For many years, a Lorentz factor of L = 1/3 has been used to describe the local electric field in thin amorphous dielectrics. However, the exact meaning of thin has been unclear. The local electric field E
loc modeling presented in this work indicates that L = 1/3 is indeed valid for very thin solid dielectrics (t
diel ≤ 20 monolayers) but significant deviations from L = 1/3 start to occur for thicker dielectrics. For example, L ≈ 2/3 for dielectric thicknesses of t
diel = 50 monolayers and increases to L ≈ 1 for dielectric thicknesses t
diel > 200 monolayers. The increase in L with t
diel means that the local electric fields are significantly higher in thicker dielectrics and explains why the breakdown strength E
bd of solid polar dielectrics generally reduces with dielectric thickness t
diel. For example, E
bd for SiO
2 reduces from approximately E
bd ≈ 25 MV/cm at t
diel = 2 nm to E
bd ≈ 10 MV/cm at t
diel = 50 nm. However, while E
bd for SiO
2 reduces with t
diel, all SiO
2 thicknesses are found to breakdown at approximately the same local electric field (E
loc)
bd ≈ 40 MV/cm. This corresponds to a coordination bond strength of 2.7 eV for the silicon-ion to transition from four-fold to three-fold coordination in the
tetrahedral structure.