Coulomb's Law of Electrostatics

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Physical Law

Force Between two Like Charges

Let $a$ and $b$ be stationary particles in a vacuum, each carrying an electric charge of $q_a$ and $q_b$ respectively.

Then $a$ and $b$ exert a force upon each other whose magnitude and direction are given by Coulomb's law (of electrostatics):

$\mathbf F_{a b} \propto \dfrac {q_a q_b {\mathbf r_{a b} } } {r^3}$


$\mathbf F_{a b}$ is the force exerted on $b$ by the electric charge on $a$
$\mathbf r_{a b}$ is the displacement vector from $a$ to $b$
$r$ is the distance between $a$ and $b$.
the constant of proportion is defined as being positive.

By exchanging $a$ and $b$ in the above, it is seen that $b$ exerts the same force on $a$ as $a$ does on $b$, but in the opposite direction.

SI Units

In SI units, the law becomes:

$\mathbf F_{a b} = \dfrac 1 {4 \pi \varepsilon_0} \dfrac {q_a q_b {\mathbf r_{a b} } } {r^3}$


$q_a$ and $q_b$ are measured in coulombs
$r$ is measured in metres
$F_{a b}$ is measured in newtons
$\varepsilon_0$ denotes the vacuum permittivity:
$\varepsilon_0 \approx 8 \cdotp 85418 \, 78128 (13) \times 10^{-12} \, \mathrm C^2 \, \mathrm N^{-1} \, \mathrm m^{-2}$

Thus the equation becomes:

$\mathbf F_{a b} = \dfrac 1 {4 \pi \varepsilon_0} \dfrac {q_a q_b {\mathbf r_{a b} } } {r^3}$

Also presented as

$\mathbf F_{a b} \propto \dfrac {q_a q_b \hat {\mathbf r}_{a b} } {r^2}$

where $\hat {\mathbf r}_{a b}$ is the unit vector in the direction from $a$ to $b$.

Also known as

Coulomb's Law of Electrostatics is also known as just Coulomb's Law.

Source of Name

This entry was named for Charles-Augustin de Coulomb.

Historical Note

Charles-Augustin de Coulomb proposed what is now referred to as Coulomb's Law in the year $1785$.