pressure of the fluid

          Isochoric: No change in volume of the fluid

          Isentropic: No change of entropy of the fluid

          Isenthalpic: No change of enthalpy of the fluid

  • : 1. engineering toolbox 2. science clarified

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