a the nusselt number and the biot number have the same formula nu or b

Question

a) The Nusselt number and the Biot number have the same formula: Nu or Bi=hL/K, where h is the convection coefficient, L is the characteristic dimension of system, and k is the thermal conductivity. Explain where the difference lies and the physical meanings of these two famous non-dimensional numbers. b) In the analysis of transient heat-transfer systems, it is sufficient to use are presentative characteristic length of the system to compute the Biot number when one has to decide whether it is valid to use the lump-system approach or not. For example, for a sphere one can use the diameter or the radius. Explain why, when using a Heisler chart, a unique specified characteristic length must instead be used.