Both the isothermal and adiabatic processes are integral parts of thermodynamics and thermodynamic systems even though both of them are entirely different and opposite to each other with some of the significant factors contributing to the difference between isothermal process and adiabatic process. In thermodynamics, the adiabatic process corresponds to the thermodynamic process which happens without any transfer of heat or mass, between the system and the surroundings instead there is only energy transfer in terms of work. This is the reason to differentiate between isothermal process and adiabatic process as the isothermal process is one in which only the temperature of the system remains constant i.e. there is no difference in the temperature during the thermodynamic process and there is heat transfer in between the system and the surroundings.
The isothermal process is the thermodynamic process in which there is no change in the temperature throughout the process i.e. ΔT = 0. This can only be maintained with the transfer of heat in-between the system and the surrounding. This condition hence, is used to distinguish between isothermal and adiabatic process. For the heat transfer to take place the system should be surrounded by a thermal reservoir. The thermodynamic changes that take place inside the system are slow enough for maintaining and adjusting a constant temperature of the system via the heat exchange with the surrounding.
From the above explanation, it is clear that isothermal processes can occur in any kind of system that has some way of recovering the lost heat and regulating the temperature be it highly structured machines or any of the living cells which is also a significant factor to distinguish between isothermal and adiabatic process. The most famous and best example of a machine carrying out isothermal processes is the Carnot Cycle. Usually when studying the chemical reactions, thermodynamically speaking, first the effects under isothermal conditions are analyzed before moving on to analyze the temperature effect on the process. Other examples of isothermal processes are phase changes such as melting or evaporation where they also occur at constant pressure. To understand non-isothermal processes, it is a general practice to solve the same process under isothermal conditions and then understand the changes in the non-isothermal processes.
Isothermal processes are an interesting choice when it comes to the study of ideal gases. Since there are no intermolecular forces in ideal gases, the internal energy of the ideal gases in an isothermal process remains constant. Also, according to Joules second law, the internal energy of a fixed amount of an ideal gas depends on the temperature only. Hence, with no change in temperature the internal energy of the isothermal process also remains constant. But this is only true for the ideal gases. For liquids, solids and real gases, the internal energy of the system depends on the temperature as well as on pressure.
To decrease the volume and increase the pressure of a system in isothermal compression, significant work needs to be done. When work is done on the gas it increases the internal energy and in turn increases the temperature. For the maintaining of the temperature energy must leave the system as heat into the surroundings. In case of ideal gas, the amount of energy leaving the system is equal to the work done on the system, as already mentioned above, the internal energy does not change. When there is isothermal expansion the energy given to the system does the work on the surrounding. With suitable help the change in the gas volume can perform useful mechanical work as well.
The difference between isothermal and adiabatic process is that for an adiabatic process there is no heat flow in and out of the system as the system is well insulated. Hence, ΔQ = 0. And if there is no work done, there is no change in the internal energy. Hence, such a process also becomes isothermal. Thus, under certain conditions to specify a unique process it is not sufficient to mention that the process is isothermal. It becomes necessary depending on the parameters to tell whether it is adiabatic as well.
As mentioned in the introduction, to differentiate between isothermal and adiabatic process thermodynamically, adiabatic process is a thermodynamic process in which there is no transfer of heat or mass in-between the system and the surroundings. As per the difference between the isothermal and adiabatic process, in an adiabatic process, the energy is transferred only in the form of work done. Because of the difference between isothermal and adiabatic, the adiabatic process is a key process also as the theory conceptually supports the explanation of the first law of thermodynamics.
As per the definition and the isothermal and adiabatic process difference, an adiabatic system in which there is no transfer of heat and no change in the heat of the system, is said to be adiabatically isolated. The assumption of an adiabatic process is for simplifying the processes and understanding it. Once the assumption is made and the process simplified makes the calculations easier. For example, the compression of gas inside the cylinder of an engine is assumed to occur very rapidly on the time scale of the compression process; some of the system’s energy can be transferred out in the form of heat to the surroundings. Although the cylinders are not insulated and are quite conductive the process is idealized as adiabatic. Same can be said for the expansion process of such a system.
The classical Carnot heat engine which is used to understand and identify the isothermal and adiabatic process difference is shown below:
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Difference Between Isothermal and Adiabatic Process
In order to differentiate between isothermal and adiabatic processes, the major difference between isothermal and adiabatic processes, as clear from the introduction, is in the matter of heat transfer in-between the surrounding and the system. The heat change by heat transfer is used to distinguish between isothermal and adiabatic processes as in isothermal process, heat transfer occurs to maintain constant temperature while there is no such heat transfer in-between the system and the surrounding. Some of the more significant difference between isothermal and adiabatic process in tabular form is given below as a means to compare between isothermal and adiabatic process:
Difference Between Isothermal Process and Adiabatic Processes