Mass thermal unit

The Joule per kilogram Kelvin (J·kg^-1·K^-1), is the unit IF of heat capacity mass and the mass Entropie. If a homogeneous body has a mass heat capacity of 1 J·kg^-1·K^-1, that means that one needs a quantity of energy of a Joule to increase by a degree the temperature of one kilogram this body.

History of the joule per kilogram Kelvin

The measuring unit of the mass heat capacity was not always the joule per kilogram Kelvin: it did not appear with the concept of specific heat (another name of the mass heat capacity), nor even with the idea of measurement of this specific heat.

The first work relating to the specific heat of the bodies does not mention in fact any unit of particular measure of this property. Here are thus how Lavoisier evokes in 1780 the measuring unit of the specific heat in its Mémoire on heat : If one supposes two equal bodies masses some, and reduced to the same temperature, the quantity of heat necessary to raise of a degree their temperature can not be not the same one for these two bodies; and, if one takes for unit that which can raise of a degree the temperature of a common water book, one conceives easily that all the other quantities of heat, relative to the various bodies, can be expressed in parts of this unit. We will hear in the continuation, by capacities of heat or specific heats, these reports/ratios of the quantities of heat necessary to raise of the same number of degrees their temperature, with equality of mass.

The joule per kilogram Kelvin appears only in second half of 19th, after work of James Prescott Joule (1818-1889). This measuring unit does not appear obviously in work founders of this scientist, but one finds it mentioned as of the second edition of almanac of Whitaker, in 1869, where it is explicitly specified that the measuring unit of the " capacity of heat spécifique" is the joule per kilogram Kelvin. For a long time, this measuring unit will coexist with other measuring units of the specific heat, made up with other basic units (in particular the Calorie).

Another scientist having contributed to the study of the concept of specific heat, Julius Robert von Mayer (1814-1878), Doctor and German Physicist which formulated in 1845 the First principle of thermodynamics, gave his name to the mass thermal unit: 1 mayer = 1 J (kg K). According to an edition of 1964 of an anglophone dictionary of the scientific measuring units, one finds this unit in a publication of 1925. The mayer is however a nonlegal Measuring unit (like the Inch, in the system of measurement length), and hardly has course today.

With the course 20th, the joule per kilogram Kelvin continued to coexist with other measuring units of the specific heat. Thus, the thermochimists use the " rather; version calorie" of this unit. However, the joule per kilogram Kelvin is recognized today by the International office of the Weights and measures like the legitimate unit of the specific heat: it is a unit of the international Système of measurement.

Legal definition

The joule per kilogram Kelvin is thus today a unit IF, and more exactly a “unit IF coherent derivative whose name and symbol include/understand units IF derivative coherent having special names and particular symbols”, precisely definite and framed legally by the texts of the International office of the Weights and measures (BIPM).

Its symbol is the J (kg K): there is not an other specific symbol for this measuring unit. Expressed in Unit SO basic, the joule per kilogram Kelvin is written in m ² s^-2K^-1.

Beside the joule per kilogram Kelvin appears the joule per mole Kelvin, which measures the molar heat capacity.

Use

With the difference in other units IF derived, there does not exist instrument graduated in joule per kilogram Kelvin: the mass heat capacity is measured indirectly.

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