Coulomb's law

History

Ancient cultures around the Mediterranean knew that certain objects, such as rods of amber, could be rubbed with cat's fur to attract light objects like feathers. Thales of Miletus made a series of observations on static electricity around 600 BC, from which he believed that friction rendered amber magnetic, in contrast to minerals such as magnetite, which needed no rubbing. Thales was incorrect in believing the attraction was due to a magnetic effect, but later science would prove a link between magnetism and electricity. Electricity would remain little more than an intellectual curiosity for millennia until 1600, when the English scientist William Gilbert made a careful study of electricity and magnetism, distinguishing the lodestone effect from static electricity produced by rubbing amber. He coined the New Latin word electricus ("of amber" or "like amber", from ἤλεκτρον [elektron], the Greek word for "amber") to refer to the property of attracting small objects after being rubbed. This association gave rise to the English words "electric" and "electricity", which made their first appearance in print in Thomas Browne's Pseudodoxia Epidemica of 1646.

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 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.

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. However, he did not generalize or elaborate on this. In 1767, he conjectured that the force between charges varied as the inverse square of the distance.

In 1769, Scottish physicist John Robison announced that, according to his measurements, the force of repulsion between two spheres with charges of the same sign varied as x^−2.06.

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.

Finally, in 1785, the French physicist Charles-Augustin de Coulomb published his first three reports of electricity and magnetism where he stated his law. This publication was essential to the development of the theory of electromagnetism. 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.

The torsion balance consists of a bar suspended from its middle by a thin fiber. The fiber acts as a very weak torsion spring. In Coulomb's experiment, the torsion balance was an insulating rod with a metal-coated ball attached to one end, suspended by a silk thread. The ball was charged with a known charge of static electricity, and a second charged ball of the same polarity was brought near it. The two charged balls repelled one another, twisting the fiber through a certain angle, which could be read from a scale on the instrument. 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.

The law

Coulomb's law states that:

The magnitude of the electrostatic force of interaction between two point charges is directly proportional to the scalar multiplication of the magnitudes of charges and inversely proportional to the square of the distance between them.

The force is along the straight line joining them. If the two charges have the same sign, the electrostatic force between them is repulsive; if they have different signs, the force between them is attractive.

Coulomb's law can also be stated as a simple mathematical expression. The scalar and vector forms of the mathematical equation are

where k_e is Coulomb's constant (k_e = 7009898755178736817♠8.9875517873681764×10⁹ N m² C⁻²), q₁ and q₂ are the signed magnitudes of the charges, the scalar r is the distance between the charges, the vector r₂₁ = r₁ − r₂ is the vectorial distance between the charges, and r̂₂₁ = r₂₁/|r₂₁| (a unit vector pointing from q₂ to q₁). The vector form of the equation calculates the force F₁ applied on q₁ by q₂. If r₁₂ is used instead, then the effect on q₂ can be found. It can be also calculated using Newton's third law: F₂ = −F₁.

Units

When the electromagnetic theory is expressed using the standard SI units, force is measured in newtons, charge in coulombs, and distance in metres. Coulomb's constant is given by k_e =  ¹⁄_4πε0. The constant ε₀ is the permittivity of free space in C² m⁻² N⁻¹. And ε is the relative permittivity of the material in which the charges are immersed, and is dimensionless.

The SI derived units for the electric field are volts per meter, newtons per coulomb, or tesla meters per second.

Coulomb's law and Coulomb's constant can also be interpreted in various terms:

Cgs units are often preferred in the treatment of electromagnetism, as they greatly simplify formulas.

Electric field

An electric field is a vector field that associates to each point in space the Coulomb force experienced by a test charge. In the simplest case, the field is considered to be generated solely by a single source point charge. The strength and direction of the Coulomb force F on a test charge q_t depends on the electric field E that it finds itself in, such that F = q_tE. If the field is generated by a positive source point charge q, the direction of the electric field points along lines directed radially outwards from it, i.e. in the direction that a positive point test charge q_t would move if placed in the field. For a negative point source charge, the direction is radially inwards.

The magnitude of the electric field E can be derived from 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:

Coulomb's constant

Coulomb's constant is a proportionality factor that appears in Coulomb's law as well as in other electric-related formulas. Denoted k_e, it is also called the electric force constant or electrostatic constant, hence the subscript e.

The exact value of Coulomb's constant is:

Conditions for validity

There are three conditions to be fulfilled for the validity of Coulomb’s law:

1. The charges must have a spherically symmetric distribution (e.g. be point charges, or a charged metal sphere).
2. The charges must not overlap (e.g. be distinct point charges).
3. The charges must be stationary with respect to each other.

Scalar form

When it is only of interest to know the magnitude of the electrostatic force (and not its direction), it may be easiest to consider a scalar version of the law. The scalar form of Coulomb's Law relates the magnitude and sign of the electrostatic force F acting simultaneously on two point charges q₁ and q₂ as follows:

where r is the separation distance and k_e is Coulomb's constant. If the product q₁q₂ is positive, the force between the two charges is repulsive; if the product is negative, the force between them is attractive.

Vector form

Coulomb's law states that the electrostatic force F₁ experienced by a charge, q₁ at position r₁, in the vicinity of another charge, q₂ at position r₂, in a vacuum is equal to:

where r₂₁ = r₁ − r₂, the unit vector r̂₂₁ = r₂₁/|r₂₁|, and ε₀ is the electric constant.

The vector form of Coulomb's law is simply the scalar definition of the law with the direction given by the unit vector, r̂₂₁, parallel with the line from charge q₂ to charge q₁. If both charges have the same sign (like charges) then the product q₁q₂ is positive and the direction of the force on q₁ is given by r̂₂₁; the charges repel each other. If the charges have opposite signs then the product q₁q₂ is negative and the direction of the force on q₁ is given by −r̂₂₁ = r̂₁₂; the charges attract each other.

The electrostatic force F₂ experienced by q₂, according to Newton's third law, is F₂ = −F₁.

System of discrete charges

The law of superposition allows Coulomb's law to be extended to include any number of point charges. 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. The resulting force vector is parallel to the electric field vector at that point, with that point charge removed.

The force F on a small charge q at position r, due to a system of N discrete charges in vacuum is:

where q_i and r_i are the magnitude and position respectively of the ith charge, R̂_i is a unit vector in the direction of R_i = r − r_i (a vector pointing from charges q_i to q).

Continuous charge distribution

In this case, the principle of linear superposition is also used. 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 dq. The distribution of charge is usually linear, surface or volumetric.

For a linear charge distribution (a good approximation for charge in a wire) where λ(r′) gives the charge per unit length at position r′, and dl′ is an infinitesimal element of length,

For a surface charge distribution (a good approximation for charge on a plate in a parallel plate capacitor) where σ(r′) gives the charge per unit area at position r′, and dA′ is an infinitesimal element of area,

For a volume charge distribution (such as charge within a bulk metal) where ρ(r′) gives the charge per unit volume at position r′, and dV′ is an infinitesimal element of volume,

The force on a small test charge q′ at position r in vacuum is given by the integral over the distribution of charge:

Simple experiment to verify Coulomb's law

It is possible to verify Coulomb's law with a simple experiment. Consider two small spheres of mass m and same-sign charge q, hanging from two ropes of negligible mass of length l. The forces acting on each sphere are three: the weight mg, the rope tension T and the electric force F.

In the equilibrium state:

and:

Dividing (1) by (2):

Let L₁ be the distance between the charged spheres; the repulsion force between them F₁, 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 q/2. In the equilibrium state, the distance between the charges will be L₂ < L₁ and the repulsion force between them will be:

We know that F₂ = mg tan θ₂. And:

Dividing (4) by (5), we get:

Measuring the angles θ₁ and θ₂ and the distance between the charges L₁ and L₂ is sufficient to verify that the equality is true taking into account the experimental error. 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 check that the division approximates the theoretical value.

Electrostatic approximation

In either formulation, Coulomb’s law is fully accurate only when the objects are stationary, and remains approximately correct only for slow movement. These conditions are collectively known as the electrostatic approximation. When movement takes place, magnetic fields that alter the force on the two objects are produced. The magnetic interaction between moving charges may be thought of as a manifestation of the force from the electrostatic field but with Einstein’s theory of relativity taken into consideration.

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. This simple law also correctly accounts for the forces that bind atoms together to form molecules and for the forces that bind atoms and molecules together to form solids and liquids. Generally, as the distance between ions increases, the force of attraction, and binding energy, approach zero and ionic bonding is less favorable. As the magnitude of opposing charges increases, energy increases and ionic bonding is more favorable.