Net Entropy Change in Heat Engine Cycle

[Sondra]

Homework Statement

A heat engine operates between a high-temperature reservoir at 634 K and a low-temperature reservoir at 320 K. In one cycle, the engine absorbs 6540 J of heat from the high-temperature reservoir and does 2200 J of work.

Homework Equations

What is the magnitude of the net change in entropy as a result of this cycle?

The Attempt at a Solution

Qh=6540J
W=2200J
Find Qc=Qh-W=4340J
Sc=Qc/Tc= 4340J/320K=13.56J/K
Sh=Qh/Th= 6540J/634K= 10.32 J/K
Net S= Sc-Sh= 3.2 J/K
Please help. I feel like i am doing this problem correctly but I seem to be missing something. Thank you!

 

[Chestermiller]

Homework Statement

A heat engine operates between a high-temperature reservoir at 634 K and a low-temperature reservoir at 320 K. In one cycle, the engine absorbs 6540 J of heat from the high-temperature reservoir and does 2200 J of work.

Homework Equations

What is the magnitude of the net change in entropy as a result of this cycle?

The Attempt at a Solution

Qh=6540J
W=2200J
Find Qc=Qh-W=4340J
Sc=Qc/Tc= 4340J/320K=13.56J/K
Sh=Qh/Th= 6540J/634K= 10.32 J/K
Net S= Sc-Sh= 3.2 J/K
Please help. I feel like i am doing this problem correctly but I seem to be missing something. Thank you!

Since the engine is operating in a cycle, what is the change in entropy of the working fluid?
What is the change in entropy of the hot reservoir? What is the change in entropy of the cold reservoir? What is the total change in entropy of the working fluid and the two reservoirs?

Chet

 

[Sondra]

Since the engine is operating in a cycle, what is the change in entropy of the working fluid?
What is the change in entropy of the hot reservoir? What is the change in entropy of the cold reservoir? What is the total change in entropy of the working fluid and the two reservoirs?

Chet

So the hot reservoir has a net (-) charge because it's energy is absorbed. The net change in the cold is (+) because work is being put into the system and needs to be exhausted..? I am not exactly sure of how a "cycle" would influence the entropy unless it is referring to a reversible engine.

 

[Chestermiller]

So the hot reservoir has a net (-) charge because it's energy is absorbed. The net change in the cold is (+) because work is being put into the system and needs to be exhausted..? I am not exactly sure of how a "cycle" would influence the entropy unless it is referring to a reversible engine.

In any cycle, the change in any state function (like entropy), for a system experiencing the cycle, is equal to zero. The hot reservoir loses heat to the engine and the cold reservoir receives heat from the system. So, the change in entropy of the hot reservoir is negative and the change in entropy of the cold reservoir is positive.

Chet

 

[Sondra]

In any cycle, the change in any state function (like entropy), for a system experiencing the cycle, is equal to zero. The hot reservoir loses heat to the engine and the cold reservoir receives heat from the system. So, the change in entropy of the hot reservoir is negative and the change in entropy of the cold reservoir is positive.

Chet

The correct answer is not zero :/

 

[Chestermiller]

The correct answer is not zero :/

I didn't say it was. I said that the entropy change of the working fluid is zero. The total entropy change, including the reservoirs, is not zero. Your original answer was correct, although you obtained this answer without understanding what is happening fundamentally. I was trying to explain the correct way to analyze the problem.

Chet

 

[Sondra]

I didn't say it was.

So yeah I understand the Sh is negative that is why i subtracted it from the positive Sc. Is my equation incorrect?