Conduction happens inside solids. Convection happens where a solid meets a moving fluid, and almost all of the resistance lives in a thin, sluggish layer of fluid clinging to the surface — the boundary-layer film. Newton's law of cooling prices that film in one number: $\dot{Q} = h A \, \Delta T$. Read aloud: Q-dot equals h A delta-T.

Letter by letter, because a formula with undefined letters is a formula you do not own. $\dot{Q}$ (say Q-dot — the dot means per second) is the heat rate in watts. $h$ is the convection coefficient, or film coefficient, in $\mathrm{W/(m^2 \cdot K)}$: the watts one square metre of surface moves for every kelvin of difference. $A$ is the wetted area in $\mathrm{m^2}$ — the area the fluid actually touches. $\Delta T$ (say delta-T) is the surface-to-fluid difference in kelvin: the surface minus the bulk fluid, and never a temperature on its own. Whichever of the four the question leaves blank is the one you solve for.

Learn the magnitudes and you will catch your own mistakes: still air gives $h$ around $5$–$25$, forced air $10$–$200$, flowing water $500$–$10{,}000$, and condensing steam higher still. A film coefficient of 3 belongs to nothing in a working plant.

The same film has a second face. Write $R = \dfrac{1}{hA}$ — R equals one over h A — and you have the film's thermal resistance $R$ in kelvin per watt, exactly the way a resistor is measured in volts per amp. One face for computing heat, one face for stacking layers in series. They are the same fact, and this chapter uses both.