Difference between Adiabatic and Isothermal Process
Thermodynamics uses the concepts isothermal process and adiabatic process to explain the behaviour of a thermodynamic system and its relation to the temperature changes. Isothermal process is a process that happens under constant temperature, but other parameters regarding the system can be changed accordingly. Adiabatic process describes a process where no heat transfer occurs between a system and its surrounding. Here, the temperature of the system should be changed in order to avoid any heat transfer. This indicates that the main difference between isothermal and adiabatic process is that isothermal process occurs under constant temperature whereas adiabatic process occurs under varying temperature.
Adiabatic :
Adiabatic change is the one in which no heat is transferred into or out of the system. Heat transfer can be mainly stopped by two ways. One is by using a thermally insulated boundary, so that no heat can enter or exist. For example, a reaction carried out in a Dewar flask is adiabatic. The other type of adiabatic process happens when a process takes place vary rapidly; thus, there is no time left to transfer heat in and out. In thermodynamics, adiabatic changes are shown by dQ=0. In these instances, there is a relationship between the pressure and temperature. Therefore, the system undergoes changes due to pressure in adiabatic conditions. This is what happens in cloud formation and large scale convectional currents. At higher altitudes, there is a lower atmospheric pressure. When air is heated, it tends to go up. Because the outside air pressure is low, the rising air parcel will try to expand. When expanding, the air molecules do work, and this will affect their temperature. That is why the temperature reduces when rising up. According to thermodynamics, the energy in the parcel is remained constant, but it can be converted to do the expansion work or maybe to maintain its temperature. There is no heat exchange with the outside. This same phenomena can be applied to air compression too (e.g.: a piston). In that situation, when the air parcel compresses temperature increases. These processes are called adiabatic heating and cooling.
Isothermal:
Isothermal change is the one in which the system remains at constant temperature. Therefore, dT=0. A process can be isothermal, if it happens very slowly and if the process is reversible. So that, the change occurs very slowly, there is enough time to adjust the temperature variations. Moreover, if a system can act like a heat sink, where it can maintain a constant temperature after absorbing heat, it is an isothermal system. For an ideal has in isothermal conditions, the pressure can be given from the following equation.
P=nRT /V
Since work, W= PdV following equation can be derived.
W= nRT ln (Vf/Vi)
Therefore, at constant temperature the expansion or compression work happens while changing the system volume. Since there is no internal energy change in an isothermal process (dU=0), all the heat supplied are used to do work. This is what happens in a heat engine.
