Internal energy of an ideal gas depends only on:

1. For an ideal gas, molecules are point particles with no inter-molecular PE. 2. Internal energy is purely the kinetic energy of random thermal motion. 3. From kinetic theory, average KE per molecule = (f/2) k_B T, function of T only. 4. Hence U = U(T) for an ideal gas — independent of P and V. _Source: syllabus Physics Ch 11, §11.5 + Ch 12 Kinetic Theory_