For
electrical engineers the word electromagnetics typically conjures up thoughts
of antennas, transmission lines, and radio waves, or maybe boring lectures and
“ all-nighters ” studying for exams.
However,
this electrical word also describes a broad range of phenomena in addition to
electronics, ranging from X-rays to optics to thermal radiation. In physics
courses, we are taught that all these phenomena concern electromagnetic waves.
Even many nontechnical people are familiar with this concept and with the electromagnetic
spectrum, which spans from electronics and radio frequencies through infrared,
visible light, and then on to ultraviolet and X-rays.
We are told
that these waves are all the same except for frequency. However, most engineers
fi nd that even after taking many physics and engineering courses, it is still
difficult to see much commonality across the electromagnetic spectrum other
than the fact that all are waves and are governed by the same mathematics
(Maxwell’s equations).
Why is
visible light so different from radio waves? I certainly have never encountered
electrical circuits or antennas for visible light. The idea seems absurd.
Conversely, I have never seen FM radio or TV band lenses for sale.
So why do light waves and radio waves behave
so differently? Of course the short answer is that it all depends on frequency,
but on its own this statement is of little utility. Here is an analogy. From
basic chemistry, we all know that all matter is made of atoms, and that atoms
contain a nucleus of protons and neutrons with orbiting electrons.
The
characteristics of each element just depend on how many protons the atom has.
Although this statement is illuminating, just knowing the number of protons in
an atom doesn’t provide much more than a framework for learning about
chemistry.
Continuing
this analogy, the electromagnetic spectrum as shown in Figure 20.1 provides a basic
framework for understanding electromagnetic waves, but there is a lot more to
learn.
To truly
understand electromagnetics, it is important to view different problems in different
ways. For any given frequency of a wave, there is also a corresponding wavelength,
time period, and quantum of energy. Their definitions are given below, with their
corresponding relationships in free space. frequency, f , the number of
oscillations per second wavelength, λ , the distance between peaks of:
λ = C/f
time period,
T , the time between peaks of a wave:
T = 1/f
photon
energy, E , the minimum value of energy that can be transferred at this
frequency:
E = h x f
where c
equals the speed of light and h is Planck’s constant.
Depending on
the application, one of these four interrelated values is probably more useful
than the others. When analyzing digital transmission lines, it helps to compare
the signal rise time to the signal transit time down the transmission line.
When dealing with infrared, optical, ultraviolet, and x-ray interactions with matter, it is often most useful to talk about the energy of each photon to relate it to the orbital energy of electrons in atoms.
No comments:
Post a Comment