Definition of Thermodynamic System
We know that there are various methods that can be taken to get a Thermodynamic system from its beginning state to its ultimate state. We'll talk about those Thermodynamic processes in this article. We'll look at what a quasi-static process is first. State variables are defined only when the Thermodynamic system is in equilibrium with its surroundings, as previously explained. A quasi-static process is one in which the system is in Thermodynamic equilibrium with its surroundings at all times.
In a refrigerator, how does food stay cold and fresh? Have you ever noticed that even when a refrigerator's entire inside compartment is chilly, the outside or back of the refrigerator is warm? Here, the refrigerator extracts heat from its interior and transmits it to the surrounding area. This is why a refrigerator's back is warm. Thermodynamic processes are the movement of heat energy within or between systems.
A Thermodynamic system is a specific space or macroscopic region in the universe, whose state can be expressed in terms of pressure, temperature, and volume, and in which one or more than one Thermodynamic process occurs. Anything external to this Thermodynamic system represents the surroundings and is separated from the system by a boundary. The surroundings, system, and the boundary, together constitute the universe. Types of systems in Thermodynamics are as follows:
Open System: It allows energy as well as mass to flow in and out of it.
Closed System: It only allows energy (work and heat) to be transferred across its boundary.
Isolated System: Neither mass nor energy is allowed to interact with it.
In the winter, rubbing your palms together makes you feel warmer. Because touching our palms produces heat, this happens. The heat of the steam is also used in steam engines to move the pistons, which causes the train wheels to rotate. But what is the actual procedure here? This is related to a phenomenon known as 'Thermodynamics.'
The study of the relationship between heat, work, temperature, and energy is known as Thermodynamics. Thermodynamics is concerned with the movement of energy from one location to another and from one form to another in its broadest definition. The essential concept is that heat is a sort of energy that correlates to a specified quantity of mechanical labor.
The heat was not formally recognized as a form of energy until around 1798, when Count Rumford (Sir Benjamin Thompson), a British military engineer, discovered that infinite amounts of heat might be produced while boring cannon barrels, and that the quantity of heat produced is proportionate to the amount of work done in spinning a blunt boring instrument. The foundation of Thermodynamics is Rumford's observation of the relation between heat created and work done. Carnot's research focused on the limits to the maximum amount of work that a steam engine can produce when using a high-temperature heat transfer as its driving force. Rudolf Clausius, a German mathematician, and physicist, refined these ideas into the first and second laws of Thermodynamics later that century.
Types of Thermodynamic Processes
The state of a given Thermodynamic system can be expressed by various parameters such as pressure (P), temperature (T), volume (V), and internal energy (U). If any two parameters are fixed, say, pressure (P) and volume (V) of a fixed mass of gas, then the temperature (T) of the gas will be automatically fixed according to the equation PV =RT. No change can be made to T without altering P and V.
The state of a system can be changed by different processes. In Thermodynamics, types of processes include:
Isobaric process in which the pressure (P) is kept constant (ΔP =0).
Isochoric process in which the volume (V) is kept constant (ΔV =0).
Isothermal process in which the temperature (T) is kept constant (ΔT =0).
Adiabatic process in which the heat transfer is zero (Q=0).
Thermodynamic process notes have been discussed later.
Work in Thermodynamic Processes
When the volume (V) of a system alters, it is said that pressure-volume work has occurred. A Thermodynamic process occurring in a closed system in such a way that the rate of volume change is slow enough for the pressure (P) to remain constant and uniform throughout the system, is a quasi-static process. In this case, work (W) is represented as:
δW = PdV, where δW is the infinitesimal work increment by the system, and dV is the infinitesimal volume increment.
Also, W = \[\int\] PdV, where W is the work the system does during the entire reversible process.
Isobaric Process
Since the pressure (P) is constant in this process, the volume of the system changes. The work (W) done can be calculated as W = P (Vfinal - Vinitial).
If ΔV is positive (expansion), the work done is positive. For negative ΔV (contraction), the work done is negative.
Isochoric Process
The volume remains constant in an isochoric process. Therefore, the system does not do any work (since ΔV = 0, PΔV or W is also zero). Such a process in which there is no change in volume can be achieved by placing a Thermodynamic system in a closed container that neither contracts nor expands. Thus, from the first law of Thermodynamics (Q = ΔU + W), the change in internal energy becomes equal to the heat transferred (ΔU = Q) for an isochoric process.
Isothermal Process
The temperature of the system remains constant in an isothermal process. We know,
W = \[\int\] PdV
From Gas Law,
PV = nRT
P = nRT/V. Using the value of P in the work equation:
W = nRT VB \[\int\]VA (dV/V)
W = nRT ln (VB/VA)
If VB is higher than VA, the work done will be positive, or else negative.
Since internal energy is temperature-dependent, ΔU = 0 because the temperature is constant, and thus, from the first law of Thermodynamics (Q = ΔU + W), we will get Q = W.
Adiabatic Process
No heat is exchanged with the system in an adiabatic process (Q = 0). Its mathematical representation is:
PVƔ = K (constant).
Also, W = \[\int\] PdV. Substituting the value of P in the work equation:
W = K Vf \[\int\]Vi (dV/VƔ)
W = K [(Vf1-Ɣ - Vi1-Ɣ)/ 1-Ɣ]
Since Q = 0 for an adiabatic process, from the first law of Thermodynamics (Q = ΔU + W), we will get ΔU = -W. Thus, the internal energy will increase if the work done is negative and vice versa.
FAQs on Thermodynamic Processes
1. What are the four primary types of thermodynamic processes as per the Class 11 Physics syllabus?
The four main types of thermodynamic processes are defined by which state variable remains constant:
- Isothermal Process: A process where the temperature of the system remains constant (ΔT = 0). Heat can be exchanged with the surroundings to maintain this constant temperature.
- Isobaric Process: A process during which the pressure of the system does not change (ΔP = 0). An example is water boiling in an open container at atmospheric pressure.
- Isochoric Process: A process that occurs at a constant volume (ΔV = 0). Since there is no change in volume, the work done by the system is zero.
- Adiabatic Process: A process where there is no heat transfer between the system and its surroundings (Q = 0). These processes usually occur very rapidly, like the bursting of a tyre.
2. What are some real-world examples of thermodynamic processes?
Thermodynamic processes are happening all around us. Here are some common examples:
- Melting of ice: This is an example of an isothermal process, as the ice melts at a constant temperature of 0°C.
- Boiling water in an open pan: This represents an isobaric process because the pressure remains constant at atmospheric pressure while the temperature increases.
- A pressure cooker: Heating food inside a sealed pressure cooker is an isochoric process because the volume is fixed.
- A refrigerator's cooling cycle: The rapid expansion of refrigerant gas is an application of the adiabatic process, which causes it to cool down and absorb heat from the inside.
3. Explain the key concepts of internal energy, enthalpy, and entropy in a thermodynamic system.
These three properties describe the energy state of a system:
- Internal Energy (U): This is the sum of the kinetic and potential energies of all the molecules within a system. It is a state function, meaning its value depends only on the current state of the system, not how it got there.
- Enthalpy (H): Enthalpy represents the total heat content of a system. It is defined as the sum of the internal energy and the product of pressure and volume (H = U + PV). It is particularly useful for studying processes at constant pressure.
- Entropy (S): Entropy is a measure of the molecular disorder or randomness of a system. It also represents the amount of energy that is unavailable to do useful work. According to the Second Law of Thermodynamics, the entropy of an isolated system always tends to increase.
4. Briefly explain the four laws of thermodynamics.
The laws of thermodynamics are fundamental principles governing energy and its transformations:
- Zeroth Law: If two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. This law provides the basis for measuring temperature.
- First Law: This is the law of conservation of energy. It states that energy cannot be created or destroyed, only changed in form. The change in a system's internal energy (ΔU) is equal to the heat added to it (Q) minus the work done by it (W).
- Second Law: This law defines the direction of heat flow and introduces the concept of entropy. It states that heat will not spontaneously flow from a colder body to a hotter one, and the total entropy of an isolated system will always increase over time.
- Third Law: This law states that as the temperature of a system approaches absolute zero (0 Kelvin), the entropy of the system approaches a minimum value. For a perfect crystal, this value is zero.
5. How is an adiabatic process fundamentally different from an isothermal process?
The key difference lies in how energy is managed. In an isothermal process, the goal is to keep the temperature constant. This is achieved by allowing the system to freely exchange heat with its surroundings. In contrast, an adiabatic process is one where there is absolutely no heat exchange with the surroundings (it is thermally insulated). As a result, in an adiabatic expansion, the system does work on the surroundings, causing its internal energy and temperature to decrease.
6. What is the difference between a state function and a path function in thermodynamics?
The distinction is crucial for understanding thermodynamic calculations. A state function (or state variable) is a property whose value depends only on the current equilibrium state of the system, regardless of the path taken to reach that state. Examples include internal energy (U), temperature (T), pressure (P), volume (V), and entropy (S). A path function, on the other hand, is a property whose value depends on the specific path followed during a process. The two most common path functions are work (W) and heat (Q).
7. Why is the concept of a quasi-static process important for understanding thermodynamics?
A quasi-static process is a theoretical concept of a process that happens infinitely slowly. Its importance lies in the fact that at every instant, the system is considered to be in thermodynamic equilibrium. This allows us to define and measure its macroscopic properties like pressure, volume, and temperature at every step of the process. Real-world processes are not quasi-static, but this idealisation is essential for deriving thermodynamic equations and understanding the maximum possible efficiency of processes, such as in the Carnot cycle.
8. Can a real-world process ever be truly reversible? Explain why.
No, a real-world process can never be truly reversible. A reversible process is an idealised concept where the system and its surroundings can be returned to their original states without any change in the universe. This requires the process to be quasi-static and free from any dissipative effects. However, all real processes involve factors like friction, viscosity, and heat loss, which generate entropy and are irreversible. Therefore, a reversible process serves as a theoretical benchmark to measure the efficiency of real, irreversible processes.
9. What is the significance of the First Law of Thermodynamics in daily life and engineering?
The First Law, the principle of energy conservation, is fundamental to nearly every aspect of our world. In daily life, it explains how our bodies convert the chemical energy in food into energy for movement and maintaining body temperature. In engineering, it is the core principle behind heat engines (like those in cars), which convert thermal energy into mechanical work, and refrigerators, which use work to transfer heat from a cold space to a warmer one. It dictates that you cannot get more energy out of a system than you put in, setting the basic rules for all energy conversion technologies.
