Electrostatic Force and Electrostatic Field


A. Electrostatic Force

Attraction or repulsion force between two charges at certain distance.


Unlike charges attract


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Like charges repel


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Attraction or repulsion force of two charges in empty space is:

\begin{equation*} \begin{split} F & = \frac{1}{4 \pi \varepsilon_o} \: \frac{Q_1 \: Q_2}{r^2} = k \: \frac{Q_1 \: Q_2}{r^2}  \\\\ k & = \frac{1}{4 \pi \varepsilon_o} = 9 \times 10^9 \\\\ \varepsilon_o & = 8,85 \times 10^{-12} \text{ (permitivitas hampa udara)} \end{split} \end{equation*}


When charge in a certain medium, the force is:

\begin{equation*} \begin{split} F_{\text{bahan}} & = \frac{1}{4 \pi \varepsilon_r \: \varepsilon_o} \: \frac{Q_1 \: Q_2}{r^2} = \frac{k}{\varepsilon_r} \: \frac{Q_1 \: Q_2}{r^2} \\\\ F_{\text{bahan}} & = \frac{F_{\text{hampa udara}}}{\varepsilon_r} \\\\ \varepsilon_r & = \text{permitivitas relatif bahan } (\varepsilon_r > 1) \end{split} \end{equation*}


Ratio of force in medium and in vacuum space:

\begin{equation*} \begin{split} \frac{F_{\text{medium}}}{F} & = \frac{\dfrac{\coret [red] {k}}{\varepsilon_r} \: \coret [red] {\dfrac{Q_1 \: Q_2}{r^2}}}{\coret [red] {k} \: \coret [red] {\dfrac{Q_1 \: Q_2}{r^2}}} \\\\ \frac{F_{\text{medium}}}{F} & = \frac{1}{\varepsilon_r} \\ \end{split} \end{equation*}


Because εr > 1, F in medium is lower than in vacuum space.


Electric Field

a region in which an electric charge (a test charge) experiences a force because of influence of another charge.


Electric field pattern of positive charge


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Electric field pattern of negative charge


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Electric field strength:

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\(E = k \: \dfrac{Q}{r^2}\)


When a charge q is in an electric field E, the charge will experience a force:

$F = q \cdot E$



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