# Heat capacity

# Heat capacity

**Heat capacity** or **thermal capacity** is a physical property of matter, defined as the amount of heat to be supplied to a given mass of a material to produce a unit change in its temperature.^{[1]}

Heat capacity is an extensive property. The corresponding intensive property is the specific heat capacity. Dividing the heat capacity by the amount of substance in moles yields its molar heat capacity. The volumetric heat capacity measures the heat capacity per volume.

Heat capacity is often said as **thermal mass** in architecture and civil engineering to refer to the heat capacity of a building .

Definition

`The heat capacity of an object, denoted by, is the limit`

`whereis the amount of heat that must be added to the object (of mass`

*M*) in order to raise its temperature by.`The value of this parameter usually varies considerably depending on the starting temperatureof the object and the pressureapplied to it. Therefore, it should be considered a functionof those two variables. However, the variation can be ignored in contexts when working with objects in narrow ranges of temperature and pressure. For example, the heat capacity of a block ofironweighing onepoundis about 204 J/K when measured from a starting temperature`

*T*=25 °C and*P*=1 atm of pressure. That approximate value is quite adequate for all temperatures between, say, 15 °C and 35 °C, and surrounding pressures from 0 to 10 atmospheres, because the exact value varies very little in those ranges. One can trust that the same heat input of 204 J will raise the temperature of the block from 15 °C to 16 °C, or from 34 °C to 35 °C, with negligible error.Heat capacities for a homogeneous system undergoing different thermodynamic processes

At constant pressure (Isobaric process)

`At constant pressure, heat supplied to the system would contribute to both theworkdone and the change ininternal energy, according to thefirst law of thermodynamics. The heat capacity would be called.`

At constant volume (Isochoric process)

`A system undergoing a process at constant volume would imply that no work would be done, so the heat supplied would contribute only to the change in internal energy. The heat capacity obtained this way is denoted. The value ofis always less than the value of.`

Calculating and for an ideal gas

`(Mayer's relation)`

where

`= number of moles of the gas,`

`=universal gas constant,`

`=heat capacity ratio(can be calculated by knowing thedegrees of freedomof the gas molecule)`

Using the above two relations, the specific heats can be deduced as follows:

At constant temperature (Isothermal process)

No change in internal energy (as the temperature of the system is constant throughout the process) leads to only work done of the total supplied heat, and thus infinite amount of heat is required to increase the temperature of the system by a unit temperature, leading to infinite or undefined heat capacity of the system.

At the time of phase change (Phase transition)

Heat capacity of a system undergoing phase transition is infinite, because the heat is utilized in changing the state of the material rather than raising the overall temperature.

Heterogeneous objects

The heat capacity may be well-defined even for heterogeneous objects, with separate parts made of different materials; such as an electric motor, a crucible with some metal, or a whole building. In many cases, the (isobaric) heat capacity of such objects can be computed by simply adding together the (isobaric) heat capacities of the individual parts.

`However, this computation is valid only all parts of the object are at the same external pressure before and after the measurement. That may not be possible in some cases. For example, when heating an amount of gas in an elastic container, its volume`

*and pressure*will both increase, even if the atmospheric pressure outside the container is kept constant. Therefore, the effective heat capacity of the gas, in that situation, will have a value intermediate between its isobaric and isochoric capacitiesand.For complex thermodynamic systems with several interacting parts and state variables, or for measurement conditions that are neither constant pressure nor constant volume, or for situations where the temperature is significantly non-uniform, the simple definitions of heat capacity above are not useful or even meaningful. The heat energy that is supplied may end up as kinetic energy (energy of motion) and potential energy (energy stored in force fields), both at macroscopic and atomic scales. Then the change in temperature will depends on the particular path that the system followed through its phase space between the initial and final states. Namely, one must somehow specify how the positions, velocities, pressures, volumes, etc. changed between the initial and final states; and use the general tools of thermodynamics to predict the system's reaction to a small energy input. The "constant volume" and "constant pressure" heating modes are just two among infinitely many paths that a simple homogeneous system can follow.

Measurement

The heat capacity can usually be measured by the method implied by its definition: 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.

Units

International system

The SI unit for heat capacity of an object is joule per kelvin (J/K, or J K−1). Since an increment of temperature of one degree Celsius is the same as an increment of one kelvin, that is the same unit as J/°C.

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 Imperial pound (lb = 0.45459237 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),^{[2]}^{[3]} as the unit of heat. In those contexts, the unit of heat capacity is BTU/°F ≈ 1900 J. The BTU was in fact defined so that the average heat capacity of one pound of water would be 1 BTU/°F.

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 small calories, 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 ("small calorie") = 4.184 J/K
- 1 kcal/°C ("large calorie") = 4184 J/K

Negative heat capacity

Most physical systems exhibit a positive heat capacity. However, even though it can seem paradoxical at first,^{[4]}^{[5]} there are some systems for which the heat capacity is *negative*. These are inhomogeneous systems that do not meet the strict definition of thermodynamic equilibrium. They include gravitating objects such as stars and galaxies, and also sometimes some nano-scale clusters of a few tens of atoms, close to a phase transition.^{[6]} A negative heat capacity can result in a negative temperature.

Stars and black holes

According to the virial theorem, for a self-gravitating body like a star or an interstellar gas cloud, the average potential energy *U*pot and the average kinetic energy *U*kin are locked together in the relation

The total energy *U* (= *U*pot + *U*kin) therefore obeys

If the system loses energy, for example, by radiating energy into space, the average kinetic energy actually increases. If a temperature is defined by the average kinetic energy, then the system therefore can be said to have a negative heat capacity.^{[7]}

A more extreme version of this occurs with black holes. According to black-hole thermodynamics, the more mass and energy a black hole absorbs, the colder it becomes. In contrast, if it is a net emitter of energy, through Hawking radiation, it will become hotter and hotter until it boils away.

See also

Quantum statistical mechanics

Heat capacity ratio

Statistical mechanics

Thermodynamic equations

Thermodynamic databases for pure substances

Heat equation

Heat transfer coefficient

Heat of mixing

Latent heat

Material properties (thermodynamics)

Joback method (Estimation of heat capacities)

Specific heat of melting (Enthalpy of fusion)

Specific heat of vaporization (Enthalpy of vaporization)

Volumetric heat capacity

Thermal mass

R-value (insulation)

Storage heater

Frenkel line

Table of specific heat capacities

## References

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*Verein Deutscher Ingenieure*(VDI).

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*Physical Review Letters*.

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*British Journal for the Philosophy of Science*.

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*Scientific Unit Conversion: A Practical Guide to Metrication*