The key difference between adiabatic and polytropic processes is that in adiabatic processes no heat transfer occurs whereas in polytropic processes heat transfer occurs.
In chemistry, we divide the universe into two parts. The part we are going to study is “a system”, and the rest is “the surrounding”. A system can be an organism, a reaction vessel or even a single cell. We can distinguish systems from each other by the kind of interactions they have or by the types of exchanges take place. We can classify systems into two groups as open and closed systems. Sometimes matters and energy can go through the system boundaries. The exchanged energy can take several forms such as light energy, heat energy, sound energy, etc. If the energy of a system changes because of a temperature difference, we say there has been a flow of heat. Adiabatic and polytropic are two thermodynamic processes that relate to heat transfer in systems.
What is Adiabatic?
Adiabatic change is the one in which no heat is transferred into or out of the system. This heat transfer limitation mainly occurs in two ways. One is by using a thermally insulated boundary so that no heat can enter or exist. For example, a reaction that we carry out in a Dewar flask is adiabatic. Secondly, an adiabatic process happens when a process takes place very rapidly; thus, there is no time left to transfer heat in and out.
In thermodynamics, we can show adiabatic changes as dQ=0 where Q is heat energy. In these instances, there is a relationship between pressure and temperature. Therefore, the system changes due to pressure in adiabatic conditions.
For example, think what happens in cloud formation and large-scale convectional currents. At higher altitudes, there is lower atmospheric pressure. When air heats, 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 alter their temperature. That is why the temperature reduces when rising.
According to thermodynamics, the energy in the air parcel remains constant, but it can be converted into different energy forms (to do the expansion work or maybe to maintain its temperature). However, there is no heat exchange with the outside. We can apply this same phenomenon 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.
What is Polytropic?
Polytropic process occurs with a heat transfer. However, the heat transfer happens reversibly in this process.
When a gas undergoes this type of heat transfer, the following equation is true for a polytropic process.
PVn = constant
Where P is the pressure, V is the volume and n is a constant. Hence, to hold PV constant in the polytropic gas expansion/compression process, both heat and work interchange takes place between the system and surrounding. Therefore, polytropic is a non-adiabatic process.
What is the Difference Between Adiabatic and Polytropic?
Adiabatic change is the one in which no heat is transferred into or out of the system while the polytropic process occurs with a heat transfer. Hence, the key difference between adiabatic and polytropic processes is that in adiabatic processes no heat transfer occurs whereas in polytropic processes heat transfer occurs. Moreover, the equation dQ=0 is true for the adiabatic process while the equation PVn=constant is true for the polytropic process.
Summary – Adiabatic vs Polytropic
Adiabatic and polytropic process are two important thermodynamic processes. The key difference between adiabatic and polytropic processes is that in adiabatic processes no heat transfer occurs whereas in polytropic processes heat transfer occurs.
1. Libretexts. “3.6: Adiabatic Processes for an Ideal Gas.” Physics LibreTexts, Libretexts, 11 Mar. 2018. Available here