Biot number

Summary

The Biot number $\mathrm{Bi} = h L / k$ is the dimensionless number that compares heat resistance inside a solid body with heat resistance at its surface. Here $h$ is the heat transfer coefficient at the surface and $k$ is the thermal conductivity of the solid. A small Biot number means that the temperature inside the body is nearly uniform. This justifies the lumped-element thermal model.