The heat capacity of a substance tells us how much heat is required to raise a certain amount of the substance by one degree. For a gas we can define a molar heat capacity C - the heat required to increase the temperature of 1 mole of the gas by 1 K.
The value of the heat capacity depends on whether the heat is added at constant volume, constant pressure, etc.
Heat Capacity at Constant Volume
For an ideal gas, applying the First Law of Thermodynamics told us that: