Why in adiabatic compression and expansion small volumetric change occurs while in isothermic compression or expansion very small pressure is applied and volume changes very significantly i am referring to PV diagram of Carnot engine?
In Adiabatic changes the system is disconnected from the surrounding therefore any change in pressure leads to work which goes to change in internal energy of the system - this change is reflected as change in temperature (one can visualize it as change in kinetic energy of the gas molecules) so the amount of work done does not lead /can not lead to large volume changes. If large volume changes will occur then the Kinetic energy can not rise to the extent demanded
The change in pressure is related to change in volume raised to a power gamma where gamma is greater than 1.0
In Isothermal situation ,the system is in thermal contact with the surrounding and the change in pressure leads to work done which can be exchanged as heat energy with the surrounding -
the internal energy remains the same so change in pressure is related inversely to the change in volume only . The thermal energy can come inside or can go from the system to the surroundiing - mechanical work is the p.dv amount.
In any engine running between two resevoirs the importance of isothermal and adiabatic parts can be underlined to say that the conversion of heat into mechanical work is done during the adiabatic paths whereas the isothermals are regions to draw energy or throw it out to the surroundings.