can anyone please help me solve this one? may it be please with a lot of good de

Question

can anyone please help me solve this one? may it be please with a lot of good details. I am bad at physic :( thank you in advance. :)

1) A substance undergoes a cyclic but overall irreversible process between two heat reser- voirs at temperatures 400 C and 200 C. During the cycle, the substance does 200 J of work and rejects 600 J of heat to the colder reservoir. a) What is the heat flow from the hot reservoir in this cycle? b) What is the efficiency in this cycle? c) Given that those "legs" (of this cyclic process) in which heat is exchanged with the two temperature reservoirs are isothermal and reversible. What is the change in entropy for the substance during the isothermal part of the cycle when in contact with just the cold reservoir

Explanation / Answer

e = W/Q_h = Q_h -Q_c /Q_h

the heat engine is working between the temperatures T1 = 400 0C ,T2 = 200 0C

e = 1-200 /400 = 1-1/2 = 0.5

now 0.5 = Q_h -Q_c /Q_h

W = 200 J , Q_c = 600 J

Q_h = ?

work done W = Q_h-Q_c

Q_h = W+Q_c

Q_h = 200+600 = 800 J

now the effiviency is eta = W/Q_h

eta = 200 /800 = 0.25  

so the heat flow from the hot reservoir is 800 J

and the efficiency is eta = 0.25

c) in isothermal process the change in temperature is zero  

if there is no change in temperature and no change in entropu

from second law of thermodynamics dQ = T*ds ==> ds = dQ/T = 0

if both the cold reservoir and the hot reservoir are at same tempereure then there i sn oflow heat

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