when we put pressure constant molar heat capacity Cp.if the objects expand then it is less than Cv.when you subtract means Cp-Cv get negative sign.But R Universal gas constant value(which comes from the subtraction of those both two, ) become negative then how it is?

Dear student,

Heat capacity (symbol C) is an extensive property , meaning it is a physical property that scales with the size of a physical system. A sample containing twice the amount of substance of another requires twice the amount of heat (ΔQ) to achieve the same rise in temperature (ΔT):

C_P AND C_V  ARE FIXED FOR MONOATOMIC, DIATOMIC ,POLYATOMIC MOLECULE SINCE CP AND CV ARE CALCULATED BE OBSERVING THEIR VIBRATIONAL MOTION , TRANSLATIONAL MOTION ETC. SO CP IS ALWAYS GREATER THAN CV AND IF YOU HAVE STUDIED FORMULA FOR ADIABATIC EXPANSION PV^GAMMA=CONSTANT WHERE GAMMA=CP/CV AND IF IN EXPANSION CV IS GREATER THAN CP THEN ACCORDING TO THE FORMULA THE FINAL VOLUME WOULD BE LESSER THAN THE INTIAL VOLUME BECAUSE CP/CV WOULD BE LESSER THAN 1 . BUT YOU HAD CONSIDERED THE PROCESS TO BE EXPANSION SO HOW COME VOLUME DECREASED THEREFORE YOUR ASSUMPTION WAS ONLY WRONG