When two physical system aer described by similar equations and have similar boundary conditions, there are said to be analogous, Heat transfer processor may be compared by analogy with the flow of electricity in an electrical resistance. As the flow of electric current in the electrical resistance is directly proportional to potential difference (dv); similarly heat flow rate Q, is directly proportional to potential to temperature difference (dt), the driving force for heat conduction through a medium.
As per ohms law (in electric – circuit theory), we have
Current (I) = (Potential difference (dv) / electrical resistance (r))
By analogy, the heat flow equation (Fourier’s equation) may be written as
Heat flow rate (Q) = (temperature difference (dt) / (dx/kA))
By comparing above equation, we find that I is analogous to Q, dv is analogous to dt and R is analogous to the quantity dx/kA. The quantity dx/kA is called thermal conduction resistance (RTH)cond. i.e.,
(RTH)cond = dx/kA
The reciprocal of the thermal resistance is called thermal conductance. It may be noted that rules for combining electrical resistance in series and parallel apply equally well to thermal resistances.