Please be clear. Problem 4. The air in a piston cylinder is compressed from P1 t
ID: 2995269 • Letter: P
Question
Please be clear.
Problem 4. The air in a piston cylinder is compressed from P1 to P2 at a constant temperature. The compression requires 12 kJ of PV work. The air temperature is maintained constant at 25C during this process as a result of heat transfer to the surrounding medium at 10°C Determine a. the change in entropy of the ideal gas during this process. b. the change in entropy of the surroundings during this process. c. the change in entropy of the world during this process. (Answer: 0.0021 kJ/K) Problem 6. Steam contained in a well-insulated cylinder piston is initially at 150 kPa, 440 K and has a volume of 1 m3. As a paddle wheel turns in the cylinder, the steam temperature rises to 720 K. During the process, the piston moves and the pressure is maintained at 150 kPa. a. Find the PV work by the steam. b. Find the shaft work by the paddle wheel. c. Find the change in entropy during the process. (Answer: 0.7453 kJ/K) Use the Problem Solving Method for this problem.Explanation / Answer
problem 4)
he process is isothermal compression.
The compression requires 12 kJ of PV work
?w = -12 kJ
From the first law, we know that:
dU = ?q + ?w
where dU is the infinitesimal change in energy due to an infinitesimal amount of work (?w) done on the system and and infinitesimal amount of heat (?q) added to the system.
We also know that the internal energy of an ideal gas depends only on temperature, so in an isothermal process, the internal energy remains constant, and we have:
dU = 0
hence, ?q = -?w = 12 kJ
(a) for a reversible process,
dS = ?q/T, where T is temp of ideal gas
T = 25 C = 298 K
the change in entropy of the ideal gas during this process = 12/298
= 0.040268 kJ/K = ?S_sys
(b) the change in entropy of the surroundings during this process = -?q/T
= -0.040268 kJ/K = ?S_surr
(c) the change in entropy of the world during this process
= ?S_world = ?S_sys + ?S_surr = 0