In some text books I've encountered the calculation of electrical work done on a point charge moving in an electric field (e.g. when demonstrating that electric force is conservative) is done by making use of Coulomb's law. However, Coulomb's law is not valid in case of moving charges, and therefore, cannot be used for this purpose. There's clearly something I am missing, but can't figure out what.

  • 1
    $\begingroup$ Neglecting Bremsstrahlung Coulomb's law is valid for moving charges as long as the electric field they are moving in is constant. For classical slowly moving charges this is an excellent approximation. $\endgroup$
    – CuriousOne
    Commented Jan 30, 2016 at 20:56
  • $\begingroup$ Also in these derivations one can think of "infinitely slow" movement, allowing arbitrarily close approximation. $\endgroup$
    – Danu
    Commented Jan 30, 2016 at 21:11
  • $\begingroup$ So it's just a low velocity approximation. I wish it were mention in those books... Thanks $\endgroup$ Commented Jan 30, 2016 at 21:23
  • $\begingroup$ No it is not an approximation: PE is defined as the work in infinitesimally slow motion. $\endgroup$
    – mike stone
    Commented Jan 8, 2021 at 22:07
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    $\begingroup$ Does this answer your question? How can moving electrons participate in electrostatic interaction? $\endgroup$ Commented Jan 8, 2021 at 22:24

2 Answers 2


Technically you are correct, but the magnetic forces generated by a slowly moving electron (slow compared to the speed of light) are quite small in comparison a robust coulomb force so its a good approximation to neglect the magnetic forces. A good example is the quantum treatment of the hydrogen atom. If you solve the Schrodinger equation for the Coulomb potential, you get very close to the Balmer spectrum. The magnetic effects are called fine and hyper-fine structure because of their small magnitude.


The relativistically correct way of treating a moving electric point charge is by using the Liénard–Wiechert potentials. These incorporate terms with retarded time which is connected with electromagnetic radiation.


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