Where  is the temperature coefficient of thermal conductivity with the dimensions , and  is the thermal conductivity at .
It must be noted that conduction of heat through gases is usually negligibly small compared with heat flow by convection and radiation.
The general equation for heat conduction is solids at steady state without any heat source is

(3)

Equation 3 applies to steady-state conduction in systems without heat source. It is also termed as Laplace equation.

Convection   
       
Heat transfer by convection results due to fluid motion. Fluid motion can be caused either by buoyancy force (due to density gradient) or by inertial force. The former is called “free” or “natural” convection and the later is “forced” convection heat transfer. In both modes of heat transfer velocity of the fluid governs the rate of heat flow. To quantify the heat transfer rate by convection, it is necessary to know the velocity of fluid. Differential approach can be used to determine the fluid velocity and the reader may refer to any text book on transport phenomena as given in the reference. However, in most engineering calculations involving heat flow between fluids and their confining surfaces, the following empirical relation has been found to be very useful:

(4)