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2.1: Coulomb's Law and the Electrostatic Potential

  • Page ID
    119817
  • Skills to Develop

    • Use Coulomb's Law to explain electrostatic potential and bonding

    Through the work of scientists in the late 18th century, the main features of the electrostatic force—the existence of two types of charge, the observation that like charges repel, unlike charges attract, and the decrease of force with distance—were eventually refined, and expressed as a mathematical formula. The mathematical formula for the electrostatic force is called Coulomb’s law after the French physicist Charles Coulomb (1736–1806), who performed experiments and first proposed a formula to calculate it.

    Figure_19_03_01a.jpg

    Figure \(\PageIndex{1}\): This NASA image of Arp 87 shows the result of a strong gravitational attraction between two galaxies. In contrast, at the subatomic level, the electrostatic attraction between two objects, such as an electron and a proton, is far greater than their mutual attraction due to gravity. (credit: NASA/HST)

    Following the work of Ernest Rutherford and his colleagues in the early twentieth century, the picture of atoms consisting of tiny dense nuclei surrounded by lighter and even tinier electrons continually moving about the nucleus was well established. This picture was called the planetary model, since it pictured the atom as a miniature “solar system” with the electrons orbiting the nucleus like planets orbiting the sun. The simplest atom is hydrogen, consisting of a single proton as the nucleus about which a single electron moves. The electrostatic force attracting the electron to the proton depends only on the distance between the two particles, based on Coulomb's Law:

    \[ F_{gravity} = G \dfrac{ m_1 m_2}{r^2} \]

    with

    • \(G\) is a gravitational constant
    • \(m_1\) and \(m_2\) are the masses of particle 1 and 2, respectively
    • \(r\) is the distance between the two particles

    The electrostatic force has the same form as the gravitational force between two mass particles except that the electrostatic force depends on the magnitudes of the charges on the particles (+1 for the proton and −1 for the electron) instead of the magnitudes of the particle masses that govern the gravitational force.

    \[ F_{electrostatic} = k \dfrac{ m_1 m_2}{r^2}\]

    with

    • \(k\) is a constant
    • \(m_1\) and \(m_2\) are the masses of particle 1 and 2, respectively
    • \(r\) is the distance between the two particles

    The electrostatic force is a vector quantity and is expressed in units of newtons. The force is understood to be along the line joining the two charges. (Figure \(\PageIndex{2}\))

    Although the formula for Coulomb’s law is simple, it was no mean task to prove it. The experiments Coulomb did, with the primitive equipment then available, were difficult. Modern experiments have verified Coulomb’s law to great precision. For example, it has been shown that the force is inversely proportional to distance between two objects squared \((F\propto 1/r^{2})\) to an accuracy of 1 part in \(10^{16}\). No exceptions have ever been found, even at the small distances within the atom.

    ​​​​​​154328493389683765.jpg

    Figure \(\PageIndex{2}\): The magnitude of the electrostatic force\(F\) between point charges \(q_{1}\) and \(q_{2}\) separated by a distance \(r\) is given by Coulomb’s law. Note that Newton’s third law (every force exerted creates an equal and opposite force) applies as usual—the force on \(q_{1}\) is equal in magnitude and opposite in direction to the force it exerts on \(q_{2}\). (a) Like charges. (b) Unlike charges.

    Since forces can be derived from potentials, it is convenient to work with potentials instead, since they are forms of energy. The electrostatic potential is also called the Coulomb potential. Because the electrostatic potential has the same form as the gravitational potential, according to classical mechanics, the equations of motion should be similar, with the electron moving around the nucleus in circular or elliptical orbits (hence the label “planetary” model of the atom). Potentials of the form V(r) that depend only on the radial distance \(r\) are known as central potentials. Central potentials have spherical symmetry, and so rather than specifying the position of the electron in the usual Cartesian coordinates (x, y, z), it is more convenient to use polar spherical coordinates centered at the nucleus, consisting of a linear coordinate r and two angular coordinates, usually specified by the Greek letters theta (θ) and phi (Φ). These coordinates are similar to the ones used in GPS devices and most smart phones that track positions on our (nearly) spherical earth, with the two angular coordinates specified by the latitude and longitude, and the linear coordinate specified by sea-level elevation. Because of the spherical symmetry of central potentials, the energy and angular momentum of the classical hydrogen atom are constants, and the orbits are constrained to lie in a plane like the planets orbiting the sun. This classical mechanics description of the atom is incomplete, however, since an electron moving in an elliptical orbit would be accelerating (by changing direction) and, according to classical electromagnetism, it should continuously emit electromagnetic radiation. This loss in orbital energy should result in the electron’s orbit getting continually smaller until it spirals into the nucleus, implying that atoms are inherently unstable.

    visualize coulomb's law with this simulation!

    Summary

    Video \(\PageIndex{1}\): A review of Coulomb's Law.

    Frenchman Charles Coulomb was the first to publish the mathematical equation that describes the electrostatic force between two objects. Coulomb’s law gives the magnitude of the force between point charges. It is \(F=k\dfrac{|q_{1}q_{2}|}{r^{2}},\) where \(q_{1}\) and \(q_{2}\) are two point charges separated by a distance \(r\), and \(k\approx 8.99\times 10^{9}N\cdot m^{2}/C^{2}\). This Coulomb force is extremely basic, since most charges are due to point-like particles. It is responsible for all electrostatic effects and underlies most macroscopic forces. The Coulomb force is extraordinarily strong compared with the gravitational force, another basic force—but unlike gravitational force it can cancel, since it can be either attractive or repulsive. The electrostatic force between two subatomic particles is far greater than the gravitational force between the same two particles.

    Glossary

    Coulomb’s law
    the mathematical equation calculating the electrostatic force vector between two charged particles
    Coulomb force
    another term for the electrostatic force
    electrostatic force
    the amount and direction of attraction or repulsion between two charged bodies

    Contributors

    • Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at http://cnx.org/contents/85abf193-2bd...a7ac8df6@9.110).

    • Paul Peter Urone (Professor Emeritus at California State University, Sacramento) and Roger Hinrichs (State University of New York, College at Oswego) with Contributing Authors: Kim Dirks (University of Auckland) and Manjula Sharma (University of Sydney). This work is licensed by OpenStax University Physics under a Creative Commons Attribution License (by 4.0).
    • Adelaide Clark, Oregon Institute of Technology
    • Crash Course Physics: Crash Course is a division of Complexly and videos are free to stream for educational purposes.

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