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**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 thermal energy per unit of area per unit of time.

\[\vec{\phi}_q\;\left[\dfrac{\textrm{W}}{\textrm{m}^2}\right]\]

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.