The figure shows the PV diagram for a heat engine. It consists of two isothermal

Question

The figure shows the PV diagram for a heat engine. It consists of two isothermal steps (a -> b) and (c -> d), and two constant volume steps (b -> c and d -> a). We are interested in the efficiency, assuming the engine uses a monatomic (f= 3) ideal gas.
a) What do we know about the temperatures?
[1]T1<T2 [2]T1=T2 [3]T1>T2
[4] need more information to decide

The figure shows the PV diagram for a heat engine. It consists of two isothermal steps (a -> b) and (c -> d), and two constant volume steps (b -> c and d -> a). We are interested in the efficiency, assuming the engine uses a monatomic (f= 3) ideal gas.
a) What do we know about the temperatures?
[1]T1<T2 [2]T1=T2 [3]T1>T2
[4] need more information to decide

Explanation / Answer

a) [3]T1 > T2 expansion at T1 compression at T2

b) [4] only (b -> c) since the temperature decreases.

c) [2] (b -> c) and (c -> d) In b to c work done ic zero and change in U is negative so Q is negative.

d) [3]W=nRTaln(Vb/Va) since the process is isothermal

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