Physics Lens

Which Equations Apply Only to Ideal Gases?

Students are sometimes unclear about which of the equations taught in the topic of Thermal Physics apply to ideal gases and which apply to all systems (whether ideal or real gas, even liquids and solid). The following table should help to clarify:


Applies to Ideal Gas only Applies to all systems
Gas Laws $$pV=nRT$$ $$pV\approx nRT$$ only for gases at low pressure and high temperatures
Average Kinetic Energy $$=\dfrac{3}{2}kT$$ $$\propto T$$
Internal Energy $$U$$ = sum of KE of molecules
$$U$$ = sum of KE and PE of molecules
First Law of Thermodynamics applies to all systems $$\Delta U=Q+W$$

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