# The energy stored in the electromagnetic field of an electron

Due to Wikipedia the total energy per unit volume stored in an electromagnetic field is

$$u_{EM}=\frac{\varepsilon}{2}|\mathbb E|^2+\frac{1}{2\mu}|\mathbb B|^2$$

How does the energy stored in the electric field of the electron relates to its rest mass? How large part of the rest mass comes from this field?

And related, how does the energy stored in the magnetic field induced by a moving electron relates to its kinetic energy?

A Rigorous Derivation of Electromagnetic Self-force seems to give relevant information related to this question.

• Please avoid list-type questions. – Gert Sep 1 '16 at 21:48
• @Gert: what is a list-type question? – flippiefanus Sep 3 '16 at 8:40

How does the energy stored in the electric field of the electron relates to its rest mass?

It depends on whether we assume the electron has finite charge density everywhere or not.

In case the charge density of electron is finite everywhere (like it is in the Lorentz and Abraham models of the electron, where charge is distributed on the surface or throughout the volume of a sphere), Poynting's equation is valid everywhere and implies expression for EM energy density you wrote above. It can be shown that net result of mutual EM forces between parts of the sphere results in increase of effective rest mass and other effects, like radiation damping. The change in the rest mass can be then related to Poynting's energy of the electron's field. However, how large these effects are depends on many details, like the size of the sphere, distribution of charge in it and nature of non-EM forces that hold the electric charge together. It is possible that change in the mass is very small part of total mass, but it could also be a substantial part.

In case the electron's charge is concentrated at some point so density is infinite, local Poynting's equation is invalid at that point and thus cannot be relied upon to calculate total EM energy. For example, if the electrons are points, one needs to use theory of point particles to calculate their EM energy. In this kind of theory, a theorem analogous to Poynting's can be derived. It implies different formula for EM energy density where one charged point particle has EM field, but there is zero EM energy associated with it. Only if there are several particles, the net EM energy can be non-zero.

For example, in Frenkel-type theory of electrons, the electrons are points with individual EM fields. The particles interact via EM forces but one electron has no parts that could interact among themselves, so there is no change in its mass due to EM interactions. Also there is no EM energy associated with the EM field of one lone electron.

How large part of the rest mass comes from this field?

We do not know if electron is extended or point-like. Consequently, we do not know what part of its mass if any can be related to EM energy stored in the space around it. In the end of the 19th century and first years of 20th century there was an hypothesis that all mass of the electron is electromagnetic mass and Kaufmann's experiments on the behaviour of fast electrons in electric and magnetic field seemed to support it. This idea was largely abandoned when special relativity got accepted, because in special relativity electromagnetic and non-electromagnetic mass behave the same. The past experiments got reinterpreted in such a way that no evidence of EM mass could be found from them.

J. Frenkel, Zur Elektrodynamik punktfoermiger Elektronen, Zeits. f. Phys., 32, (1925), p. 518-534. http://dx.doi.org/10.1007/BF01331692

J. A. Wheeler, R. P. Feynman, Classical Electrodynamics in Terms of Direct Interparticle Interaction, Rev. Mod. Phys., 21, 3, (1949), p. 425-433. http://dx.doi.org/10.1103/RevModPhys.21.425

https://en.wikipedia.org/wiki/Kaufmann%E2%80%93Bucherer%E2%80%93Neumann_experiments

• Besides from the idea of electromagnetic mass, which was proposing that all mass was electromagnetic, how to explain the measureable electric field around a charged body (which contain energy) if the energy of the electrons charging this body, doesn't contribute to the energy of this field? – Lehs Sep 5 '16 at 2:19
• @Lehs, in above theories, electromagnetic energy is not a function of the total electromagnetic field. It is zero for one lone particle, because there is no work needed to form it - it has no parts. But bringing two charged particles close to each other does take some work and so the net electromagnetic energy of such a system is positive. It can be expressed as function of positions of particles or functional of their individual fields. This functional is zero for one lone particle, but can be positive or negative for system of two and more particles. – Ján Lalinský Sep 5 '16 at 10:25
• I like your point of view that the question is model dependent. – Lehs Sep 5 '16 at 10:59

The energy in an electron E=me c^2, where me is the mass of the electron. Simple calculation shows that the energy required to bring an electron from infinity against another electron repulsion is Integral(F.dr)=Int( (k e^2/r^2) dr)= k e^2/r, evaluated from infinity to r(resulting in -ve sign cancellation), and k is the electrostatic coupling constant. Equate the two energies and you find; m c^2= -k e^2 /r; giving r=2.82 e-15 m, which is the classical electron radius. This shows that the energy equivalent of the mass is the same that is in the field of the electron.(this also shows that two electrons can never collide).

Note also that a force is not the same as energy. The force from one electron can extend to infinity, but still have finite energy. Only when a force acts on a distance we get energy.. E is the integral of force x distance.Thus as long as there is no accompanying motion, there are no energy limits on how much force there is or for what extent it acts.

How large part of the rest mass comes from this field?

This is still a good question, because we know that energy stored in electromagnetic field is real. When we store energy in a capacitor that energy is 1/2 ED V, where V is the volume of the capacitor. We can than convert this energy into mass connecting capacitor to the electric bulb which will radiate this energy in the form of photons. Energy stored in the field of the electron is at least α*me/2, where α is fine structure constant (approximately equal 1/137). We have integrated energy density around an electron from infinity up to the so called reduced Compton length of the electron (386 fm) i.e. to the limit of localisation of electron. So the answer is that minimum contribution of classical electromagnetic energy to the electron mass is 1/274 of electron mass. Below the distance 386 fm (fermi) we have still divergences in calculations on the field theory level ..