= Heat transfer
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<Heat transfer> asks how quickly <heat> crosses a boundary and how the <temperature> varies along its path. Its principal mechanisms are <thermal conduction>, <convection> and <thermal radiation>. A warm wall can conduct <heat> through its interior while exchanging <heat> with moving air and emitting <thermal radiation> at its surfaces. Identifying these paths lets us estimate heating requirements and cooling times.
Distinguish the accumulated <heat> $Q$ from its rate $\dot Q=dQ/dt$, a <power> measured in $\mathrm W$. The <heat flux> $q$ is the rate per unit <area>, measured in $\mathrm{W\,m^{-2}}$: a uniform normal <heat flux> across <area> $A$ gives $\dot Q=Aq$. The steady bar in <thermal conduction> carries $q=400\,\mathrm{W\,m^{-2}}$ through $A=0.01\,\mathrm{m^2}$, hence $\dot Q=4\,\mathrm W$; maintained for $60\,\mathrm s$, it transfers $Q=240\,\mathrm J$. This distinction makes a surface's intensity and a whole object's heating load easy to compare.
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