Question

Why is intenral energy of an ideal gas only changed by gas temperature and not by pressure change or volume change?

Answer 1

I think it is easier to understand if we realize that pressure(P) and volume(V) are intimately linked and then think about the effect of interactions of pressure and volume on the internal energy.

If you reduce the volume of the system so particles are closer together and thus have a higher molecular interaction between the particle and high interaction energy.(Real gas)

In the ideal gas we assume that there is no molecular interaction between the particle ,so if we reduce the volume then it doesn't have the effect on the internal energy.(ideal gas). In this case internal energy change by the following equation.

U= 3/2*NKT

conclusion: if we consider the interaction between the molecle then internal energy is function of temp,pressure,volume(Real gas) But if we neglect the interaction between the molecule then internal energy is only the function of temperature(ideal gas)