Entropy change : I have a simple qustion about the entropy change when we determ

Question

Entropy change :

I have a simple qustion about the entropy change

when we determine the entropy change , "sometimes it is multiplyed by M sometimes it is not" .

what is the concept ? and when should I multiply by M ? .( if there is a fast way to know , my exam is tomorrow)

(note :not nessasery S2-S1)

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Explanation / Answer

It is simple entropy without (or divided by mass) mass is called specific entropy or entropy per unit mass. It is same for heat, work, enthalpy and internal energy. When you divide energy (kJ) by mass it became specific energy (kJ/kg). You have seen in many problems. Without mass you are finding specific entropy. In example one you given, they didn't give the mass of air so you can leave the mass i.e. you are calculating specific entropy (kJ/kgK). In second and third example they have given mass flow rate and mass respectively multiply it.i.e you are finding entropy (kJ/kg).

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