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I was reading up on how hertz discovered the electric and magnetic nature of radio waves. In my textbook it says that the electric nature could be demonstrated using another dipole parallel to the first. Im guessing that the electric field will interact with this and thus cause an AC current but why could this not be the magnetic component? And also when demonstrating the magnetic component they use a circular loop contraption which will also generate a spark after the original spark gap. But why could this not be the electric field doing this? I don't understand how either demonstrate that it has each component and how they distinguish between each other. I've tried googling this but my textbook doesn't have the greatest description of things so it's been quite difficult to find any good sources on the exact information I want.

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  • $\begingroup$ Hertz did not accidentally discover radio waves: He was trying to confirm Maxwell's theory. $\endgroup$ – Solomon Slow Jun 27 '17 at 20:34
  • $\begingroup$ Even so that doesn't answer my question. Merely just correcting a historical inaccuracy on my behalf. $\endgroup$ – Jake Rose Jun 27 '17 at 20:47
  • $\begingroup$ @JakeRose Do you have a source for the claim that the circular loop contraption was supposed to demonstrate the magnetic component? $\endgroup$ – probably_someone Jun 27 '17 at 20:50
  • $\begingroup$ My textbook gives two variations on it. It firstly says the loop demonstrated the B nature and the dipole demonstrated the E nature. It's not the most comprehensive and it is only A-level though so it could have simplified it a bit in all honesty $\endgroup$ – Jake Rose Jun 27 '17 at 20:53
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    $\begingroup$ @JakeRose Which textbook is it? $\endgroup$ – probably_someone Jun 27 '17 at 21:12
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The dipole

A dipole under AC current creates an oscillating electric field parallel to the current direction, and an oscillating magnetic field perpendicular to the current direction. The charges in a dipole parallel to the first dipole will, as you noted, receive an oscillating electric field in the direction of the wire, which pushes on the charges to generate an oscillating current.

If you consider the effect of the oscillating magnetic field in the absence of the electric field, we must remember that the direction of magnetic force on a charge depends on its velocity. Since the velocity of charges in a wire is distributed pretty randomly, there will be no net current imparted by the magnetic field in the absence of an electric field (of course, this situation is impossible in reality due to Maxwell's laws, but as a commenter remarked, this is what Hertz was trying to prove).

Even if you add in the oscillating magnetic field, the magnetic force on the charges will be perpendicular to both the magnetic field and the current (i.e. the velocity of the charges). So, once again, the magnetic field adds nothing to the current in the dipole.

The loop

A loop of wire under AC current generates an oscillating magnetic field along the axis of the loop, as the field components for all sections of the wire contribute in the same direction inside the loop. Note that the electric field follows the direction of the current, which is circular. As such, every section of the wire generates an electric field that, reasonably far from the loop, is approximately canceled by the field from the section directly opposite it. As such, the electric field received by the other loop is negligible.

As the oscillating axial magnetic fields travel through the loop, they create a changing magnetic flux, which induces a current in the wire, which, at a certain magnitude, produces sparks.

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  • $\begingroup$ With regards to your second to last paragraph. Are you saying that since the B force is at right angles to the current and since the dipole is a relatively straight contraption that there would be no real effect on the current? Or am I interpreting it wrong $\endgroup$ – Jake Rose Jun 27 '17 at 20:56
  • $\begingroup$ Also, if my textbook is wrong in regards to the spark gap then how did hertz confirm the magnetic nature of the wave? $\endgroup$ – Jake Rose Jun 27 '17 at 21:00
  • $\begingroup$ The 'magnetic loop antenna' is even now a recognized and viable antenna for, e.g., ham radio. $\endgroup$ – Jon Custer Jun 27 '17 at 21:39
  • $\begingroup$ I'm having a little difficulty imagining the loop section, could you possibly draw a diagram? $\endgroup$ – Jake Rose Jun 28 '17 at 12:26
  • $\begingroup$ See the images at Loop antenna. $\endgroup$ – Thomas Fritsch Jun 30 at 15:08
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If a conductor (e.g. a piece of metal) does not move, any electric current in it can be induced only by electric forces (in this case, due to the radiation). Magnetic forces of the radiation do act on the current carriers in conductors too, they can alter current distribution in conductor slightly, but this effect in wires is usually negligible. Much more important is so-called skin-effect, which means for high-frequency radiation, the conductor's charges push on each other in such a way the electric current will be concentrated close to the surface of the conductor, and inside of the conductor it will be much weaker.

People talk about antenna being sensitive to magnetic field but this does not mean that the current is started and maintained by magnetic forces. It is due to fact that in some small receiving loop antennas, the induced voltage on its terminals could not be explained just by electrostatic induction due to conservative electric field (whose lines of force have zero circulation over any closed loop) as in case of a direct dipole antenna, but takes advantage of presence of solenoidal electric field (whose lines of force are close loops or vortices), which always goes with presence of magnetic field varying in time.

People call this effect of solenoidal electric field "magnetic" for at least two reasons:

1) there is still a very strong emphasis in classes and literature on Faraday's original formulation of his law, where the varying magnetic flux is given as the reason for induced voltage or current in a coil. This is not entirely wrong, because that varying magnetic flux is present, but it has some drawbacks; it does not provide any local mechanism for the induction effect, it just states the observation. In the modern physics formulation, in phenomena of electromagnetic induction in stationary conductors, the forces that make the charge carriers move and produce voltage or current are due to solenoidal electric field that appears with the changing magnetic field.

2) the solenoidal electric field can be enhanced by presence of magnetic materials, such as ferromagnets (ferrite rod). The standard description of this effect is in terms of magnetic field, ferrite rod increases externally provided magnetic field by orders of magnitude. Of course, if the magnetic field oscillates with high frequency, it also increases the solenoidal electric field by orders of magnitude, but this effect is usually not stressed as much and all is described as magnetic effect, due to 1).

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