- #1
21joanna12
- 126
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I am teaching myself thermodynamics (and really enjoying it!) but am slightly confused about Carnot's engine. From the equation efficiency=1-T(cold reservoir)/T(hot reservoir), I see that the most efficient engine is one where the difference in temperature between the cold and hot reservoirs is the greatest. However I also read this:
"It was Carnot who realized that the most efficient heat engine of all was a ‘reversible’ heat engine. In other words – one that got the same amount of work out of the heat transfer as would be needed to operate a perfect fridge to undo its operation.
In order to do this, it is necessary for all the heat transfers (between one object and another) to take place with as small a temperature difference as possible. If this is not done, heat will flow from hot objects to cold – a process which could have been used to do work, but wasn’t. Therefore not enough work will be done to enable the fridge to return the heat to the hot object. "
And to me this Implies that the most efficient engine is where the temperature different is smallest because then less heat flows straight from the hot to cold reservoir without being used to do work.
So my first question is about the above, but I also wanted to ask about how Carnot's engine links to the third law of thermodynamics. I know That the equations for Carnot's engine are supposed to somehow demonstrate that absolute zero can never be reached in a finite number of steps, but I don't understand how?
Thank you in advance :)
"It was Carnot who realized that the most efficient heat engine of all was a ‘reversible’ heat engine. In other words – one that got the same amount of work out of the heat transfer as would be needed to operate a perfect fridge to undo its operation.
In order to do this, it is necessary for all the heat transfers (between one object and another) to take place with as small a temperature difference as possible. If this is not done, heat will flow from hot objects to cold – a process which could have been used to do work, but wasn’t. Therefore not enough work will be done to enable the fridge to return the heat to the hot object. "
And to me this Implies that the most efficient engine is where the temperature different is smallest because then less heat flows straight from the hot to cold reservoir without being used to do work.
So my first question is about the above, but I also wanted to ask about how Carnot's engine links to the third law of thermodynamics. I know That the equations for Carnot's engine are supposed to somehow demonstrate that absolute zero can never be reached in a finite number of steps, but I don't understand how?
Thank you in advance :)