Introduction to Thermodynamics
Thermodynamics is said to be a branch of physics that deals with heat, work, and temperature and their relation with the energy and radiation and physical properties of matter. The behaviour that is of these quantities is governed by the four laws that are of thermodynamics which convey a quantitative description using measurable macroscopic quantities in physics. But these all can also be explained in terms of microscopic constituents by statistical mechanics. The phenomenon of thermodynamics applies to a wide variety of topics in science and engineering, especially in physical chemistry, chemical engineering and mechanical engineering as well in other complex fields such as meteorology. Here, we will discuss thermodynamics in detail.
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Origin of Thermodynamics
Before 1798, people were not aware of utilising heat as the source of energy. After that, a British military engineer, Count Rumford, noticed the numerous amounts of heat generated in the boring of cannon barrels. He also identified that the heat generated from the boring cannon barrels are proportional to the work done in turning a blunt boring tool. His observation turned into the foundation of thermodynamics. The concept of the heat-engine cycle and the principle of reversibility was introduced by the French military engineer Sadi Carnot in 1824. But he failed to be concerned about the limitations and the maximum work that can be obtained from a steam engine with a high temperature.
Later, a German mathematician and physicist, Rudolf Clausius, developed the ideas of Sadi Carnot and stated the first and second laws of thermodynamics. During the 19th century, the concept of thermodynamics increased rapidly and increased the performance of steam engines and the application of thermodynamics was increased in all physical and biological systems.
Laws of Thermodynamics and Limitations
The four important laws of thermodynamics and their limitations are explained below
First Law Thermodynamics
Second Law Of Thermodynamics
Third Law Of Thermodynamics
1. First Law Thermodynamics
Law of Conservation of Energy states that energy can neither be created nor can it be destroyed. Energy can only be transferred or changed that too from one form to another. For example, we can say that turning on a light would seem to produce energy, however, it is the electrical energy which is converted.
A way of expressing the first law of thermodynamics is that any change which is in the internal energy denoted by ∆E of a system is given by the sum of the heat that is denoted by q that flows across its boundaries and the work (w) done on the system by the surroundings:
We write it as : ∆E = q + w
This law tells us that there are two kinds of processes: the work process and the heat process that can lead to a change in the internal energy of a system. Since we can see that both heat and work can be measured and quantified so this is the same as saying that any change which is occurring in the energy of a system. In other words, we can say that energy cannot be created or destroyed. If heat usually flows into a system or we can say that the surroundings do work on it then the internal energy increases and the sign q and w are positive. Conversely, we can say that the heat flowing out of the system or work which is done by the system that too on the surroundings will be at the expense of the internal energy and q and w will therefore be negative.
Limitations of First Law of Thermodynamics
The first law of thermodynamics does not mention anything about the direction of the flow of heat energy. Also, they did not mention whether the process is spontaneous or not, reversible or not. Practically, the entire heat energy cannot be converted into mechanical energy.
2.The Second Law of Thermodynamics
The second law of thermodynamics says that the entropy of any system which is isolated always increases. The system which is the isolated system spontaneously evolves towards a thermal equilibrium that is the state of entropy which is the maximum of the system. We can say that the entropy of the universe which is of the ultimate isolated system only increases and never decreases.
A simple way in which we can think of the second law of thermodynamics is that of a room. If not cleaned and tidied, then the room will invariably become more messy and disorderly with time – that is regardless of how careful one is to keep it clean. When the room is cleaned then its entropy decreases but the effort to clean it has resulted in an increase in entropy outside the room that exceeds the loss of the entropy.
Limitations of Second Law of Thermodynamics
The second law of thermodynamics does not have limitations. But this law is applicable only for the closed system.
3.The Third Law of Thermodynamics
The entropy that is of a system at absolute zero is typically said to be zero. And we can say that in all cases it is determined only by the number of different ground states it has. Specifically, the entropy of a pure crystalline substance in the perfect order which is at absolute zero temperature is zero. This statement is to hold true if the perfect crystal has only one state with energy that is minimum.
Limitations of Third Law of Thermodynamics
The third law of thermodynamics limits the behaviour of the system because the temperature of the system approaches absolute zero.
4.The Zeroth Law of Thermodynamics
This law identifies the thermal equilibrium and introduces temperature as a tool for identifying equilibrium. According to this law, “we can say that if two systems are in thermal equilibrium with a third system then those two systems themselves are in equilibrium.”
An assembly that is of a very large number of particles whose state can be expressed in terms of pressure and volume and temperature, is known as a thermodynamic system.
Limitations of Zeroth Law of Thermodynamics
The zeroth law of thermodynamics is not applicable for all kinds of equilibrium. Other laws cannot be derived from the zeroth law of thermodynamics.
The Thermodynamic System is Said to Be Classified Into the Following Three Systems
(i) Open System: It exchanges both energy and matter with the surrounding.
(ii) Closed System: It exchanges only energy but not matter with surroundings.
(iii) Isolated System: It exchanges neither energy nor matter with the surrounding.
Thermodynamics Notes PDF
The branch of physics which deals with the study of the transformation of heat into other different forms of energy and vice-versa is known as thermodynamics.
The phenomenon of thermodynamics is macroscopic science. It generally deals with bulk systems and does not go into the molecular constitution of the matter.
A collection which is of an extremely large number of molecules or atoms is said to be confined within certain boundaries
such that it has a certain value of pressure denoted by P volume denoted by V and temperature denoted by T is known as a thermodynamic system.
The Thermal Equilibrium
A system of thermodynamics is in a state of equilibrium if the macroscopic variables such as pressure and volume and temperature, mass composition etc. that generally characterise the system do not change in time. In equilibrium that is thermal equilibrium, the temperature of the two systems is said to be equal.
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Fundamental Concepts Of Thermodynamics
Thermodynamic States
The applications of thermodynamic principles completely depend on the system and its surroundings. For example, the system can be a cylinder with gas, movable piston, steam engine, marathon runner etc. Usually, a system will exchange its heat, energy or another form of work with its surroundings. The condition of the system at the given time is called the thermodynamic state. The changes in the value of the system are completely dependent on their initial and final state. One system cannot follow the path of the other system. In the atmosphere of the earth, the thermodynamic states completely depend on the properties of the components. The thermodynamic state may vary for water and water vapour.
Thermodynamic Equilibrium
Thermodynamic equilibrium is one of the important concepts of thermodynamics. According to thermodynamic equilibrium, no system has the tendency to change the state of a system spontaneously. For example, the gas present inside the cylinder with a moveable piston will remain at equilibrium only if the pressure and temperature inside the cylinder are constant. When the external pressure is imposed on the system, it may change the thermodynamic equilibrium of the system. Usually, no system can be in the equilibrium state, it will adjust according to the changes in the environment. In some systems, it may return to its original state after a small increment, and such a system is said to be a reversible system.
Temperature
If the two objects are brought together to have a thermal contact, then the heat will flow from one object to another, still attaining its equilibrium state. If the transfer of heat between two objects stops, it means that the temperature of the two objects becomes the same.
The work done by the system is directly proportional to the force applied to the system. The energy of the system refers to the capacity of the system to do certain work. The energy of the system is classified into two types, one is potential energy, and the other is kinetic energy. All the objects will have some potential energy, and the energy developed during the motion of the object is known as kinetic energy. If the object does not have any friction, then the energy of the object is never lost. Albert Einstein also said that it is possible to store the energy in the form of mass and can be converted back into energy through the formula, E = mc2.
Total Internal Energy
Thermodynamics also deals with the macroscopic properties of the materials such as temperature, volume and pressure. The thermal energy of the object will increase as the increase in kinetic energy. The total energy of a system is equal to the sum of internal energy of a system with other forms of energy like kinetic energy, gravitational potential energy etc.
This article explained the origin of thermodynamics, fundamental concepts of heat and thermodynamics, laws of thermodynamics in detail with their limitations.
FAQs on Thermodynamics
1. What is the standard mathematical expression for the First Law of Thermodynamics used in Physics for JEE Advanced, and how does its sign convention for work differ from Chemistry?
For JEE Advanced Physics, the First Law of Thermodynamics is expressed as ΔQ = ΔU + W. In this convention:
- ΔQ is the heat supplied to the system (positive).
- ΔU is the change in internal energy of the system (positive for an increase).
- W is the work done by the system on its surroundings (positive for expansion).
2. How do you calculate the work done during an isothermal versus an adiabatic expansion for an ideal gas?
The calculation of work done depends on the thermodynamic process:
- For a reversible isothermal expansion from volume V₁ to V₂, the work done is given by the integral of P dV, which results in: W = nRT ln(V₂/V₁). Here, temperature (T) is constant.
- For a reversible adiabatic expansion, where no heat is exchanged (ΔQ=0), the relationship is PVγ = constant. The work done is: W = (P₁V₁ - P₂V₂)/(γ - 1) or W = nR(T₁ - T₂)/(γ - 1), where γ is the adiabatic index (Cp/Cv).
3. What is a Carnot engine, and what is the formula for its maximum theoretical efficiency?
A Carnot engine is an idealised, reversible heat engine that operates in a cycle between two temperature reservoirs—a hot source at temperature Tₕ and a cold sink at temperature Tc. It represents the upper limit of efficiency that any heat engine can achieve between these two temperatures. Its efficiency (η) is determined solely by these temperatures (in Kelvin) and is given by the formula: η = 1 - (Tc / Tₕ).
4. Why is the efficiency of any practical engine always lower than that of a Carnot engine operating between the same two temperatures?
The Carnot engine's efficiency is a theoretical maximum because it assumes perfectly ideal conditions that are unattainable in reality. Practical engines have lower efficiencies due to:
- Irreversible Processes: Real-world processes like combustion and heat transfer occur at finite rates and involve dissipative forces like friction, making them irreversible.
- Heat Loss: Unavoidable heat loss to the surroundings occurs through conduction and radiation, which is not accounted for in the ideal Carnot cycle.
- Mechanical Friction: Moving parts in an engine, like pistons and bearings, generate friction, which converts useful work into wasted heat.
5. How is the adiabatic exponent (γ) related to the degrees of freedom of a gas, and why is this critical for JEE Advanced problems?
The adiabatic exponent, γ (gamma), is the ratio of specific heats (Cp/Cv). It is fundamentally linked to the molecular degrees of freedom (f) of the gas by the relation: γ = 1 + (2/f). This is critical because:
- For a monatomic gas (like He, Ar), f=3, so γ = 5/3.
- For a diatomic gas (like N₂, O₂) at moderate temperatures, f=5, so γ = 7/5.
- For a polyatomic gas, f can be higher.
6. What is the physical significance of entropy in the context of the Second Law of Thermodynamics?
Entropy (S) is a thermodynamic state function that quantifies the degree of disorder or randomness in a system. Its significance is central to the Second Law of Thermodynamics, which states that the total entropy of an isolated system can never decrease over time. For any spontaneous or natural process, the total entropy of the universe (system + surroundings) always increases. For a reversible process, the change in entropy of the universe is zero.
7. Why does the internal energy of an ideal gas depend only on its temperature?
The internal energy (U) of a system is the sum of the kinetic and potential energies of its constituent particles. For an ideal gas, a key assumption is that there are no intermolecular forces of attraction or repulsion between the gas particles. Consequently, the potential energy of the system is zero. This means the internal energy is composed entirely of the kinetic energy of the molecules, which, according to the kinetic theory of gases, is directly proportional to the absolute temperature (T). Therefore, for an ideal gas, U is a function of T only.
8. How do thermodynamic state functions differ from path functions, and why is this distinction important?
The distinction is crucial for understanding energy changes in a system:
- State Functions: These are properties that depend only on the current equilibrium state of the system, regardless of how it got there. Examples include Internal Energy (U), Enthalpy (H), and Entropy (S). The change in a state function (e.g., ΔU) depends only on the initial and final states.
- Path Functions: These are quantities whose values depend on the specific path taken to get from the initial to the final state. Examples include Heat (Q) and Work (W). Different processes (e.g., isothermal vs. adiabatic) between the same two states will involve different amounts of heat and work.
