# Coulomb law

**Coulomb law** is fundamental scientific concept about interaction of electric charges.

The special conserving quantity is postulated, called electric charge.

Potential energy of interaction of two charges $q_1$ and $d_2$ localised in regions small compared to the distance $r$ between them, is postulated to be

$\displaystyle U=k \frac{q_1 q_2}{r}$

where $k$ is fundamental constant determined by unit of measurement of the electric charge.

In the system SI, unit of charge is Coulomb, and

$\displaystyle k=\frac{1}{4 \pi \varepsilon_0} \approx 8.9875517873681764\times 10^9 \mathrm{Newton\, Meter^2\, Coulomb}^{-2} $

## Do physicians know physics?

Since century 20, the popular questions for students of medical institutes was:
**How many physicians know physics?**

The similar question refer to Matehmatica with respect to the example below:

Needs["PhysicalConstants`"] VacuumPermittivity N[VacuumPermittivity] ElectronCharge ElectronCharge^2/(4 Pi VacuumPermittivity )

The last line produces output $\mathrm{ (2.30708\times 10^{-28} Coulomb^2~ Meter~ Volt)/(Ampere~ Second) }$

instead of expected

$\mathrm{ 2.30708\times 10^{-28}~ Joule~ Meter }$