Field Generated by Surface Charge Density

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Theorem

Let $B$ be a body of matter.

Let $P$ be a point inside $B$ whose position vector is $\mathbf r$.


The electric field at $P$ generated by the surface charge density over $B$ is given by:

$\ds \map {\mathbf E} {\mathbf r} = \dfrac 1 {4 \pi \varepsilon_0} \int_{\text {all surfaces} } \dfrac {\paren {\mathbf r - \mathbf r'} \map \sigma {\mathbf r'} } {\size {\mathbf r - \mathbf r'}^3} \rd S'$

where:

$\d S'$ is an infinitesimal area element
$\mathbf r'$ is the position vector of $\d S'$
$\map \sigma {\mathbf r'}$ is the surface charge density at $\mathbf r'$
$\varepsilon_0$ denotes the vacuum permittivity.


Proof

From Electric Field Strength from Assemblage of Point Charges, the electric field strength caused by an assemblage of point charges $q_1, q_2, \ldots, q_n$ is given by:

$\ds \map {\mathbf E} {\mathbf r} = \dfrac 1 {4 \pi \epsilon_0} \sum_i \dfrac {\paren {\mathbf r - \mathbf r_i} q_i} {\size {\mathbf r - \mathbf r_i}^3}$

where $\mathbf r_1, \mathbf r_2, \ldots, \mathbf r_n$ are the position vectors of $q_1, q_2, \ldots, q_n$ respectively.

We apply the same principle to the surface charge density and convert the summation into a definite integral, as follows:


Consider an area element $\d S'$.

The electric field strength caused by $\d S'$ is:

$\map {\mathbf E} {\mathbf r'} = \dfrac 1 {4 \pi \epsilon_0} \dfrac {\paren {\mathbf r - \mathbf r'} \rd q} {\size {\mathbf r - \mathbf r'}^3}$

where $\d q$ is the electric charge on $\d S'$.

By definition of surface charge density:

$\map \sigma {\mathbf r'} = \dfrac {\d q} {\d S'}$

and so:

$\map {\mathbf E} {\mathbf r'} = \dfrac 1 {4 \pi \epsilon_0} \dfrac {\paren {\mathbf r - \mathbf r'} \map \sigma {\mathbf r'} } {\size {\mathbf r - \mathbf r'}^3} \rd S'$


Integrating over all surfaces of $B$:

$\ds \map {\mathbf E} {\mathbf r} = \dfrac 1 {4 \pi \varepsilon_0} \int_{\text {all surfaces} } \dfrac {\paren {\mathbf r - \mathbf r'} \map \rho {\mathbf r'} } {\size {\mathbf r - \mathbf r'}^3} \rd S'$

Hence the result.

$\blacksquare$


Examples

Arbitrary Rectangular Area

Consider a rectangular surface $S$ embedded in the $x$-$y$ plane in a Cartesian $3$-space.

Let the corners of $S$ be at $x = \pm a$ and $y = \pm b$.


The electric field at $P$ generated by the surface charge density over $S$ is given by:


$\ds \map {\mathbf E} {\mathbf r} = \dfrac 1 {4 \pi \varepsilon_0} \int_{x \mathop = -a}^a \int_{y \mathop = -b}^b \dfrac {\paren {\mathbf r - \mathbf r'} \map \sigma {\mathbf r'} } {\size {\mathbf r - \mathbf r'}^3} \rd y' \rd x'$

where:

$\d y' \rd x'$ is an infinitesimal area element of $S$
$\mathbf r'$ is the position vector of $\d y' \rd x'$
$\map \sigma {\mathbf r'}$ is the surface charge density at $\mathbf r'$
$\varepsilon_0$ denotes the vacuum permittivity.


Sources

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