ELECTRIC field in vacuum \[{{E}_{0}}=\frac{\SIGMA }{2{{\varepsilon }_{0}}}\] When the PLATES are dipped in KEROSENE oil tank, then the electric field between the plate will be \[E=\frac{\sigma }{2{{\varepsilon }_{0}}k}\] \[\because \] \[k>1\] \[\therefore \] \[E<{{E}_{0}}\] So, the electric field between the plates will decrease.