In this paper we shall examine the nematodynamics which involves both the microscopic order parameters and the macroscopic director based on Landau-de Gennes theory of nematics. Introducing a Lagrange multiplier corresponding to the conservation of trace of the tensor order parameter, the threshold condition of planar alignment cell will be analytically explored to show the agreement with the conventional vector approach on the bases of Oseen-Zocher-Frank theory. In addition some practical numerical computations will be presented so as to describe the relationship between the microscopic orders and the macroscopic director alignments under a certain electric field. In practice, the microscopic orders are found to be affected by the external electric field as well as the anchoring strength at the boundaries.