Electromagnetic Radiation Fundamentals
Date: Summer 2012
Please explain to me the differences and the meaning of the two components of electromagnetic radiation -- the electric and the magnetic components.
The electronic component,Eo is at right angles to and in phase with the magnetic component, Bo. These have a wavelength and a magnitude of +/- q. However, these magnitudes are restrictive relative to the direction of the propagating wave and the matrix it is traveling in.
If allowed to boil down the vector identities and differentials: Eo = cBo. It is difficult to separate the two and speak meaningfully about each one, but:
Generally we may think of light polarization as isolation of the direction of the Eo component and its measurement as V/m, think of a pressure.
Magnetic components are a bit more involved: B, magnetic flux density and H, magnetic field intensity. Flux density is measured in teslas (10K Gauss). Field density is measured in A/m, think of a volume.
We use the convenience of Eo measurements in V/m and Bo measurements in A/m for our student labs. We regard the V as the electronic component and I as the magnetic components. We use an electromagnet(student measured and wound) to pick up different weights as the students vary V and I independently. The students seem to absorb the material and concept well.
Hoping this helps! PEHughes, Ph.D. Milford, NH
Electromagnetic radiation (light) is composed of an oscillating electric field and an oscillating magnetic field perpendicular to one another and out of phase with one another. That is the “short” answer. You can find much more detailed info from a web search on the terms “electromagnetic waves”.
The electric and magnetic components of the radiated field are really two aspects of the same field. Their separation comes from historical reasons and how they were first observed as separate physical effects. Permanent magnets, as might be found in natural lodestones, were observed to attract and repel giving rise to the idea of “magnetism.” Similarly, static fields, such as might be found by rubbing certain materials together, gave rise to the idea of “electricity.” Both of these fields are “static” in nature, meaning they do not change in time. In this case, the two fields behave as though they are independent from each other. We know, however, that moving a magnetic across a wire will induce a current and produce a voltage. This effect had to be discovered, however, and it is not obvious. This simple effect implies that there is some connection between a magnetic field and electricity. Maxwell, in working with the equations explaining this connection posited the existence of a connection going the other way—a changing electric field can give rise to a magnetic field. In doing so, he realized that the two effects will couple and allow for wave propagation. In other words, a time-varying magnetic field will give rise to a time-varying electric field which in turn will give rise to a time-varying magnetic field. In this way, the two fields will propagate in space by themselves. Thus, he developed “Maxwell’s equations” which combines the electric and magnetic fields into an “electromagnetic” field. They reduce to separate fields (electric or magnetic) in static situations. To answer your original question as to the nature of the two fields, the two fields are the same as you observe in other situations. The electric field is the same as that giving rise to an electric shock (as in static electricity). The magnetic field is the same as that created by bar magnetics. When these are changing in time, however, they can couple to each other and cause an electromagnetic wave. This wave is of the same nature whether we are talking of radio waves, microwaves, infrared wave, visible light waves, x-rays, and so on. As an educator, this one observation is both very simple and profound and should be stressed. All these different phenomena are aspects of the same electromagnetic field. The two aspects, electric and magnetic, are like the two sides of a coin: we see both sides in our daily lives, but they are really the same coin.
The electric component corresponds to a kind of field that can be produced by the presence of an electric charge. The magnetic field component results from the motion of an electric charge. Also, electric field interacts with the presence of a charge, while magnetic field interacts with the motion of a charge. The electric component and magnetic component are what make up electric and magnetic fields. When the radiation is in the form of a wave, the electric and magnetic fields interact with each other. Faraday’s Law and Ampere’s Law (including the induction term) describe one way to analyze this interaction. When viewed in terms of motion, the orientation of electric and magnetic components relative to one another indicates the direction that energy is transported through the wave (Poynting vector).
Dr. Ken Mellendorf
Illinois Central College
When electric charges move along a conductor, a magnetic field forms.
This Wikipedia article can fill you in on the formal details that I miss:
So, if we run a current of electric charges down a conductor, a magnetic field forms.
If we run an alternating current up and down a conductor, an alternating magnetic field forms around the conductor.
If the wavelength of the alternating current is fitted to the length (tuned) of the conductor (an antenna) such that standing waves form (resonance),
an electromagnetic wave gets radiated into space.
So, this is what it comes down to:
The electric field on the antenna generates a magnetic field in space
The magnetic field in space generates another electric field in space
The electric field in space generates another magnetic field in space
And on and on and on until a “lengthwise physically tuned” “antenna” is encountered in space, where
The magnetic field in space induces a resonant electric field/current in the antenna (conductor)
That is processed by a receiver.
I hope this explains the relationship of the two components of electromagnetism for you.
And that is how radio and television electromagnetic signals “propagate” over space.
If you want to ask why this is so, you will have to ask your religious leader or speculate your own answer because that is beyond our frontier of knowledge at this time.
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Update: November 2011