[en] Linear temporal instabilities of a two-dimensional planar liquid sheet in a static electric field are investigated when the relaxation and convection of surface electric charges are considered. Both viscous sheet liquid and inviscid surrounding liquid are placed between two parallel sheath walls, on which an external electric field is imposed. In particular, effects of the electric Peclet number $\mathrm{P}\mathrm{e}$ (charge relaxation time/convection time) and the electric Euler number Λ (electric pressure/liquid inertial) on the instabilities are emphasized for the symmetric and antisymmetric deformations of the sheet. It is found that the unstable mode is composed of the aerodynamic and electric modes, which are merged with each other for the symmetric deformation and separated for the antisymmetric deformation. For the symmetric deformation, the combined mode is more destabilized with the decrease of $\mathrm{P}\mathrm{e}$ and the increase of Λ. On the other hand, for the antisymmetric deformation, the electric mode is more destabilized and the aerodynamic mode is left unchanged with the decrease of $\mathrm{P}\mathrm{e}$, while the electric mode is more destabilized but the aerodynamic mode is more stabilized with the increase of Λ. It is also found for both symmetric and antisymmetric deformations that the instabilities are most suppressed when ${\mathrm{\sigma <!--\; ??\; -->}}_R\simeq <!--\; ???\; -->1/{\mathrm{ϵ<!--\; ??\; -->}}_P$ (${\mathrm{\sigma <!--\; ??\; -->}}_R$: conductivity ratio of the surrounding to the sheet liquid, ${\mathrm{ϵ<!--\; ??\; -->}}_P$: permittivity ratio of the sheet to the surrounding liquid), whose trend of the instabilities is more enhanced with the decrease of $\mathrm{P}\mathrm{e}$ except for vanishingly small $\mathrm{P}\mathrm{e}$. (paper)