Thermodynamics - please guide me through

So find the maximum efficiency of the engine (meaning it uses a carnot cycle) and check the difference.

What is the definition of the coefficient of performance?

What is the first law of thermodynamics for a closed cycle?

 

The efficiency of the engine is given by

[tex]e= \frac{W_{eng}}{|Q_h|}=\frac{|Q_h|-|Q_c|}{|Q_h|}= 1-\frac{|Q_c|}{|Q_h|}[/tex]

We are only told that the they claim the engine has efficiency 72.0%

[tex]0.72=\frac{W_{eng}}{|Q_h|}[/tex]

But we don't have any other information. So what do we need to do?

The coefficient of preformance is given by

[tex]COP=\frac{|Q_c|}{W}[/tex] (cooling mode)

4.3= 258/W

W=60

It think for cyclic process, the internal energy is equal to 0. Because Q=-W

But here Q=-W=-60

But this is wrong, the correct answer has to be 318.0 J.

 

(A) The maximum efficiency of a thermal engine operating between Ta and Tb>Ta is 1-Ta/Tb (see Carnot efficiency at http://en.wikipedia.org/wiki/Thermal_efficiency)

(C) The internal energy is decreased at each cycle by 258J, you didn't consider that...

 

Thank you so much, I've got it!