State functions and thermodynamic properties of ideal gases:

• Pressure alone does not dictate internal energy; at constant temperature, isothermal pressure changes cause zero change in ideal gas internal energy.
• Volume alone does not dictate internal energy; free expansion into vacuum produces no change in temperature or internal energy.
• According to Joule's law of ideal gases, the internal energy $U$ of an ideal gas depends solely and exclusively on its absolute temperature: $U = f(T)$, with $dU = m c_v dT$. Since intermolecular attractive forces are negligible in ideal gas models, changing volume or pressure without changing temperature does not alter molecular kinetic energy.
• For real gases, $U$ depends on $T$ and $v$, but for an ideal gas it is strictly a function of absolute temperature alone.