Adiabatic vs Polytropic
For the purpose of chemistry, the universe is divided into two parts. The part we are interested in is called a system, and the rest is called the surrounding. A system can be an organism, a reaction vessel or even a single cell. The systems are distinguished by the kind of interactions they have or by the types of exchanges take place. The systems can be classified into two as open systems and closed systems. Sometimes matters and energy can be exchanged 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 related to heat transfer in systems.
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.
Heat transfer happens reversibly in this process. When a gas undergoes this type of heat transfer, following equation is true for a polytropic process.
PVn = constant, where P is the pressure, V is the volume and n is a constant.
Both heat and work interchange take place between the system and surrounding, to hold PV constant in the polytropic gas expansion / compression process. Therefore, polytropic is a non-adiabatic process.
What is the difference between Adiabatic and polytropic?
• In adiabatic processes, heat transfer between the system and the surrounding does not happen. However, in polytropic processes heat is transferred.
• Therefore, polytropic is a non-adiabatic process.