(a(i))[2]
State what the internal energy of a system means.
(a(ii))[3]
Explain why the change in internal energy of an ideal gas is directly proportional to the change in its thermodynamic temperature.
(b)[3]
A cylinder with volume $1.8 \times 10^{4}\,\text{cm}^3$ contains helium gas at pressure $6.4 \times 10^{6}\,\text{Pa}$ and temperature $25\,^{\circ}\text{C}$. Helium gas may be treated as an ideal gas made up of single atoms. Calculate the number of helium atoms in the cylinder.