# coulomb’s law

• By choosing one of the point charges to be the source, and the other to be the test charge, it follows from Coulomb’s law that the magnitude of the electric field E created
by a single source point charge Q at a certain distance from it r in vacuum is given by A system of n discrete charges stationed at produces an electric field whose magnitude and direction is, by superposition Atomic forces Coulomb’s law holds
even within atoms, correctly describing the force between the positively charged atomic nucleus and each of the negatively charged electrons.

• For a linear charge distribution (a good approximation for charge in a wire) where gives the charge per unit length at position , and is an infinitesimal element of length,[22]
For a surface charge distribution (a good approximation for charge on a plate in a parallel plate capacitor) where gives the charge per unit area at position , and is an infinitesimal element of area, For a volume charge distribution (such
as charge within a bulk metal) where gives the charge per unit volume at position , and is an infinitesimal element of volume,[21] The force on a small test charge at position in vacuum is given by the integral over the distribution of charge
The “continuous charge” version of Coulomb’s law is never supposed to be applied to locations for which because that location would directly overlap with the location of a charged particle (e.g.

• Considering the charge to be invariant of observer, the electric and magnetic fields of a uniformly moving point charge can hence be derived by the Lorentz transformation
of the four force on the test charge in the charge’s frame of reference given by Coulomb’s law and attributing magnetic and electric fields by their definitions given by the form of Lorentz force.

• For slow movement, the magnetic force is minimal and Coulomb’s law can still be considered approximately correct, but when the charges are moving more quickly in relation
to each other, the full electrodynamics rules (incorporating the magnetic force) must be considered.

• In relativity Coulomb’s law can be used to gain insight into the form of the magnetic field generated by moving charges since by special relativity, in certain cases the magnetic
field can be shown to be a transformation of forces caused by the electric field.

• The vector form of Coulomb’s law is simply the scalar definition of the law with the direction given by the unit vector, , parallel with the line from charge to charge .

• Force on a small charge at position , due to a system of discrete charges in vacuum is[20] where and are the magnitude and position respectively of the ith charge, is a unit
vector in the direction of , a vector pointing from charges to .

• [30] The fields hence found for uniformly moving point charges are given by:[31] where is the charge of the point source, is the position vector from the point source to the
point in space, is the velocity vector of the charged particle, is the ratio of speed of the charged particle divided by the speed of light and is the angle between and .

• The force acting on a point charge due to a system of point charges is simply the vector addition of the individual forces acting alone on that point charge due to each one
of the charges.

• [4] Coulomb discovered that bodies with like electrical charges repel: It follows therefore from these three tests, that the repulsive force that the two balls – [that were]
electrified with the same kind of electricity – exert on each other, follows the inverse proportion of the square of the distance.

• [4] He used a torsion balance to study the repulsion and attraction forces of charged particles, and determined that the magnitude of the electric force between two point
charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

• In the equilibrium state: and Dividing (1) by (2): Let be the distance between the charged spheres; the repulsion force between them , assuming Coulomb’s law is correct, is
equal to so: If we now discharge one of the spheres, and we put it in contact with the charged sphere, each one of them acquires a charge .

• [20] The strength and direction of the Coulomb force on a charge depends on the electric field established by other charges that it finds itself in, such that .

• However, Coulomb’s law can be proven from Gauss’s law if it is assumed, in addition, that the electric field from a point charge is spherically symmetric (this assumption,
like Coulomb’s law itself, is exactly true if the charge is stationary, and approximately true if the charge is in motion).

• [19] Vector form Coulomb’s law in vector form states that the electrostatic force experienced by a charge, at position , in the vicinity of another charge, at position , in
a vacuum is equal to[20] where is the displacement vector between the charges, a unit vector pointing from to , and the electric constant.

• When movement takes place, Einstein’s theory of relativity must be taken into consideration, and a result, an extra factor is introduced, which alters the force produced on
the two objects.

• In practice, angles can be difficult to measure, so if the length of the ropes is sufficiently great, the angles will be small enough to make the following approximation:
Using this approximation, the relationship (6) becomes the much simpler expression: In this way, the verification is limited to measuring the distance between the charges and checking that the division approximates the theoretical value.

• These solutions, when expressed in retarded time also correspond to the general solution of Maxwell’s equations given by solutions of Liénard–Wiechert potential, due to the
validity of Coulomb’s law within its specific range of application.

• Outline of proof Coulomb’s law states that the electric field due to a stationary point charge is: where • er is the radial unit vector, • r is the radius, |r|, • ε0 is the
electric constant, • q is the charge of the particle, which is assumed to be located at the origin.

• Using the expression from Coulomb’s law, we get the total field at r by using an integral to sum the field at r due to the infinitesimal charge at each other point s in space,
to give where ρ is the charge density.

• In the case of a single point charge at rest, the two laws are equivalent, expressing the same physical law in different ways.

• Coulomb’s law was essential to the development of the theory of electromagnetism and maybe even its starting point,[1] as it allowed meaningful discussions of the amount of
electric charge in a particle.

• In the equilibrium state, the distance between the charges will be and the repulsion force between them will be: We know that and: Dividing (4) by (5), we get: Measuring the
angles and and the distance between the charges and is sufficient to verify that the equality is true taking into account the experimental error.

• For a continuous charge distribution, an integral over the region containing the charge is equivalent to an infinite summation, treating each infinitesimal element of space
as a point charge .

• Relation to Gauss’s law Deriving Gauss’s law from Coulomb’s law Gauss’s law can be derived from Coulomb’s law and the assumption that electric field obeys the superposition
principle, which says that the resulting field is the vector sum of fields generated by each particle (or the integral, if the charges are distributed in a region of space).

• Note that since Coulomb’s law only applies to stationary charges, there is no reason to expect Gauss’s law to hold for moving charges based on this derivation alone.

• [3] The law states that the magnitude, or absolute value, of the attractive or repulsive electrostatic force between two point charges is directly proportional to the product
of the magnitudes of their charges and inversely proportional to the squared distance between them.

• [21] If both charges have the same sign (like charges) then the product is positive and the direction of the force on is given by ; the charges repel each other.

• This form of solutions need not obey Newton’s third law as is the case in the framework of special relativity (yet without violating relativistic-energy momentum conservation).

• If the field is generated by a positive source point charge , the direction of the electric field points along lines directed radially outwards from it, i.e.

• [32] Note that the expression for electric field reduces to Coulomb’s law for non-relativistic speeds of the point charge and that the magnetic field in non-relativistic limit
(approximating ) can be applied to electric currents to get the Biot–Savart law.

• Outline of proof Taking S in the integral form of Gauss’ law to be a spherical surface of radius r, centered at the point charge Q, we have By the assumption of spherical
symmetry, the integrand is a constant which can be taken out of the integral.

• In the simplest case, the field is considered to be generated solely by a single source point charge.

• [18] In his notes, Cavendish wrote, “We may therefore conclude that the electric attraction and repulsion must be inversely as some power of the distance between that of the
and that of the , and there is no reason to think that it differs at all from the inverse duplicate ratio”.

• [11] Early investigators of the 18th century who suspected that the electrical force diminished with distance as the force of gravity did (i.e., as the inverse square of the
distance) included Daniel Bernoulli[12] and Alessandro Volta, both of whom measured the force between plates of a capacitor, and Franz Aepinus who supposed the inverse-square law in 1758.

• Consider two small spheres of mass and same-sign charge , hanging from two ropes of negligible mass of length .

• System of discrete charges The law of superposition allows Coulomb’s law to be extended to include any number of point charges.

• in the direction that a positive point test charge would move if placed in the field.

• [34] It can also be derived within the non-relativistic limit between two charged particles, as follows: Under Born approximation, in non-relativistic quantum mechanics, the
scattering amplitude is: This is to be compared to the: where we look at the (connected) S-matrix entry for two electrons scattering off each other, treating one with “fixed” momentum as the source of the potential, and the other scattering
off that potential.

• If the charges have opposite signs then the product is negative and the direction of the force on is ; the charges attract each other.

• [13] Based on experiments with electrically charged spheres, Joseph Priestley of England was among the first to propose that electrical force followed an inverse-square law,
similar to Newton’s law of universal gravitation.

• Being an inverse-square law, the law is similar to Isaac Newton’s inverse-square law of universal gravitation, but gravitational forces always make things attract, while electrostatic
forces make charges attract or repel.

• By knowing how much force it took to twist the fiber through a given angle, Coulomb was able to calculate the force between the balls and derive his inverse-square proportionality
law.

• [17] In the early 1770s, the dependence of the force between charged bodies upon both distance and charge had already been discovered, but not published, by Henry Cavendish
of England.

• Also note that the spherical symmetry for gauss law on stationary charges is not valid for moving charges owing to the breaking of symmetry by the specification of direction
of velocity in the problem.

• If the charges have the same sign, the electrostatic force between them makes them repel; if they have different signs, the force between them makes them attract.

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Photo credit: https://www.flickr.com/photos/thegrid-ch/4945315538/’]