permittivity of free space is a constant \(\epsilon_{0} \approx 8.85 \times 10^{-12} \frac{C^{2}}{N \cdot m^{2}}\).
redefinition of Coulomb’s Constant based on permittivity of free space
\begin{equation} k = \frac{1}{4\pi \epsilon_{0}} \end{equation}