as the force from the pressure displaces the enclosure. That work must come from the heat energy provided. The value thus obtained is said to be the molar heat capacity at constant pressure (or isobaric), and is often denoted cP,m, cp,m, cP,m, etc. On the other hand, if the expansion is prevented — for example by a sufficiently rigid enclosure, or by increasing the external pressure to counteract the internal one — no work is generated, and the heat energy that would have gone into it must instead contribute to the internal energy of the object, including raising its temperature by an extra amount. The value obtained this way is said to be the molar heat capacity at constant volume (or isochoric) and denoted cV,m, cv,m, cv,m, etc. The value of cV,m is always less than the value of cP,m. This difference is particularly notable in gases where values under constant pressure are typically 30% to 66.7% greater than those at constant volume. All methods for the measurement of specific heat apply to molar heat capacity as well. Units The SI unit of molar heat capacity heat is joule per kelvin per mole (J/(K⋅mol), J/(K mol), J K−1 mol−1, etc.). Since an increment of temperature of one degree Celsius is the same as an increment of one kelvin, that is the same as joule per degree Celsius per mole (J/(°C⋅mol)). In chemistry, heat amounts are still often measured in calories. Confusingly, two units with that name, denoted "cal" or "Cal", have been commonly used to measure amounts of heat: the "small calorie" (or "gram-calorie", "cal") is 4.184 J, exactly. The "grand calorie" (also "kilocalorie", "kilogram-calorie", or "food calorie"; "kcal" or "Cal") is 1000 small calories, that is, 4184 J, exactly. When heat is measured in these units, the unit of specific heat is usually 1 cal/(°C⋅mol) ("small calorie") = 4.184 J⋅K−1⋅mol−1 1 kcal/(°C⋅mol) ("large calorie") = 4184 J⋅K−1⋅mol−1. The molar heat capacity of a substance has the same dimension as the heat capacity of an object; namely, L2⋅M⋅T−2⋅Θ−1, or M(L/T)2/Θ. (Indeed, it is the heat capacity of the object that consists of an Avogadro number of molecules of the substance.) Therefore, the SI unit J⋅K−1⋅mol−1 is equivalent to kilogram metre squared per second squared per kelvin (kg⋅m2⋅K−1⋅s−2). Physical basis Monatomic gases The temperature of a sample of a substance reflects the average kinetic energy of its constituent particles (atoms or