Three processes compose a thermodynamic cycle shown in the accompany P-V, diagram of an ideal gas :

Process 1\(\rightarrow\)2 takes place at constant temperature, during this process 60 J of heat enters the system

Process 2\(\rightarrow\)3 takes place at constant volume, during this process 40 J of heat leaves the system

Process 3\(\rightarrow\)1 is adiabatic

What is the change in internal energy of the system during process 3\(\rightarrow\)1 ?

+ 40 J

1\(\rightarrow\)2 : isothermal; \(\Delta U_{12}\) = 0

2\(\rightarrow\)3 : isochoric; \(\Delta W\) = 0

\(\Rightarrow \Delta Q_{23} \Rightarrow -40 = \Delta U_{23}\)

For a cyclic process : \(\Delta U\) = 0

\(\Delta U_{12} + U_{23} + U_{31} = 0\)

0 + (-40) + \(\Delta U_{31}\) = 0

\(\Delta U_{31}\) = + 40 J