AExpanded by adding more molecules to it
a
(a) Internal energy of an ideal gas is given by
$U = \frac{f}{2}\mu RT = \frac{f}{2}\left( {\frac{N}{{{N_A}}}} \right)\,RT$ ==> $U \propto NT.$
In isothermal process $T =$ constant ==> $U \propto N.$
i.e. internal energy increases by increasing number of molecules $(N).$