Mayer's Relation for one mole of an ideal gas states: $C_p-C_v=R$

• $C_p$: Molar heat capacity at constant pressure.
• $C_v$: Molar heat capacity at constant volume.
• $R$: Universal gas constant.
• Physically, $C_p$ is greater because at constant pressure, some absorbed heat is used for expansion work against the surroundings, whereas at constant volume, all heat goes into increasing internal energy.