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# Heat flux

Heat flux or thermal flux is a change in energy over a given area and time. Heat is represented by the symbol Q.

The heat transfer rate, or heat flow per unit time, is denoted by:

$\dot{Q} = {dQ\over dt} \,\!$.

It is measured in watts in SI units (a watt is one joule per second).

Heat flux is defined as rate of heat transfer per unit cross-sectional area, and is denoted q, resulting in units of watts per square meter, though slightly different notation conventions can be used.