Dulong-Petit Law

DULONG-PETIT LAW (Dulong, Petit), concerns the heat capacities of solid bodies. The product of the atomic weight of any element multiplied by its heat capacity in solid state, according to the D.-P. law, should equal the constant value of 5.955 calories. The D.-P. law is theoretically a consequence of the classical principle of uniform distribution of energy among degrees of freedom. For a monatomic gas, according to this theorem, the product of atomic weight multiplied by heat capacity (at constant volume) should equal ³/₂R (R is the gas constant in thermal units, equal to 1.985 cal.), and for solid state - 3 R, i.e., the value mentioned above. The D.-P. law is well satisfied for many elements at comparatively high temperatures; as the temperature decreases, it begins to be sharply violated, and the heat capacity, as it approaches absolute zero, also tends toward zero. These deviations from the D.-P. law are explained by quantum theory, in which the theorem of uniform distribution of energy among degrees of freedom loses its force. The Dulong-Petit law can be to some extent generalized to the case of solid compounds: the molecular heats of solid compounds equal the sum of the atomic heats of the elements contained in them (Kopp's rule).