Magnetic Field and Electric Field: Two Sides Of A same Coin
Probably, I won’t be able to give a satisfying answer to this question, but it’s the risk we take when we ask the Why of a physical phenomenon —instead of how physics can describe a natural phenomenon— and we don’t want a metaphysical answer. I don’t know how to give a direct answer, so I’m going to make a brief review of the milestones that have characterized this branch of physics (electromagnetism), with the hope to remove some doubt.
Until the early decades of the 1800’s, experiments conducted to investigate electrical phenomena were completely separate from those focused on magnetic phenomena. Then, in 1820, Hans Christian Oersted —a Danish physicist and chemist— saw by chance that the needle of a compass is deflected from magnetic north if placed near a wire in which an electric current is flowing. Repeating the experience several times in a rigorous way, he demonstrated that an electric current generates a magnetic field, establishing for the first time a relationship between magnetism and electricity.
As soon as the French physicist and mathematician André-Marie Ampere learned of Oersted’s discovery, he began his experiments, in turn coming to some amazing conclusions. Not only he established in a formal way that a current-carrying circuit behaves like a magnet, but he also hypothesized that in a permanent magnet there are “electrodynamic molecules” responsible for the magnetic field at the macroscopic level.
And that is exactly what happens.
Although the phenomenon is more complex, we can say that, in ferromagnetic materials, spin and motion of electrons generate small magnetic fields which, aligning along a privileged direction (magnetic polarization), add themselves vectorially, producing the macroscopic magnetic field.
The amazing thing is that Ampere’s intuition took place about 70 years before the discovery of electron by J. J. Thomson (1897).
Ampere, however, was not the only one interested in the interaction between current and magnetism. After his colleagues’ discoveries, Michael Faraday —an English chemist and physicist— wondered if it was possible to reverse the phenomenon, that is, if it were possible to generate electricity from a magnetic field. In 1831, after several experiments, he reached the conclusion that, in a circuit, an induced electromotive force is created each time the magnetic flux (number of lines) through the surface bounded by the circuit is changed. And it’s important to remember that Faraday’s law of induction is at the bottom of electric motors, alternators, electric generators and transformers’ working principles.
Thus, up to this point, the magicians of physics had determined that an electric field is linked to an electric charge and that a magnetic field is linked to a moving charge.
Then, in 1864, the Scottish mathematician and physicist James Clerk Maxwell developed a theory that includes all the observations, the experiments and the laws known until the first half of the nineteenth century, regarding this branch of physics. In fact, with his system of equations, he proved that electricity and magnetism (and light) are manifestations of the same phenomenon: the electromagnetic field. With them, he also introduced the concept of electromagnetic waves which, about 20 years after, were discovered experimentally by the German physicist Heinrich Rudolf Hertz, opening the door to modern technology (from radio to cellular phone and beyond).
With Maxwell’s equations (that predict the existence of waves having a speed equal to that of light) and all the charges’ “activity”, it was no longer possible to avoid the big question:
With respect to which inertial frame of reference are they moving?
A charge may be in motion with respect to the observer, likewise, the latter can be in movement with respect to the charge. Now, can you guess who was the first theoretical physicist to give a formal statement of this ambiguity?
Right! In 1905, Albert Einstein published a paper entitled On the Electrodynamics of Moving Bodies exposing the theory known later as special relativity.
As already mentioned, a magnetic field and an electric field are two different manifestations of the electromagnetic field. But in other words, we can say that it’s the electromagnetic field to have physical relevance and, according to the inertial reference frame in which we observe a phenomenon, it may appear as an electric field, a magnetic field or a superposition of the two.