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DefinitionEdit

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

 

where   is 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 temperature   of the object and the pressure   applied to it. Therefore, it should be considered a function  of 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 of iron weighing one pound is 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 processesEdit

At constant pressure (Isobaric process)Edit

At constant pressure, heat supplied to the system would contribute to both the work done and the change in internal energy, according to the first law of thermodynamics. The heat capacity would be called   .

At constant volume (Isochoric process)Edit

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 of  is always less than the value of  .

Calculating  and  for an ideal gasEdit

  (Mayer's relation)

 

where

 = number of moles of the gas,

 =universal gas constant,

 = heat capacity ratio (can be calculated by knowing the degrees of freedom of the gas molecule)

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

 

 

At constant temperature (Isothermal process)Edit

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)Edit

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 objectsEdit

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 capacities   and  .

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.

MeasurementEdit

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.

UnitsEdit

International systemEdit

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.

The heat capacity of an object is an amount of energy divided by a temperature change, which has the dimension L2·M·T−2·Θ−1. Therefore, the SI unit J/K is equivalent to kilogram meter squared per second squared per kelvin (kg m2 s−2 K−1 ).

English (Imperial) engineering unitsEdit

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.

CaloriesEdit

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 capacityEdit

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 holesEdit

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

 

The total energy U (= Upot + Ukin) 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 alsoEdit

ReferencesEdit

  1. ^ Halliday, David; Resnick, Robert (2013). Fundamentals of Physics. Wiley. p. 524.
  2. ^ Koch, Werner (2013). VDI Steam Tables (4 ed.). Springer. p. 8. Published under the auspices of the Verein Deutscher Ingenieure (VDI).
  3. ^ Cardarelli, Francois (2012). Scientific Unit Conversion: A Practical Guide to Metrication. M.J. Shields (translation) (2 ed.). Springer. p. 19.
  4. ^ D. Lynden-Bell; R. M. Lynden-Bell (Nov 1977). "On the negative specific heat paradox". Monthly Notices of the Royal Astronomical Society. 181 (3): 405–419. Bibcode:1977MNRAS.181..405L. doi:10.1093/mnras/181.3.405.
  5. ^ Lynden-Bell, D. (Dec 1998). "Negative Specific Heat in Astronomy, Physics and Chemistry". Physica A. 263 (1–4): 293–304. arXiv:cond-mat/9812172v1. Bibcode:1999PhyA..263..293L. doi:10.1016/S0378-4371(98)00518-4.
  6. ^ Schmidt, Martin; Kusche, Robert; Hippler, Thomas; Donges, Jörn; Kronmüller, Werner; Issendorff, von, Bernd; Haberland, Hellmut (2001). "Negative Heat Capacity for a Cluster of 147 Sodium Atoms". Physical Review Letters. 86 (7): 1191–4. Bibcode:2001PhRvL..86.1191S. doi:10.1103/PhysRevLett.86.1191. PMID 11178041.
  7. ^ See e.g., Wallace, David (2010). "Gravity, entropy, and cosmology: in search of clarity" (preprint). British Journal for the Philosophy of Science. 61 (3): 513. arXiv:0907.0659. Bibcode:2010BJPS...61..513W. CiteSeerX 10.1.1.314.5655. doi:10.1093/bjps/axp048. Section 4 and onwards.

Further readingEdit