What is the working substance in an external combustion engine and an internal combustion engine?

Three moles of an ideal gas undergo a cyclic process $ABCA$ as shown in the figure. The pressure, volume, and absolute temperature at points $A, B,$ and $C$ are respectively $(P_1, V_1, T_1)$, $(P_2, 3V_1, T_1)$, and $(P_2, V_1, T_2)$. Then the total work done in the cycle $ABCA$ is (where $R$ is the universal gas constant).

Two gases $A$ and $B$ are filled at the same pressure in separate cylinders with movable pistons of radius $r_A$ and $r_B$,respectively. On supplying an equal amount of heat to both the systems reversibly under constant pressure,the pistons of gas $A$ and $B$ are displaced by $16 \ cm$ and $9 \ cm$,respectively. If the change in their internal energy is the same,then the ratio $\frac{r_A}{r_B}$ is equal to

What are the differences between mechanics and thermodynamics?

Heat is applied to a rigid diatomic gas at constant pressure. The ratio $\Delta Q : \Delta U : \Delta W$ is

Helium gas undergoes a cycle $ABCDA$ (consisting of two isochoric and two isobaric lines) as shown in the figure. The efficiency of this cycle is nearly ....... $\%$ (Assume the gas to be an ideal gas).