The flux density \(N_{\rm{B}}\) is related to the velocity by the equation \[N_{\rm{B}}=c_{\rm{B}}\ v_{\rm{B}}\] The vector \(v_{\rm{B}}\) is the macroscopic average velocity at which the species B moves (which is to be distinguished from the random molecular velocity); \(c_{\rm{B}}\) is the concentration of species B (\(\pu{mol m-3}\)). Note that the velocity is defined with respect to a frame of reference.