Adiabatic compression and expansion and isothermal expansion? Please help me in understanding the concepts 


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*Why in adiabatic compression and expansion small volumetric change occurs while in isothermic compression or expansion very small pressure is applied and why the volume changes very significantly? Here, I am referring to PV diagram of Carnot engine? 

*Why in adiabatic process compression and expansion should be made very fast and vice versa in that of isothermic process?

 A: Let us look to your first question.  
In an adiabatic process, pressure change is due to two factors: change in volume (due to work done) and internal energy change (due to the temperature change), but in an isothermal change, the pressure change is due to change of volume only (since temperature is a constant). Thus, for a fixed change in volume, pressure changes deeply in an adiabatic change because there is a change in internal energy also. But in iso thermal change, there is only a small change in pressure, because internal energy change is zero. That is why the adiabatic prossess is deeper than isothermal.  
Let us go to your next question.
An isothermal change is a change occure at constant temperature. So a system to remain in same temperature till the completion of a process, the system must go through infinte slow step, with each steps have the same temperature as before. For this to happen, the steps takesplce so slowly such that there is enough time to convert the heat gained/lose to the work done, without altering the internal energy. Thus an isothermal change should takesplace so slowly.
For an adiabatic change, the process should takesplace so fast that there must no time for the system to exchange the heat with the surrounding. 
