What is the true statement about Carnot engine efficiency?

[newyork]

Homework Statement

Which of the following statements about a Carnot engine is true?
a) Any Carnot engine has an efficiency of 100%
b) Irreversible energies have the greatest possible efficiency.
c) Heat can spontaneously flow from a cold reservoir to a hotter reservoir.
d) If a process is reversible, the efficiency of an engine is maximum.
e) All energies are reversible.

Homework Equations

None

The Attempt at a Solution

Well, I think the answer is E. I canceled out choice b because no where in my textbook did it mention irreversible energies having high efficiency. I don't think it's choice a because the third law of thermodynamics says reaching a temperature of absolute zero is impossible and makes it impossible for a Carnot cycle engine to achieve 100% efficiency. So if I'm right, why is E right? If I'm wrong, then why would it be one of the other choices? I pretty much just used process of elimination.

 

[Marco_84]

i suggest you to check much better the differences beetween rev and irrev process.
regards

 

[achilleus]

i suggest you to check much better the differences beetween rev and irrev process.
regards

i've got a better idea. research what a carnot engine is. it's a lot faster than checking the differences between rev and irrev processes. it will actually help you solve the question

 

[achilleus]

i think the answer is that all engines are reversible. the one that states that "if a process is reversible, the efficiency of an engine is maximum" wouldn't be accurate because ALL processes have to be reversible to obtain maximum efficiency, not just one.

 

[rhyso88]

I would say d is the correct answer here, a is completely false due to the second law of thermodynamcs(never can all heat be converted into pure work),b is obviously not true, c would not have a positive entropy change andhence does not occur spontaneously, e doesn't make sense...hence my choice d, however the statements are rather ambiguous