Which law describes convection heat transfer?

• A. Newton's Law of Cooling
• B. Fourier's Law
• C. Stefan-Boltzmann Law
• D. Kirchhoff's Law

Answer: (A)

Convection heat transfer is described by Newton's law of cooling, which says the rate of heat transfer between a surface and a moving fluid is proportional to the surface area and to the temperature difference between the surface and the fluid, with the proportionality given by the convective heat transfer coefficient h. In equation form, Q = h A (T_surface − T_fluid). The coefficient h encapsulates how effective the fluid flow is at removing or supplying heat, and it depends on whether the convection is natural or forced, the flow velocity, fluid properties, and the geometry involved. This distinguishes convection from conduction, which is described by Fourier’s law (q = -k ∇T) as heat flow within a solid or stationary fluid, and from radiation, which follows the Stefan-Boltzmann law (q ∝ T^4) for energy transfer by electromagnetic waves. Kirchhoff’s law relates to surface radiative properties rather than convective transfer. Newton’s law is the practical description used for convection because it ties the heat transfer rate directly to a temperature difference with a coefficient that reflects the fluid flow conditions.