6.1.2. The interaction force between two identical charges at a distance of $1\text{ m}$ is $1\text{ N}$. Determine these charges in SI and CGS systems.
Solution
1. In the SI system: From Coulomb's law: $$F = \frac{1}{4\pi\varepsilon_0} \cdot \frac{q^2}{r^2}$$ Expressing the charge $q$: $$q = \sqrt{4\pi\varepsilon_0 r^2 F}$$ Substitute the numerical values ($F = 1\text{ N}$,$r = 1\text{ m}$,$\frac{1}{4\pi\varepsilon_0} = 9 \cdot 10^9\text{ N}\cdot\text{m}^2/\text{C}^2$): $$q = \sqrt{\frac{1 \cdot 1^2}{9 \cdot 10^9}} = \frac{1}{3 \cdot 10^4 \cdot \sqrt{10}} \approx 1.05 \cdot 10^{-5}\text{ C}$$
2. In the CGS system (electrostatic): In the CGS system, the coefficient is $1$, and Coulomb's law is written as: $$F = \frac{q^2}{r^2}$$ Expressing the charge $q_{\text{cgs}}$: $$q_{\text{cgs}} = r\sqrt{F}$$ Keep in mind that in CGS, force is measured in dynes ($1\text{ N} = 10^5\text{ dyn}$), and distance is measured in centimeters ($1\text{ m} = 100\text{ cm}$). $$q_{\text{cgs}} = 100\text{ cm} \cdot \sqrt{10^5\text{ dyn}} = 100 \cdot 316.2 = 3.16 \cdot 10^4\text{ statC (esu)}$$