Heat capacity facts for kids

Heat capacity is a measure of how much materials can store up heat as they change temperature.

Materials with high heat capacities require a lot of heat to be stored up before a small change. Materials with low heat capacities require very little heat to be stored up before a large change.

The heat capacity of one gram (or some other unit of mass) of a material is called the specific heat capacity of the material, so that the heat capacity of something is its mass times its specific heat capacity.

In architecture and civil engineering, the heat capacity of a building is often referred to as its thermal mass.

Measurement

The SI unit of heat capacity is joule per kelvin (J/K). That is the same unit as J/°C.

The SI unit J/K is equivalent to kilogram meter squared per second squared per kelvin (kg⋅m²⋅s⁻²⋅K⁻¹ ).

The heat capacity can usually be measured by the following method: start with the object at a known uniform temperature, add a known amount of heat energy to it, wait for its temperature to become uniform, and measure the change in its temperature. This method can give moderately accurate values for many solids; however, it cannot provide very precise measurements, especially for gases.

English (Imperial) engineering units

Professionals in construction, civil engineering, chemical engineering, and other technical disciplines, especially in the United States, may use the so-called English Engineering units, that include the pound (lb = 0.45359237 kg) as the unit of mass, the degree Fahrenheit or Rankine (5/9°K, about 0.55556 °K) as the unit of temperature increment, and the British thermal unit (BTU ≈ 1055.06 J), as the unit of heat.

In those contexts, the unit of heat capacity is 1 BTU/°R ≈ 1900 J/°K. The BTU was in fact defined so that the average heat capacity of one pound of water would be 1 BTU/°F. In this regard, with respect to mass, note conversion of 1 Btu/lb⋅°R ≈ 4,187 J/kg⋅°K and the calorie (below).

Calories

In chemistry, heat amounts are 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. It was originally defined so that the heat capacity of 1 gram of liquid water would be 1 cal/°C.
• The "grand calorie" (also "kilocalorie", "kilogram-calorie", or "food calorie"; "kcal" or "Cal") is 1000 cal, that is, 4184 J, exactly. It was originally defined so that the heat capacity of 1 kg of water would be 1 kcal/°C.

With these units of heat energy, the units of heat capacity are

1 cal/°C = 4.184 J/K

1 kcal/°C = 4184 J/K

Negative heat capacity

Most physical systems have a positive heat capacity. However, there are some systems for which the heat capacity is negative. Some inhomogeneous systems do not meet the strict definition of thermodynamic equilibrium. They include gravitating objects such as stars and galaxies, and also some nano-scale clusters of a few tens of atoms close to a phase transition. A negative heat capacity can result in a negative temperature.

For example, according to black-hole thermodynamics, the more mass and energy a black hole absorbs, the colder it becomes. In contrast, if it emits energy, through Hawking radiation, it will become hotter and hotter until it boils away.

Thermal equilibrium

According to the Second Law of Thermodynamics, when two systems with different temperatures interact via a purely thermal connection, heat will flow from the hotter system to the cooler one. If such systems have equal temperatures, they are at thermal equilibrium.

This equilibrium is stable only if the systems have positive heat capacities. For such systems, when heat flows from a higher temperature system to a lower temperature one, the temperature of the first decreases and that of the latter increases, so that both approach equilibrium.

For systems with negative heat capacities the process is different. The temperature of the hotter system will further increase as it loses heat, and that of the colder will further decrease, so that they will move farther from equilibrium. This means that the equilibrium is unstable.

See also

In Spanish: Capacidad calorífica para niños