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If the electric field hits the receiving antenna (in form of pressure or voltage) to agitate the electrons in the receiving antenna, then what is the purpose of magnetic field then? What it serves for?

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  • $\begingroup$ What if your receiving antenna is a loop (see here: en.wikipedia.org/wiki/Loop_antenna)? As varying magnetic field through the loop will induce a current in the loop, what's the purpose of the electric field then? This section: en.wikipedia.org/wiki/… is also instructive. $\endgroup$ Commented Apr 16, 2017 at 3:22
  • $\begingroup$ So, do you accept the sense of redundancy for some instance? or, Do you imply the other partner field is useless for some instance? $\endgroup$
    – Pisiko
    Commented Apr 16, 2017 at 4:43
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    $\begingroup$ My advice to you, depending on your age, is to study the necessary mathematics for the physics you need to understand, in university courses , there are free ones on line if you are of an older persuasion. $\endgroup$
    – anna v
    Commented Apr 16, 2017 at 8:17
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    $\begingroup$ Actually, I have advanced mathematics background. But, my realization is, every time we analyze certain phenomenon with the aid of mathematics , the deep understanding or the essence of its existence is very much important rather than relying only the tool and interpreting its results and stop there. Even if we expressed our answer in mathematical equations, we still need to touch the real meaning behind that mathematical equations. $\endgroup$
    – Pisiko
    Commented Apr 16, 2017 at 13:10
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    $\begingroup$ Example. What is mechanical force? and we answer, Mechanical Force is mass times acceleration and we stop there. with that, we are not showing the essential meaning of it. We could be interpreted as master in certain subject matter if we could able to go deeper with it. Like questioning ourselves, what cause that mechanical force to exist? If it is a product of mass and acceleration, which of the two that brings the force? Is it the mass or the acceleration? If it is the mass, then what makes the mass to exist? and then you realize it goes down to gravity. This is what I mean. $\endgroup$
    – Pisiko
    Commented Apr 16, 2017 at 13:11

2 Answers 2

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Electromagnetic waves are independent of the source, once generated, because varying electric fields and magnetic fields can move through the vacuum without need of reinforcement by a source ( as happens in an electric circuit). They become independent. In the classical mathematical description of light

emwave

Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This 3D animation shows a plane linearly polarized wave propagating from left to right. Note that the electric and magnetic fields in such a wave are in-phase with each other, reaching minima and maxima together

So the magnetic field is necessary for the existence of the self propagating wave.

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  • $\begingroup$ You main magnetic field is necessary to preserve its existence while propagating? Is it necessary to do that for the magnetic field if that electric field is existed by electrical charges? Would you mean that electric field diminishes or decreases as it travels away and thus magnetic field is necessary to maintain its existence? $\endgroup$
    – Pisiko
    Commented Apr 16, 2017 at 4:49
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    $\begingroup$ all the above falls beautifully in place when studying maxwell's equation. Before his brilliant studies, there was no concept of electromagnetic waves. Light is a higher frequency electromagnetic wave. $\endgroup$
    – anna v
    Commented Apr 16, 2017 at 5:34
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    $\begingroup$ I already encountered that Maxwell's equation. But, my question is at the point of arrival , these two partners hit the receiving antenna, how does the two work together? We know that electric field is a force (Newton per Coulomb, Volts per meter) , so there is a pressure or voltage at the antenna that pushes or agitates the electrons by this electric field; now, here is magnetic field, which also induces voltage, are these two overlapping their responsibilities? What is really their respective functions at this point? $\endgroup$
    – Pisiko
    Commented Apr 16, 2017 at 5:47
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    $\begingroup$ I appreciate you graphical representation of electromagnetic wave. It could give more clear to understand if you can explain like this: That up and down wave means amount or quantity of fields that varies periodically as it travels from transmitter to receiver. This wave can be visualize as light. Some is omni directional like star and some is directional like laser light. So it is true that both fields reach the antenna like laser light hit the antenna but the type of antenna limits its ability to take either one of the two fields. Correct me if I'm wrong. Thank you for your helpful answer. $\endgroup$
    – Pisiko
    Commented Apr 16, 2017 at 6:38
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    $\begingroup$ yes , the type of antenna will decide whether the magentic field part of the wave will be received and processed further ( the electrical engineering part, more mathematics). $\endgroup$
    – anna v
    Commented Apr 16, 2017 at 8:10
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@annav 's answer illustrates very nicely that, for the wave to propagate from the emitter to the receiver, you need both the electric and magnetic field.

To simplify matter, suppose the wave propagates in air. Then the ratio of the magnitudes of the electric to magnetic field is constant (and about 377 if you care to know). The electric and magnetic field both decrease in amplitude at the same rate, which by geometry is usually $1/r^2$ with $r$ the distance from the emitter to the receiver, i.e. if a first receiver is twice as far a second receiver, the first will detect an amplitude of 1/4 compared to the second.

The propagation is such that you cannot propagate an electromagnetic wave by propagating the electric or magnetic field alone: both are required. You need the varying $\vec E$ to produce a varying $\vec B$ which in turn produces a varying $\vec E$ in perpertual induction one to the other.

Once the wave reaches the receiving antenna, only one or the other field will usually "couple" to the receiving antenna. The details depend on the antenna but the simplest cases are the short line antenna (or Hertzian dipole) or the small loop antenna. In the case of the latter, it is the magnetic field that drives the antenna, while in the former it is the electric field.

Please be aware that the above two cases are just the simplest and that antenna design is quite a specialize field of electrical engineering, with some antenna design cleverly optimized for specific applications.

Thus, neither field is really redundant since

  1. they are both required for propagation, and
  2. which one will "activate" the antenna actually depends on the antenna, and there are probably models where both "activate" the antenna.
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