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Coulomb's law

Charged particles exert an electric force on each other that falls off with the square of the distance. Like charges repel each other, while opposite charges attract. The forces on the two charges are always equal in size and opposite in direction, as with Newton's third law.

F = k\dfrac{q_1q_2}{r^2}

Coulomb's law,

k = 8.99\cdot 10^9

N·m²/C²

Symbols

$q_1,\ q_2$	charges	C
$r$	distance between the charges	m
$k$	Coulomb's constant	N·m²/C²

Example

Two charges $q_1 = q_2 = 2.0\ \mu$ C are $0.30$ m apart. $F = 8.99\cdot 10^9\cdot\dfrac{(2.0\cdot 10^{-6})^2}{0.30^2} \approx 0.40$ N.

Always convert µC to C and cm to m before substituting numbers into Coulomb's law.

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