The heat **flux is** amount of thermal energy which is transmitted through an isothermal surface per unit of time. The main characteristic of this concept is density.

## Instruction

1. Warmth is total kinetic energy of molecules of a body which transition from some molecules to others or from one body to another can be carried out by means of three types of transfer: heat conductivity, convection and thermal radiation.

2. At heat conductivity thermal energy passes from more heated parts of a body to colder. The intensity of its transfer depends on gradient temperatures, namely on the relation of a difference of temperatures and also cross-sectional area and coefficient of heat conductivity. In that case the formula for definition of a heat flux of q looks so: q = - kS (∆T / ∆ x), where: k - coefficient of heat conductivity of material; S – cross-sectional area.

3. This formula is called the law of heat conductivity of Fourier, and the minus sign specifies the direction of a vector of a heat flux which is opposite to temperature gradient in a formula. According to this law, decrease in a heat flux it is possible to achieve, having reduced one of its components. For example, it is possible to use material with other coefficient of heat conductivity, smaller cross section or the difference of temperatures.

4. The convective heat flux is carried out in gaseous and liquid substances. In this case speak about transfer of thermal energy from the heater by Wednesday which depends on set of factors: the size and a form of the heating element, the speed of motion of the molecules, density and viscosity of the environment and so forth. In this case Newton's formula is applicable: q = hS (Tae - Tsr), where: h – the coefficient of convective transfer reflecting properties of the heated environment; S – surface area of a heating element; Tae – temperature of a heating element; Tsr – ambient temperature.

5. Thermal radiation – a method of transfer of heat which are a kind of electromagnetic radiation. The size of a heat flux at such heat transfer submits to Stefan-Boltsman's law: q = σS (Ti^4 – Tsr^4), where: σ – constant Stephana-Boltsman; S – emitter surface area; Ti – emitter temperature; Tsr – ambient temperature, absorbing radiation.