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Coulomb's law is an inverse-square law indicating the magnitude and direction of electrical force that one stationary, electrically charged substance of small volume (ideally, a point source) exerts on another.
Coulomb's Law may be stated as follows:
"The magnitude of the electrostatic force between two charges is directly proportional to the magnitude of the charges and inversely proportional to the square of the distances between the charges."
When one is interested only in the magnitude of the force (and not in its direction), it may be easiest to consider a simplified, scalar version of the law
where (in SI units):
is the magnitude of the force exerted, measured in newtons
is the charge on one body, measured in coulombs
is the charge on the other body, also measured in coulombs
is the distance between them measured in metres
is the electrostatic constant or Coulomb force constant, ≈ 8,987,742,438 F-1·m where F is the unit of capacitance, the farad or C-2·N·m2 where C is the unit of charge, the coulomb. In cgs units, the unit charge, esu of charge or statcoulomb, is defined so that this Coulomb force constant is 1.For calculating the direction and magnitude of the force simultaneously, one will wish to consult the full-blown vector version of the Law
Note that
is also written as
where
is a fundamental physical constant, the permittivity of free space. ≈ 8.854 × 10-12 F·m-1 or C2·N-1·m2In the full-blown vector version of the Law
is the electrostatic force vector,
is the vector between the two charges, such that
where
is vector indicating the position of the charge on which the force acts
is the vector indicating the position of the other charge.This vector equation indicates that opposite charges attract, and like charges repel. When q1q2 is negative, the force is attractive. When positive, the force is repulsive.
has been raised to the third power instead of the second in the denominator in order to normalize the length of the vector in the numerator to 1.In either formulation, Coulomb's law is fully accurate only when the substances are static (stationary), and remains approximately correct only for slow movement. When movement takes place, magnetic fields are produced that alter the force on the two substances. Especially when rapid movement takes place, the electric field will also undergo a transformation described by Einstein's theory of relativity.
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "Coulomb's law"
