Jefimenko's equations

In electromagnetism, Jefimenko's equations (named after Oleg D. Jefimenko) give the electric field and magnetic field due to a distribution of electric charges and electric current in space, that takes into account the propagation delay (retarded time) of the fields due to the finite speed of light and relativistic effects. Therefore they can be used for moving charges and currents. They are the general solutions to Maxwell's equations for any arbitrary distribution of charges and currents.[1]

Equations

Electric and magnetic fields

Position vectors r and r′ used in the calculation

Jefimenko's equations give the E field and B field produced by an arbitrary charge or current distribution, of charge density ρ and current density J:[2]

${\displaystyle \mathbf {E} (\mathbf {r} ,t)={\frac {1}{4\pi \epsilon _{0}}}\int \left[{\frac {\mathbf {r} -\mathbf {r} '}{|\mathbf {r} -\mathbf {r} '|^{3}}}\rho (\mathbf {r} ',t_{r})+{\frac {\mathbf {r} -\mathbf {r} '}{|\mathbf {r} -\mathbf {r} '|^{2}}}{\frac {1}{c}}{\frac {\partial \rho (\mathbf {r} ',t_{r})}{\partial t}}-{\frac {1}{|\mathbf {r} -\mathbf {r} '|}}{\frac {1}{c^{2}}}{\frac {\partial \mathbf {J} (\mathbf {r} ',t_{r})}{\partial t}}\right]\mathrm {d} ^{3}\mathbf {r} ',}$

${\displaystyle \mathbf {B} (\mathbf {r} ,t)=-{\frac {\mu _{0}}{4\pi }}\int \left[{\frac {\mathbf {r} -\mathbf {r} '}{|\mathbf {r} -\mathbf {r} '|^{3}}}\times \mathbf {J} (\mathbf {r} ',t_{r})+{\frac {\mathbf {r} -\mathbf {r} '}{|\mathbf {r} -\mathbf {r} '|^{2}}}\times {\frac {1}{c}}{\frac {\partial \mathbf {J} (\mathbf {r} ',t_{r})}{\partial t}}\right]\mathrm {d} ^{3}\mathbf {r} ',}$

where r′ is a point in the charge distribution, r is a point in space, and

${\displaystyle t_{r}=t-{\frac {|\mathbf {r} -\mathbf {r} '|}{c}}}$

is the retarded time. There are similar expressions for D and H.[3]

These equations are the time-dependent generalization of Coulomb's law and the Biot–Savart law to electrodynamics, which were originally true only for electrostatic and magnetostatic fields, and steady currents.

Origin from retarded potentials

Jefimenko's equations can be found[2] from the retarded potentials φ and A:

${\displaystyle {\begin{aligned}&\varphi (\mathbf {r} ,t)={\dfrac {1}{4\pi \epsilon _{0}}}\int {\dfrac {\rho (\mathbf {r} ',t_{r})}{|\mathbf {r} -\mathbf {r} '|}}\mathrm {d} ^{3}\mathbf {r} ',\\&\mathbf {A} (\mathbf {r} ,t)={\dfrac {\mu _{0}}{4\pi }}\int {\dfrac {\mathbf {J} (\mathbf {r} ',t_{r})}{|\mathbf {r} -\mathbf {r} '|}}\mathrm {d} ^{3}\mathbf {r} ',\end{aligned}}}$

which are the solutions to Maxwell's equations in the potential formulation, then substituting in the definitions of the electromagnetic potentials themselves:

${\displaystyle \mathbf {E} =-\nabla \varphi -{\dfrac {\partial \mathbf {A} }{\partial t}}\,,\quad \mathbf {B} =\nabla \times \mathbf {A} }$

and using the relation

${\displaystyle c^{2}={\frac {1}{\epsilon _{0}\mu _{0}}}}$

replaces the potentials φ and A by the fields E and B.

Discussion

There is a widespread interpretation of Maxwell's equations indicating that spatially varying electric and magnetic fields can cause each other to change in time, thus giving rise to a propagating electromagnetic wave[4] (electromagnetism). However, Jefimenko's equations show an alternative point of view.[5] Jefimenko says, "...neither Maxwell's equations nor their solutions indicate an existence of causal links between electric and magnetic fields. Therefore, we must conclude that an electromagnetic field is a dual entity always having an electric and a magnetic component simultaneously created by their common sources: time-variable electric charges and currents."[6]

As pointed out by McDonald,[7] Jefimenko's equations seem to appear first in 1962 in the second edition of Panofsky and Phillips's classic textbook.[8] David Griffiths, however, clarifies that "the earliest explicit statement of which I am aware was by Oleg Jefimenko, in 1966" and characterizes equations in Panofsky and Phillips's textbook as only "closely related expressions"[2]. According to Andrew Zangwill, the equations analogous to Jefimenko's but in the Fourier frequency domain were first derived by George Adolphus Schott in his treatise Electromagnetic Radiation (University Press, Cambridge, 1912).[9]

Essential features of these equations are easily observed which is that the right hand sides involve "retarded" time which reflects the "causality" of the expressions. In other words, the left side of each equation is actually "caused" by the right side, unlike the normal differential expressions for Maxwell's equations where both sides take place simultaneously. In the typical expressions for Maxwell's equations there is no doubt that both sides are equal to each other, but as Jefimenko notes, "... since each of these equations connects quantities simultaneous in time, none of these equations can represent a causal relation."[10] The second feature is that the expression for E does not depend upon B and vice versa. Hence, it is impossible for E and B fields to be "creating" each other. Charge density and current density are creating them both.

See also

• Liénard–Wiechert potential

Notes

1. Oleg D. Jefimenko, Electricity and Magnetism: An Introduction to the Theory of Electric and Magnetic Fields, Appleton-Century-Crofts (New-York - 1966). 2nd ed.: Electret Scientific (Star City - 1989), ISBN 978-0-917406-08-9. See also: David J. Griffiths, Mark A. Heald, Time-dependent generalizations of the Biot–Savart and Coulomb laws, American Journal of Physics 59 (2) (1991), 111-117.
2. Introduction to Electrodynamics (3rd Edition), D. J. Griffiths, Pearson Education, Dorling Kindersley, 2007, ISBN 81-7758-293-3.
3. Oleg D. Jefimenko, Solutions of Maxwell's equations for electric and magnetic fields in arbitrary media, American Journal of Physics 60 (10) (1992), 899–902.
4. Kinsler, P. (2011). "How to be causal: time, spacetime, and spectra". Eur. J. Phys. 32: 1687. arXiv:1106.1792. Bibcode:2011EJPh...32.1687K. doi:10.1088/0143-0807/32/6/022.
5. Oleg D. Jefimenko, Causality Electromagnetic Induction and Gravitation, 2nd ed.: Electret Scientific (Star City - 2000) Chapter 1, Sec. 1-4, page 16 ISBN 0-917406-23-0.
6. Oleg D. Jefimenko, Causality Electromagnetic Induction and Gravitation, 2nd ed.: Electret Scientific (Star City - 2000) Chapter 1, Sec. 1-5, page 16 ISBN 0-917406-23-0.
7. Kirk T. McDonald, The relation between expressions for time-dependent electromagnetic fields given by Jefimenko and by Panofsky and Phillips, American Journal of Physics 65 (11) (1997), 1074-1076.
8. Wolfgang K. H. Panofsky, Melba Phillips, Classical Electricity And Magnetism, Addison-Wesley (2nd. ed - 1962), Section 14.3. The electric field is written in a slightly different - but completely equivalent - form. Reprint: Dover Publications (2005), ISBN 978-0-486-43924-2.
9. Andrew Zangwill, Modern Electrodynamics, Cambridge University Press, 1st edition (2013), pp. 726—727, 765
10. Oleg D. Jefimenko, Causality Electromagnetic Induction and Gravitation, 2nd ed.: Electret Scientific (Star City - 2000) Chapter 1, Sec. 1-1, page 6 ISBN 0-917406-23-0.