What is thermodynamics in Mechanical Engineering?
Thermodynamics: Definition, Laws, Basics & Importance in Engineering
The name ‘thermodynamics’ stems from the Greek words therme (heat) and dynamics (power/movement), which are most descriptive of the early efforts to convert heat into power. Thermodynamics is the branch of science that deals with energy and its interactions. More specifically, it deals with energy conversions, energy exchange, and the direction of exchange. Basically, it is the science of energy transfer and its effect on the physical properties of substances. Generally, thermodynamics is a subject of mechanical engineering and chemical engineering at the undergraduate level. Apart from mechanical engineers, undergraduate students of physics and some other courses also study thermodynamics as part of their curriculum.
Thermodynamics laws are some basic theories that are deduced from observation of common experiences and have been formulated into laws. These laws govern the principles of energy conversion.
Laws, Applications & Importance
Thermodynamics, basically entails four laws or axioms known as Zeroth, first, second and third laws of thermodynamics. These laws are the set of basic principles that leads to the definition of thermodynamic properties which help us to understand and predict the operation of a physical system. These laws are based on experimental observations and have no mathematical proof. Like all physical laws, these laws are based on logical reasoning.
Let us look at each of the four laws one by one.
Zeroth Law of thermodynamics: Zeroth law of thermodynamics deals with the thermal equilibrium and establishes a concept of temperature. According to zeroth law of thermodynamics, if objects ‘A’ and ‘C’ are in thermal equilibrium with ‘B’, then object ‘A’ is in thermal equilibrium with object ‘C’. Practically this means all three objects are at the same temperature and it forms the basis for comparison of temperatures.
For example: Thermometer: In case of mercury thermometer, the body temperature (A) is in thermal equilibrium with thermometer tube (C) and thermometer tube (C) is in thermal equilibrium with body (B) i.e. mercury. So body temperature and mercury are in thermal equilibrium as shown in figure.
Explanation
Let us say TA, TB and TC are the temperatures of the bodies A, B, and C respectively.
Given: A and C are in thermal equilibrium i.e., TA=TC and B and C are in thermal equilibrium i.e., TB = TC then as a consequence of zeroth law, A and B will also be in thermal equilibrium i.e., TA=TB
First law of thermodynamics:
The First Law of Thermodynamics is the ‘law of conservation of energy’ in the context of thermodynamic processes. It states that the net energy of any system remains the same. Energy may be exchanged between a system and its surroundings, but it can neither be created nor destroyed. The First Law of Thermodynamics relates the change in energy to work and heat transfer.
Second law of thermodynamics:
Second law of thermodynamics states that the entropy or degree or randomness of a closed system always increases over time. The second law of thermodynamics explains why natural process tends to move towards the state of equilibrium. However, the second law of thermodynamics is not limited to identifying the direction of processes. It asserts that process occur in a particular direction and that the energy has quality and quantity as well.
The first law of thermodynamics establishes the equivalence between the quantity of heat used and the mechanical work but doesn’t specify the conditions under which conversion of heat into work is possible, nether the direction, in which heat transfer can take place. This gap has been bridged by the second law of thermodynamics.
Consider a container of hot water left in a colder room. The water eventually cools off and the process satisfies the first law of thermodynamics since the amount of energy lost by the water is equal to the amount gained by the surrounding air. Now let us consider the reverse process, the hot water is getting even hotter in a colder room as a result of heat transfer from the room air. We all know this process never takes place. Yet, doing so would not violate the first law of thermodynamics as long as amount of energy lost by air is equal to the amount gained by the hot water. It’s actually the second law that decides the feasibility of the process.
Preserving the quality of energy is a major concern to engineers, and the second law provides the necessary means to determine the quality as well as the degree of degradation of energy during a process.
A process cannot occur until it satisfies both first and second law of thermodynamics.
The second law of thermodynamics can be illustrated with the help of two statements.
- Kelvin-Planck statement
- Clausius statement
According to the Kelvin-Planck statement, “it is impossible for a y system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy by heat transfer from a single thermal reservoir”. The Kelvin-Plank statement does not rule out the possibility of a system developing a net amount of work from a heat transfer drawn from a single reservoir. It only denies the possibility if the system undergoes a thermodynamic cycle.
According to Clausius statement, “it is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower temperature body to a higher temperature body.
The Kelvin-Planck’s and Clausius’ statement of the second law of thermodynamics are just two of the several alternative ways to express it. It’s just that these two are frequently used in engineering thermodynamics.
The second law of thermodynamics helps in
- Prediction the direction of process.
- Establishing conditions of equilibrium.
- Determining the best theoretical performance of cyclic devices like heat engine.
- Defining temperature scale independent of the properties of a thermometric substance.
- Developing means for evaluation properties such as ‘u’ and ‘h’ in terms of properties that are more readily obtainable experimentally.
- Quantifying the level of perfection of a process, and the point the direction to eliminate imperfections effectively.
Third law of thermodynamics
The third law of thermodynamics states that the entropy of a perfect crystal at a temperature of zero kelvin is equal to zero.
What is entropy in thermodynamics?
Entropy is the degree of randomness of any system. It is an abstract quantity. Unlike energy, entropy is a non-conserved property.
Entropy is:
- A property (extensive)
- A point or state function
- A exact differential
The SI unit of entropy is J/K. Entropy per unit mass, designated as s, is an intensive property and the SI unit is J/kg-K.
Key points:
Importance of Thermodynamics for Engineering Students
Thermodynamics is an important subject for engineering students, especially mechanical engineers. It is one of the core subjects of mechanical engineering. Thermodynamics holds significant weightage in competitive exams like GATE, ESE, SSC JE, RRB JE, and other engineering-related exams. The weightage of thermodynamics in different competitive exams is given below.
- GATE: 5-6 marks
- ESE: 10-12 marks
- SSC JE: 10-12 marks
- RRB JE: 6-7 marks
Other than the above-mentioned exams, it holds medium weightage in other exams. However, even with medium weightage, it cannot be ignored because thermodynamics serves as the basis for other subjects, which hold significant weightage.
Apart from the competitive exam point of view, thermodynamics still holds a special place in the mechanical engineering course because of the following reasons.
- Serves as a basis for understanding of other subjects.
- Important for understanding the concepts involving energy and heat.
- Helps in understanding the things that are not possible.
- Help in understanding of the Carnot engine and other processes
- Helps in determining the possibility of the system/machine.
These are some of the reasons that explain the importance of thermodynamics for engineering students.
Career Scope with Thermodynamics Knowledge
Thermodynamics is the basic subject in mechanical engineering. Apart from its academic benefits, including weightage in various competitive exams, knowledge of thermodynamics helps in understanding various other subjects of mechanical engineering.
Knowledge of thermodynamics is essential for understanding various processes of power plants, ISRO, BHEL, NTPC, DRDO, BARC, etc.
Thermodynamics is also useful in the design, R&D, and manufacturing of systems/machines, etc. Students who are interested in thermodynamics can also pursue higher courses like M.E., M.Tech, and PhD in thermal engineering to upgrade their knowledge and better understanding of concepts.
Online Courses for thermodynamics (ME):
If you want to learn about thermodynamics through Online course, click on the given below links:
Other than the above mentioned courses, there are many other courses on NPTEL offered by faculties of different IITs. Candidates can view these courses from here.
Recommended Books for thermodynamics
Candidates can refer to MADE EASY postal course books to learn in detail about thermodynamics.
FAQs:
Q. 1. What are surroundings in thermodynamics?
Answer: In thermodynamics, everything outside thermodynamic system being studied is called surroundings.
Q. 2. What is the zeroth law of thermodynamics?
Answer: According to zeroth law of thermodynamics, if objects ‘A’ and ‘C’ are in thermal equilibrium with ‘B’, then object ‘A’ is in thermal equilibrium with object ‘C’. Practically this means all three objects are at the same temperature and it forms the basis for comparison of temperatures.
Q. 3. What is a thermodynamic process?
Answer: Thermodynamic process is a process in which the thermodynamic state of a body/system is changed.
Q. 4. What is enthalpy in thermodynamics?
Answer: Enthalpy is the total heat energy of a thermodynamic system.
Q. 5. What is a thermodynamic system?
Answer: Thermodynamic system is a system/body/ matter separated from its surroundings and can be used for studying/understanding thermodynamic laws.
Q. 6. What are the 4 types of thermodynamics laws?
Answer: Four thermodynamics laws are:
- Zeroth law of thermodynamics
- First law of thermodynamics
- Second law of thermodynamics
- Third law of thermodynamics
Q. 7. How much weightage does thermodynamics carry in GATE ME?
Answer: The weightage of thermodynamics in GATE ME is 5-6 marks.
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