The first law of thermodynamics may be written as $\Delta U = q + w$. Say what the symbols $+\Delta U$, $+q$ and $+w$ mean in this equation.
A fixed mass of an ideal gas completes the cycle ABCA of changes, as illustrated in Fig. 2.1.
For the change from A to B, the energy given to the gas by heating is $442\,\text{J}$. Use the first law of thermodynamics to demonstrate that the internal energy of the gas rises by $265\,\text{J}$.
For the change from B to C, the internal energy of the gas falls by $313\,\text{J}$. Using molecular energy ideas, state and explain qualitatively what happens, if anything, to the gas temperature.
For the change from C to A, use the data in (b)(i) and (b)(ii) to work out the change in internal energy.
At point A, the gas temperature is $227^\circ\text{C}$. Calculate the number of molecules in the fixed mass of gas.