Definition: | vector quantity with magnitude equal to the heat flow rate dΦ through a surface element divided by the area dA of the element, and direction eφ in the direction of propagation of heat, thus NOTE 1 At a fixed point in a medium, the direction of propagation of heat is opposite to the temperature gradient. At a point on the surface separating two media with different temperatures, the direction of propagation of heat is normal to the surface, from higher to lower temperatures. NOTE 2 The coherent SI unit of density of heat flux rate is watt per square metre, W/m2.
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