# Can the magnetic fields of EM radiation be harnessed or measured?

We use the electric component of EM radiation to create the EM radiation and to detect it (antennas and Etc.), but does anyone know of a situation where the magnetic component of EM radiation is used? I've read that it is so vanishingly small that it is almost non measurable, but this seems odd because they both hold the same amount of energy. Thanks!

• It is incorrect to say that we only "use" the electric field to create electromagnetic radiation. In fact, we create such radiation by accelerating charges. Both the electric and magnetic fields around the particle are changing in time, so there is no reason to say that electric fields are preferred over magnetic ones. I think we commonly tend to think of an oscillating electric field when thinking of EM radiation, but the truth of the matter is that both fields are equally represented in all cases. Also, you're exactly right about the equal energy contribution. – Ultima Jul 30 '14 at 13:46
• EM radiation is both of oscillating electric and magnetic filed. Moreover, these fields have the same energy density. But we mostly focus on electric field, because (a) if we know one of them, then we can obtain another by Maxwell's equations and (b) in most cases, when we investigate the interaction of EM radiation with matter, the force due to the electric filed is much larger than the force due to the magnetic filed (in Gaussian unit, $F_e=eE$ and $F_m=e\frac{v}{c}\times B$ where $\frac{v}{c} \ll 1$ while $E=B$), so we mostly focus on the electric filed. – Mojtaba Golshani Jul 30 '14 at 16:01
• Moreover, note that in some studies (for example for studying the radiation pressure) we must consider both. – Mojtaba Golshani Jul 30 '14 at 16:12
• Note that the magnetic field is generally only negligible because most ordinary objects travel with speeds $\ll c$. For objects moving relativistically, then since the magnetic force $\propto vB$, and since $E/B =c$ tells us that the magnitude of the electric field is greater than that of the magnetic field by a factor of $c$, we see that the forces are then roughly equal. – Jerry Schirmer Jul 30 '14 at 21:40
• We routinely use magnetic field detectors all the time (e.g., fluxgate and search coil magnetometers) on spacecraft and in lab plasmas (though lab plasma magnetometers are often much smaller and different in design than those on spacecraft). So yes, we can and do measure the magnetic fields of electromagnetic radiation. – honeste_vivere Apr 26 '15 at 12:24

I think a ferrite rod antenna in a radio receiver is an example where the magnetic component of an EM-field is picked up.

• This is misleading, the addition of a ferrite rod increases the performance of the tunning circuit, but does not change the way the EM waves interact with it. The interaction of EM waves with matter (charges) is both with the electric and magnetic components. Is not possible that a charge is not affected by both. – rmhleo Jul 31 '14 at 7:38
• I intended to give an example and that happens to contain a ferrite rod. Standard elecromagnetic theory states that the coil surrounding the rod picks up the magnetic component of the field and produces a tiny current. In my response it is nowhere stated that the electric field component is absent. – Urgje Aug 1 '14 at 9:03

We do not use just the electric component, this is not possible.

An EM radiation is an oscillation in media or void of electric and magnetic fields. As described by Maxwell equations, the electric field oscillation generating the magnetic one and vice versa. So is kind of an electric wave and a magnetic one co-dependent with each other.

It is true that we generate EM signals with antennas by means of an oscillating current, i.e. movement of charges generating electrical field oscillation, but immediately this electric field generates the magnetic one and so forth.

Thus when the radiation interacts it loses all its energy to the interacting medium, i.e. not just that of one component. And in fact, the interaction with charges in media occurs with both magnetic and electric components of the wave.

You can check Wikipedia which is very well represented and explained.