Heat flow

This article covers the importance in physics. The philosophical term heat flow can be found under heat flow / cold 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 .

See also

• Heat flux density
• Conduction
• Heat transfer coefficient - description of the heat transfer at interfaces
• Thermal performance

Individual evidence

1. ^ Paul Allen Tipler, Gene Mosca: Physics for Scientists and Engineers . Ed .: Jenny Wagner. 7th German edition Springer Spectrum, Berlin Heidelberg 2015, ISBN 978-3-642-54165-0 .