بمفاضلة المعادلة (3) وباستعمال قانون الغاز المثالي, formula_7, ينتج:formula_8المعادلة (4) يعبر عنها غالبا formula_9لأن formula_10.
Differentiating Equation (3) and use of the ideal gas law, formula_22, yields:formula_23Equation (4) is often expressed as formula_24because formula_25.