specific heat

specific heat, ratio of the heat capacity of a substance to the heat capacity of a reference substance, usually water. Heat capacity is the amount of heat needed to change the temperature of a unit mass 1°. The heat capacity of water is 1 calorie per gram per degree Celsius (1 cal/g-°C;) or 1 British thermal unit per pound per degree Fahrenheit (1 Btu/lb-°F;). Thus, the specific heat of some other substance relative to water will be numerically equal to its heat capacity; for this reason, "specific heat" is often used when the heat capacity actually is meant. Because the heat capacities of most substances vary with changes in temperature, the temperatures of both the specified substance and the reference substance must be known in order to give a precise value for the specific heat. The heat capacity of water at 15°C; is a frequently used value. Like specific gravity, specific heat is a dimensionless quantity, i.e., a pure number having no unit of measurement associated with it.

---

specific heat

Ratio of the quantity of heat required to raise the temperature of a body one degree to that required to raise the temperature of an equal mass of water one degree. The term is also used to mean the amount of heat, in calories, required to raise the temperature of one gram of a substance by one Celsius degree.

---

specific heat [spə′sif·ik ′hēt]

(thermodynamics)

The ratio of the amount of heat required to raise a mass of material 1 degree in temperature to the amount of heat required to raise an equal mass of a reference substance, usually water, 1 degree in temperature; both measurements are made at a reference temperature, usually at constant pressure or constant volume.

The quantity of heat required to raise a unit mass of homogeneous material one degree in temperature in a specified way; it is assumed that during the process no phase or chemical change occurs.

---

Specific heat

A measure of the heat required to raise the temperature of a substance. When the heat ΔQ is added to a body of mass m, raising its temperature by ΔT, the ratio C given in Eq. (1) is defined as the heat capacity of the body. The quantity c defined in Eq. (2) is

(1) 

(2) 

called the specific heat capacity or specific heat. A commonly used unit for heat capacity is joule · kelvin^-1 ( J · K^-1); for specific heat capacity, the unit joule · gram^-1 · K^-1 ( J · g^-1 · K^-1) is often used. Joule should be preferred over the unit calorie = 4.18 J. As a unit of specific heat capacity, Btu · lb^-1 · °F^-1 = 4.21 J · g^-1 · K^-1 is also still in use in English-language engineering literature. If the heat capacity is referred to the amount of substance in the body, the molar heat capacity c_m results, with the unit J · mol^-1 · K^-1.

If the volume of the body is kept constant as the energy ΔQ is added, the entire energy will go into raising its temperature. If, however, the body is kept at a constant pressure, it will change its volume, usually expanding as it is heated, thus converting some of the heat ΔQ into mechanical energy. Consequently, its temperature increase will be less than if the volume is kept constant. It is therefore necessary to distinguish between these two processes, which are identified with the subscripts V (constant volume) and p (constant pressure): C_V, c_V, and C_p, c_p. For gases at low pressures, which obey the ideal gas law, the molar heat capacities differ by R, the molar gas constant, as given in Eq. (3), where R = 8.31 J · mol^-1 · K^-1; that is, the expanding gas heats up less.

(3) 

For solids, the difference between c_pand c_Vis of the order of 1% of the specific heat capacities at room temperature. This small difference can often be ignored. See Heat capacity, Thermodynamic processes

---

specific heat

The ratio of the quantity of heat required to raise the temperature of a given mass of any substance 1 degree to the quantity required to raise the temperature of an equal mass of water 1 degree.