Heat is defined as the transfer of thermal energy across a well-defined boundary around a thermodynamic system; in the kinetic theory, heat is explained in terms of energy stored in the temperature-dependent microscopic motion and interaction of constituent particles, such as electrons, atoms, and molecules.

The immediate meaning of the kinetic energy of the constituent particles is not as heat. It is a component of internal energy. In microscopic terms, heat is a transfer quantity and is described by a transport theory, not as a steadily localized kinetic energy of particles. Heat transfer arises from temperature gradients or differences, through the diffuse exchange of microscopic kinetic and potential particle energy, by particle collisions and other interactions.

Heat is transferred spontaneously from a hotter to a colder system or body as a result of thermal interactions.

The state of energy stored within matter, or transported by the carriers, is described by a combination of classical and quantum statistical mechanics. The energy is also transformed (converted) among various carriers. The heat transfer processes (or kinetics) are governed by the rates at which various related physical phenomena occur, such as (for example) the rate of particle collisions in classical mechanics. These various states and kinetics determine heat transfer. Governing these processes from the atomic level (atom or molecule length scale) to macroscale are the laws of thermodynamics, including conservation of energy.

Heat transfer is a process function (or path function), as opposed to functions of state; therefore, the amount of heat transferred in a thermodynamic process that changes the state of a system depends on how that process occurs, not only the net difference between the initial and final states of the process.

Thermodynamic and mechanical heat transfer is calculated with the heat transfer coefficient, the proportionality between the heat flux and the thermodynamic driving force for the flow of heat. Heat flux is a quantitative, vectorial representation of heat-flow through a surface.

Heat transfer

Heat transfer is classified into various mechanisms, the fundamentals are:

  • Advection: is the transport mechanism of a fluid from one location to another, and is dependent on the motion and momentum of that fluid.
  • Thermal conduction or diffusion: the transfer of energy between objects that are in physical contact. Thermal conductivity is the property of a material to conduct heat and evaluated primarily in terms of Fourier’s Law for heat conduction.
  • Convection: the transfer of energy between an object and its environment, due to fluid motion. The average temperature is a reference for evaluating properties related to convective heat transfer.
  • Thermal radiation: the transfer of energy by the emission of electromagnetic radiation.

Heat flux

Heat flux (or thermal flux, sometimes also referred to as heat flux density or heat flow rate intensity) is a flow of energy per unit of area per unit of time.


the subscript q specifying heat flux, as opposed to mass or momentum flux. Fourier’s law is an important application of these concepts. It has both a direction and a magnitude, and so it is a vector quantity. To define the heat flux at a certain point in space, one takes the limiting case where the size of the surface becomes infinitesimally small.

Heat capacity

Heat capacity (C) also called thermal capacity, is the amount of heat needed to raise the temperature of an object through 1 °C, either at constant pressure or at constant volume and without inducing chemical changes or a change of phase. Numerically it is equal to the product of the mass (m) of the object and its specific heat (c).

\[C=m\cdot c=\dfrac{\Delta Q}{\Delta T}\]

Heat capacity is measured in joules per kelvin (J/K) (SI units).

Latent heat

Latent heat is the quantity of heat required to bring about a change of state of a unit mass of a substance from solid to liquid (latent heat of fusion) or from liquid to gas (latent heat of vaporization or of condensation) or from solid to gas directly (latent heat of vaporization) without change of temperature (i.e., isothermally).

At the freezing point and boiling point of a substance, adding heat produces no rise in temperature until the change of state is complete. The energy required to effect the change of state is absorbed in the form of latent heat, and an equal amount of heat is liberated in the reverse process.

In the case of ice changing to water at 0 °C, for example, the heat energy absorbed during melting is used exclusively to overcome the intermolecular forces in the order ice structure; only when this is accomplished, and further heat is added does the kinetic energy of the water molecules and, hence, the temperature of the water begin to rise above 0 °C.

Latent heat of fusion

The latent heat of fusion of a substance (or enthalpy of fusion), is the change in its enthalpy resulting from providing energy, typically heat, to a specific quantity of the substance to change its state from a solid to a liquid, at constant pressure.

Latent heat of condensation

The latent heat of condensation (or enthalpy of condensation) is by definition equal to the enthalpy of vaporization with the opposite sign: enthalpy changes of vaporization are always positive (the substance absorbs heat), whereas enthalpy changes of condensation are always negative (the substance releases heat).

Latent heat of sublimation

The latent heat of sublimation (or enthalpy of sublimation) is the heat required to change one mole of a substance from solid state to gaseous state at a given combination of temperature and pressure, usually standard temperature and pressure (STP). The heat of sublimation is usually expressed in kJ/mol, although the less customary kJ/kg is also encountered.

Latent heat of vaporization

The latent heat of vaporization (or enthalpy of vaporization) is the amount of energy needed to change a liquid into vapor once it has reached its boiling point. Together with heat capacity, it is an essential property in determining how effectively a solvent can regulate the internal temperature of an organism.

The fact that water has both a high heat of vaporization and a high heat capacity makes it ideal in this respect and is one of the reasons it is so essential to life as we know it.

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