# Definitions - Coulomb ($\text{C}$) = unit of charge - charge of electron = $-1.6\times 10^{-19}\text{ C}$ > The force $F$ between two point charges $Q_1$, $Q_2$ in free space, separated by a distance $r$ is given by: > $ > F=k\frac{Q_1Q_2}{r^2} > $ > $ > k=\frac{1}{4\pi\epsilon_0}\approx9\times10^9\text{ N}\cdot\text{m}^2/\text{C}^2 > $ > - Implies that a charge of $1\text{ C}$ placed a distance of $1\text{ m}$ away from an equal charge will result in a force of $9\times10^9\text{ N}$ > - $\epsilon_0$ defined as *permittivity* of free space