# Heat flow

Physical size
Surname Heat flow
Formula symbol ${\ displaystyle {\ mathit {\ Phi}}}$ , ${\ displaystyle {\ dot {Q}}}$ Size and
unit system
unit dimension
SI W  = kg · m 2 · s -3 L 2 · M · T −3

The heat flow or heat flux ( Symbol to DIN 1304 : , or ) is a physical quantity for the quantitative description of heat transfer processes . It is defined as in the time .delta.t transmitted heat energy .DELTA.Q : ${\ displaystyle {\ dot {Q}}}$ ${\ displaystyle \ Phi _ {\ mathrm {th}}}$ ${\ displaystyle \ Phi}$ ${\ displaystyle I = {\ dot {Q}} = {\ frac {\ delta Q} {\ delta t}}}$ .

The notation with symbolizes that heat is not a state variable , but a process variable . ${\ displaystyle \ delta}$ The heat flow is a heat output and is given in watts : ${\ displaystyle {\ dot {Q}}}$ ${\ displaystyle \ mathrm {W} = {\ frac {\ mathrm {J}} {\ mathrm {s}}} = {\ frac {\ mathrm {kg} \, \ mathrm {m} ^ {2}} { \ mathrm {s} ^ {3}}}}$ .

It cannot be measured directly, but is always based on temperature difference measurement, for example in calorimeters . In addition, it is proportional to the material-dependent thermal conductivity .

The heat energy of a natural system always flows by itself from the area with the higher temperature to the area with the lower temperature, which is described by the increase in entropy . If heat energy is to be transported in the opposite direction, additional work has to be done, which further increases the entropy that is absorbed by another system (here: environment or universe, see also heat pump ). The observable realities of thermodynamic temperature , the flow of heat energy as if by itself and the splitting of the heat energy into an exergonic and anergonic part are described in the second law of thermodynamics .