Conceptual thermodynamics question

[kuahji]

http://wps.prenhall.com/esm_giancoli_physicsppa_6/0,8713,1115091-,00.html (under practice questions)

On questions 16,17, & 18 I'm trying to figure what is the difference between them (Clearly Tc is different, but how does that change the problem?). The question asks
The heat engine shown above is

a) a reversible (Carnot) heat engine.
b) an irreversible (Carnot) heat engine.
c) a hoax.
d) none of the above.

I'm guessing I just don't understand the concepts, as I thought a Carnot engine was suppose to avoid irreversible processes, like an idealization. Anyway, the answer to 17 & 18 are "a hoax." They all seem to have thermal efficiencies of less than 1, so how should I look at these questions?

 

[alphysicist]

Hi kuahji,

http://wps.prenhall.com/esm_giancoli_physicsppa_6/0,8713,1115091-,00.html (under practice questions)

On questions 16,17, & 18 I'm trying to figure what is the difference between them (Clearly Tc is different, but how does that change the problem?). The question asks
The heat engine shown above is

a) a reversible (Carnot) heat engine.
b) an irreversible (Carnot) heat engine.
c) a hoax.
d) none of the above.

I'm guessing I just don't understand the concepts, as I thought a Carnot engine was suppose to avoid irreversible processes, like an idealization. Anyway, the answer to 17 & 18 are "a hoax." They all seem to have thermal efficiencies of less than 1, so how should I look at these questions?

For a heat engine operating between two heat reservoirs, what is the maximum efficiency possible?

 

[thomate1]

The difference between the three questions lies in the temperature at which the heat engine operates. In question 16, the temperature of the hot reservoir (Th) is higher than the temperature of the cold reservoir (Tc), which is the typical setup for a Carnot heat engine. In question 17, the temperature of the hot reservoir (Th) is equal to the temperature of the cold reservoir (Tc), which is not possible for a Carnot engine. In question 18, the temperature of the hot reservoir (Th) is lower than the temperature of the cold reservoir (Tc), which is also not possible for a Carnot engine.

A Carnot heat engine is an idealized model that operates between two thermal reservoirs at different temperatures and achieves maximum efficiency. This means that it has no losses due to friction or other irreversible processes. However, in the real world, all engines have some level of irreversibility and therefore, cannot achieve the efficiency of a Carnot engine. This is why the answer to questions 17 and 18 is "a hoax," as they describe scenarios that are not possible for a Carnot engine.

To understand these questions, you should focus on the concept of thermal efficiency. This is the ratio of the work output to the heat input, and for a Carnot engine, it is equal to 1 - (Tc/Th), where Tc is the temperature of the cold reservoir and Th is the temperature of the hot reservoir. As you mentioned, all three scenarios have thermal efficiencies of less than 1, indicating that they are not ideal Carnot engines. Therefore, the correct answer is "none of the above" for all three questions.