Question

Question attached.

Answer 1

Alright, let's solve this part by part.

Answer 2

Hmmm.

Answer 3

@preskill89 I am here, available to be taught. :)

Answer 4

I'm having a hard time understanding exactly what you need help on; you only seemed to miss the question on engine efficiency.

Answer 5

In part A) I do not understand why the net heat added during the cycle is Q1-Q3

Answer 6

You have two separate variables for heat, referred to Qin and Qout. During the process of Isovolumic cooling, the engine cooled to 300k, meaning it lost half of it's original energy. After the stage of compression in Q3, the engine re-entered it's original state. By subtracting Q1 from Q3, it would be possible to calculate the net heat.

Answer 7

In Q1, we don't add any heat to the system, right? the heat got from the expansion process.
In Q3, to increase the temperature from 300 K to 600K , we must add heat. This amount of heat is the heat we add.
So that, I do not know why Q1 involves to the "net heat add".

Answer 8

Yes you are correct, the heat derived from Q1 does not add any heat to the system. Imagine a cylinder that is capable of compression. If the volume of the gas is expanded, the heat required to fill that volume would be less.

Answer 9

From this stuck, I do not know how to calculate the engine efficiency
the amount of heat available from Q1 is Q1, through Q3, we add some more amount of heat, it is Q3, so that, the total of heat the net absorb is Q1+Q3, and efficiency = W/(Q1+Q3)= , but it is not correct.
or if it is just Q3 (the amount of heat we add to the system, efficiency = W/Q3 , but it is not correct either. :)

Answer 10

Heat engines convert heat (Qin) into work (Wout). They cannot do this task perfectly, and normally their efficiency is well under 50%. The equation you used to calculate thermal efficiency is \[\eta=\frac{ w }{ Q1+Q3}\]
The net heat absorbed was only 300k. However, after compression, 300k was restored.