Which statement best describes the Biot number?

• A. A unit of length
• B. A dimensionless parameter used in heat transfer
• C. A type of temperature scale
• D. A material property

Answer: (B)

The Biot number is a dimensionless ratio that compares the internal conductive resistance within a solid to the convective resistance at its surface. It is defined as Bi = hL/k, where h is the convective heat transfer coefficient at the surface, L is a characteristic length (often V/A for the body), and k is the thermal conductivity of the material. Because h has units of W/m^2-K, L is in meters, and k is in W/m-K, these units cancel and Bi is dimensionless.

This number tells you how easily heat can travel inside the object compared with how quickly heat can leave it at the surface. If Bi is much less than 1, most of the temperature drop occurs at the surface, and the interior stays nearly uniform—making lumped-capacitance analyses appropriate. If Bi is much greater than 1, significant temperature gradients exist inside the solid, and internal conduction dominates the cooling or heating behavior.

So the statement that describes the Biot number best is that it is a dimensionless parameter used in heat transfer. It’s not a unit of length, not a temperature scale, and not a material property.