Thermodynamics and P-V Diagrams 

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Isentropic is a special case of polytropic. Isentropic means constant entropy, which happens for reversible processes with no heat transfer (i.e. adiabatic/quasistatic or adiabatic/reversible). Polytropic processes are governed by the relationship of P*V^n = constant, where n can be any positive real number including infinity. For n=0, we have a constant pressure process. V^n reduces to 1 no matter what volume is, implying that P=constant For n=1, we have a constant temperature process assuming an ideal gas. The process is equally sensitive to changes in volume and pressure, such that pressure and volume each change by each others' reciprocals. For n=infinity, we have a constant volume process. No matter what P is, V^n will dominate the expression, implying that V is constant. The case of isentropic processes occurs when n=k (some books use gamma), where k is the heat capacity ratio. k is a substance-specific value. For air and other 2-atom elemental gasses like it, k=1.4. For a single atom gas like helium, k=1.67. It has to do with the ways that the gas molecule can store energy in addition to pure translation.

Not enough information. We'd have to know where n of the polytropic process is, relative to the adiabatic index k.

Then there was no work done w=-P∆V 0=x×O 0=0

How much work do you do pushing against a rigid wall? That's how much work a static atmosphere is doing on our planet.

Well u r right ,the main formula. Is U= q+w but when we put formula for work ie. - P∆v it will become U=q-w

Heat exchange dq is, dq=du + pdv or du=dq+w because w=-pdv.

Vaibhav Rokde It should be Q=U-W

Depends on what sign convention you assign for the meaning of positive work, positive heat, and change in internal energy. The sign convention is arbitrary, and it is best to always interpret the sign of your results in words, to avoid confusion. And follow the sign convention given in your class. I'm accustomed to the heat engine sign convention, where heat added to the system is positive, and work done by the system is positive. Thus, Q=deltaU + W is correct in that sign convention.

Yes it is.

Depends on what sign convention you give for +Q and +W. I'm accustomed to +Q being heat added to the system, and +W being work done by the system.

The sign convention also varies in chem and phy textbook all u need to focus is that when to use when