Difference Between Isothermal and Adiabatic Process
The word ‘isothermal’ means constant temperature. An isothermal process is a thermodynamic process occurring at a constant temperature.
The word ‘adiabatic’ means isolated from surroundings. Adiabatic process means a process that neither allows the heat to transfer inside nor lets the heat out of the system.
For example, a reaction that takes place in a Dewar Flask is adiabatic.
In this article, we will discuss adiabatic and isothermal, distinguish between isothermal and adiabatic processes, isobaric isochoric isothermal and adiabatic processes, and understand the process of isothermal adiabatic in detail.
Adiabatic and Isothermal Processes
Now, we will understand the difference between adiabatic and isothermal:
Thermodynamics uses the concepts of the isothermal process, isochoric process, isobaric process, and adiabatic process to explain the behaviour of a thermodynamic system and its relationship to temperature changes.
An isothermal process is a process that happens when there are no variations in the temperature of the system, but other parameters (volume, pressure) regarding the system can be changed accordingly.
Adiabatic process describes a process that remains aloof from its surroundings. It is a process in which no heat transfer occurs between a system and its surroundings. Here, the temperature of the system can vary in order to avoid any heat transfer. This indicates that the main difference between isothermal and adiabatic processes is that the isothermal process takes place under constant temperature whereas the adiabatic process occurs under changing temperature.
Now, let’s Compare Isothermal And Adiabatic processes in a tabular form to understand isothermal and adiabatic processes:
Difference Between Isothermal Process and Adiabatic Process
Now, we will understand a bit more about isobaric isochoric isothermal and adiabatic processes:
Adiabatic, Isothermal, Isobaric, and Isochoric, Processes
Adiabatic Process
An adiabatic process is a thermodynamic process that can take place without any heat transfer between a system and its surrounding. Here, neither heat nor energy is not transferred into or out of the system. Therefore, in an adiabatic process, the only way the energy transfer takes place between a system and its surroundings is the work. It is either an irreversible or reversible process and follows the below-mentioned conditions:
The time required to carry out the process should be minimal so that it should be carried out quickly so as to reduce the chances of heat getting transferred.
The instrument used to carry out the process should be insulated perfectly from the surrounding environment.
An adiabatic process can be quickly maintained by doing the process. For example, if we quickly press the piston in a cylinder filled with gas, there is not enough time for the system to transfer heat energy to the surroundings. In adiabatic processes, the work done by the system alters the internal energy of the system. Always remember that the total heat of the system is constant in an adiabatic process.
The below diagram shows the adiabatic process:
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Adiabatic equation
The adiabatic equation is: PVγ = constant
Here P stands for the pressure of the system
V stands for volume of the system
γ stands for adiabatic index
Adiabatic index
It is the ratio of heat capacity at constant pressure Cp to heat capacity at constant volume Cv.
\[\gamma = \frac{C_{P}}{C_{V}}\]
Adiabatic Expansion
The ideal behaviour of a system where the temperature keeps on increasing and pressure remains constant is termed adiabatic expansion. It refers generally to a closed system.
Adiabatic Compression: The external work done is equal to the increased internal energy of the air in the system. Here the heat is neither subtracted or added from the surrounding air to the system air. As there is an increase in the temperature of the system the pressure of the system tends to be more than the volume.
Examples:
Oscillation of a pendulum in a vertical plane.
No heat is released or absorbed when an ice cube is put inside an icebox.
Nozzles, compressors and turbines work on the principle of adiabatic efficiency.
Reversible Adiabatic Process
It is also known as the Isentropic process. In this process, there is neither transfer of matter nor heat. Hence it is known to be a reversible process. It could also be defined as a thermodynamic process in which the work transfers are frictionless and the system is adiabatic. It is vitally used as a model to show real process models in engineering and also for major comparisons between the systems.
Isothermal Process
An isothermal process is a thermodynamic process that takes place at a constant temperature. It means that an isothermal process occurs in a system where the temperature remains constant. However, to keep the temperature of the system constant, heat must be transferred into the system or shifted out of the system.
Generally, there are two conditions under which the isothermal process can work:
The system slowly adjusts the temperature of the system with the temperature of its surrounding by releasing heat. This happens when the surrounding temperature (T) is less than the temperature of the system (TS) i.e., T < TS and there is no thermal equilibrium maintained.
In the other case when the system slowly adjusts the temperature of the system with the temperature of its surrounding by absorbing heat. This happens when the surrounding temperature is greater than the temperature of the system and there is no thermal equilibrium maintained.
In simple terms, in the isothermal process:
T = constant
This implies, the change in temperature will be zero i.e., \[\Delta T=0\] or \[dT=0\]
For ideal gasses we know that the internal energy (U) will be constant, then \[\Delta U=0\].
Apart from that, many factors of the system also vary during the continuation of an isothermal process such as internal energy. To maintain the temperature of the system, it can be kept in a tight cylinder. Then, by regulating the temperature of the cylinder, we can control the temperature of the system to an optimal level.
Below are the examples of the isothermal process:
A phase change of matter
Melting of matter, and
Evaporation, etc.
Working in the refrigerator is an isothermal process. The temperature of the surroundings changes irrespective of changes in the internal temperature of the refrigerator. Excess heat is removed and transmitted to the surrounding.
Working of a heat pump is again an isothermal process in this the heat from the surrounding is either brought inside the house or released outside the house depending upon the needs.
A common industrial use of the isothermal process is the Carnot heat engine.
To maintain the temperature of the system, work should either be done on the system or be done by the system on the surrounding; doing work on the gas increases the internal energy, which, in turn, increases the temperature.
However, if the temperature rises more than the required range, then work is done by the system on the surroundings. However, when the temperature of the system decreases, the energy is released to the surroundings in the form of heat.
Isobaric Process
An isobaric process has the word ‘bar’, where the bar is the unit of pressure. Another three letters added ‘iso’ make a process called the isobaric process. An isobaric process is a process that takes place under constant pressure.
Example:
Freezing of water to ice or boiling of water to steam. In either of the scenarios gas, either contacts or expands and a net amount of work is done so as to maintain constant pressure.
Isochoric Process
The word ‘choric’ in isochoric stands for volume and the word ‘iso’ stands for equal. An isochoric process is one that takes place at a constant volume. It is also known as isometric process or constant-volume process.
If the work done, i.e., W = PΔV
At constant volume, ΔV = 0, i.e., no work is done by the system.
FAQs on Isothermal and Adiabatic Process
1. What is an isothermal process in thermodynamics?
An isothermal process is a type of thermodynamic process in which the temperature of the system remains constant (ΔT = 0). For this to occur, the process must be carried out very slowly to allow the system to continuously exchange heat with its surroundings. According to the First Law of Thermodynamics, since the internal energy of an ideal gas depends only on temperature, the change in internal energy (ΔU) is zero. Thus, any heat added to the system is entirely used to do work (Q = W).
2. What is an adiabatic process and how does it differ from an isothermal process?
An adiabatic process is one where there is no heat transfer between the system and its surroundings (Q = 0). This typically happens when a process occurs very rapidly, or the system is perfectly insulated. The key differences are:
- Heat Exchange: In an isothermal process, heat is exchanged to keep the temperature constant. In an adiabatic process, there is zero heat exchange.
- Temperature: Temperature is constant in an isothermal process, but it changes in an adiabatic process (e.g., a gas cools during adiabatic expansion).
- Governing Equation: For an ideal gas, an isothermal process follows PV = constant, while an adiabatic process follows PVγ = constant, where γ is the ratio of specific heats.
- Speed: Isothermal processes are generally very slow, whereas adiabatic processes are very fast or sudden.
3. What are the essential conditions for a process to be isothermal or adiabatic?
The conditions depend on controlling heat flow.
- For an Isothermal Process: The primary condition is that the walls of the container must be perfectly conducting to allow free heat exchange with the surroundings. Additionally, the process must be carried out extremely slowly to provide enough time for this heat transfer to occur and maintain a constant temperature.
- For an Adiabatic Process: The main condition is that the walls of the container must be perfectly insulating to prevent any heat from entering or leaving the system. The process must also be carried out very rapidly (suddenly) so that there is insufficient time for any significant heat exchange to take place.
4. How is the work done calculated in an isothermal process compared to an adiabatic one?
The work done is represented by the area under the P-V curve, and the formulas differ because the paths are different.
- Work Done in an Isothermal Process: The work done (W) during a reversible isothermal process is given by the formula W = nRT ln(V₂/V₁), where n is the number of moles, R is the universal gas constant, T is the constant temperature, and V₁ and V₂ are the initial and final volumes.
- Work Done in an Adiabatic Process: The work done (W) during a reversible adiabatic process is calculated as W = (P₁V₁ - P₂V₂)/(γ - 1) or W = nR(T₁ - T₂)/(γ - 1). Here, both pressure and temperature change.
5. Can you provide some real-world examples of isothermal and adiabatic processes?
Yes, these processes can be observed in various natural and man-made systems.
- Isothermal Examples: The melting of ice at 0°C or the boiling of water at 100°C are examples, as the temperature stays constant during the phase change. A refrigerator's cycle, where heat is removed at a constant low temperature, is another practical application.
- Adiabatic Examples: The sudden bursting of a bicycle tube is a classic example of adiabatic expansion, which is why the escaping air feels cool. The propagation of sound waves in air and the rapid compression and expansion of fuel-air mixture in an internal combustion engine are also treated as adiabatic processes.
6. Why is the adiabatic curve steeper than the isothermal curve on a P-V diagram?
The adiabatic curve is steeper because pressure changes more significantly for a given change in volume compared to an isothermal process. In an isothermal expansion, as volume increases, pressure drops. Heat flows into the system to keep the temperature constant. In an adiabatic expansion, as volume increases, pressure drops, but no heat flows in. Instead, the system does work at the expense of its own internal energy, causing the temperature to drop as well. This additional temperature drop leads to a much larger drop in pressure, making its slope on the P-V graph steeper. Mathematically, the slope of the adiabatic curve is γ times the slope of the isothermal curve at any given point.
7. How does the First Law of Thermodynamics apply differently to these two processes for an ideal gas?
The First Law of Thermodynamics, ΔU = Q - W, is fundamental but simplifies differently for each process:
- For an Isothermal Process: The temperature is constant, so the change in internal energy (ΔU) is zero. The equation becomes 0 = Q - W, or Q = W. This means all the heat energy supplied to the system is converted into work done by the system.
- For an Adiabatic Process: There is no heat exchange, so Q = 0. The equation becomes ΔU = 0 - W, or ΔU = -W. This implies that the work done by the system is performed entirely at the cost of its own internal energy. If the system expands and does work, its internal energy and temperature decrease.
