Electromagnetic Theory
Hi. Let's play a game and
see what's your electromagnetic knowledge... I will ask you some questions. After that,
you should pause the video and think about the answers.
Then, go on and see the real answer. Ready?
First one. Do you know what electromagnetism is?
Electromagnetism is a branch of physics involving the study of electromagnetic force.
Do you know what an electromagnetic force is?
It is a type of physical interaction that occurs between electrically charged particles.
Well...do you know what electrically charged particles are?
I bet you know some of them: stuff as protons and electrons.
The next question could be: who carries the electromagnetic force?
The electromagnetic force is carried by electromagnetic fields, composed of electric
fields and magnetic fields, and it is responsible for electromagnetic radiation such as light.
WAIT, WHAAAT? Is light a type of electromagnetic radiation?
We will be back to it later in this video.
I'm glad you decided to play with us.
Now, if you want to take things more seriously and interesting,
we suggest you keep watching this video about electric and magnetic theory!
Electromagnetism is one of the four fundamental interactions (commonly called forces) in nature,
together with the strong interaction, the weak interaction, and gravitation.
The laws that lie behind Electromagnetic phenomena are elegant.
They are the so-called Maxwell's equations and the Lorentz Force.
Actually, there are numerous mathematical descriptions of the electromagnetic field,
and Maxwell's equations describe how electric and magnetic fields
are generated and altered by each other and by charges and currents.
Maxwell was an insane thinker and scientist. His theoretical and
observational work led to the development of special relativity by Albert Einstein in 1905.
In fact, Maxwell saw that electromagnetic waves go through space at the speed of
light. This was a crucial result, that entered amazingly in the special relativity theory.
But let's go back in time, and see how we got there. What did the world know about
electromagnetic theory, before Maxwell? What were the first electromagnetic phenomena ever
observed and studied? Long before any knowledge of
electromagnetism existed as a concept, people were aware of the effects of electricity.
Now we know that we can think of electromagnetism as a combo of Electricity and Magnetism, but
people at the time thought they were two distinct things, that had nothing to do with each other.
Lightning and other manifestations of electricity were known in ancient times,
but it was not understood that these phenomena had a common origin.
One possible approach to the discovery of the identity of lightning and electricity
is to be attributed to the Arabs, who before the 15th century used the same Arabic word
for lightning (barq) and the electric ray. Thales of Miletus noted that rubbing fur on
various substances such as amber would cause them to attract specks of dust and other light objects.
Thales wrote on the effect now known as static electricity. The Greeks noted
that if they rubbed the amber for long enough they could even get an electric spark to jump.
The electrostatic phenomena were also reported millennia later by Roman
and Arabic naturalists and physicians.
It was no magical trick. It was a new phenomenon that urged to be studied!
Just to understand the importance of electromagnetic theory, recall that the
electromagnetic force is responsible for practically all phenomena one encounters
in daily life above the nuclear scale, with the exception of gravity. Roughly speaking,
all the forces involved in interactions between atoms can be explained by the electromagnetic
force acting between the electrically charged atomic nuclei and electrons of the atoms.
Electromagnetic forces also explain how these particles carry momentum by their movement. This
includes the forces we experience in "pushing" or "pulling" ordinary material objects, which result
from the intermolecular forces that act between the individual molecules in our bodies and those
in the objects. The electromagnetic force is also involved in all forms of chemical phenomena.
People didn't know any of that, but with time, someone started to
feel that there was much at stake. The first remarkable in depth-study
of electric and magnetic phenomena as a whole started around 1800.
In April 1820, scientist Hans Christian Oersted observed that an electrical current
in a wire caused something strange: a nearby compass needle was moving!
Again, it was no magical trick. It was just...science!
Did he know that? Yes, and at first he struggled in order to
find a satisfactory explanation of the phenomenon, but he could not find it.
Also, he didn't try to represent the phenomenon in a mathematical framework.
However, three months later he began more intensive investigations,
which led to the publishing of his findings, proving that an electric current produces
a magnetic field as it flows through a wire. That's why the CGS unit of magnetic induction
is today named Oersted. He literally got what he deserved!
After that, Oersted findings resulted in intensive research throughout the scientific community.
They influenced French physicist Ampère's developments of a mathematical form to represent
the magnetic forces between current-carrying conductors. Oersted's discovery also represented
a major step toward a unified concept of energy. This unification, which was observed by Michael
Faraday, extended by James Clerk Maxwell, and partially reformulated by Oliver Heaviside and
Heinrich Hertz, is one of the key accomplishments of 19th-century mathematical physics.
In particular, things really changed with the publication of James Clerk Maxwell's 1873
A Treatise on Electricity and Magnetism in which the interactions of positive and
negative charges were shown to be mediated by one force. There are four main effects
resulting from these interactions, all of which have been clearly demonstrated by experiments:
1. Electric charges attract or repel one another with a force inversely proportional
to the square of the distance between them: unlike charges attract, like ones repel.
This is a pretty common thing nowadays, everybody knows that “opposites attract”.
2. Also, magnetic poles (or states of polarization at individual points) attract or repel one
another in a manner similar to positive and negative charges and always exist as pairs:
every north pole is yoked to a south pole. 3. If electric current flows inside a wire,
it creates a corresponding circumferential magnetic field outside the wire. Its direction
could be clockwise or counter-clockwise, and it depends on the direction of the current in
the wire by the famous “right-hand rule”! 4.A current is induced in a loop of wire
when it is moved toward or away from a magnetic field, or a magnet
is moved towards or away from it; the direction of current depends on that of the movement.
All of this has had far-reaching consequences,
one of which was the understanding of the nature of light.
The understanding of the nature of light is perhaps one of the most
important things in all physic's history. Do you want to know why?
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The answer is pretty simple yet pretty simple, but subtle: when we discovered that light was
nothing but an electromagnetic wave, we had a mathematical framework to treat it.
For example, we know how to describe the speed of an electromagnetic wave, and Maxwell found
that all of the electromagnetic waves travelled at a velocity of about 300.000 km/s in a vacuum.
One scientist found that light was an electromagnetic wave, the game was made: even the
light was travelling at that peculiar velocity. That's why we call it “the speed of light”.
Also, the nature of light has always been controversial. Today we know
that photons (which made up the light) can actually behave as waves or as particles.
In fact, unlike what was proposed by the electromagnetic theory of that time,
light and other electromagnetic waves are at present seen as taking the form of quantized,
self-propagating oscillatory electromagnetic field disturbances called photons. Different
frequencies of oscillation give rise to the different forms of electromagnetic radiation,
from radio waves at the lowest frequencies to visible light at intermediate frequencies,
to gamma rays at the highest frequencies.
Anyway, taking into account quantum behaviours would be too much for the purpose of this video.
Let's go back and see how Maxwell's law shook
classical mechanics. Are you ready for the revolution?
One of the peculiarities of classical electromagnetism
is that it is difficult to reconcile with classical mechanics,
but it is compatible with special relativity. How is that possible?
It has something to do with the changing of the reference system.
According to Maxwell's equations, the speed of light in a vacuum is a universal constant that
is dependent only on the electrical permittivity and magnetic permeability of free space.
This violates Galilean invariance, a long-standing cornerstone of classical mechanics. One way to
reconcile the two theories (electromagnetism and classical mechanics) was to assume the
existence of a substance called “luminiferous aether”, through which the light propagates.
But experimental efforts failed to detect the presence of the aether.
One of the most famous experiment to prove or disprove the existence of the aether was
the Michelson & Morley experiment. It was an attempt to detect the
velocity of Earth with respect to the hypothetical luminiferous ether.
The procedure depended on a Michelson interferometer, a sensitive optical device
that compares the optical path lengths for light moving in two mutually perpendicular directions.
Michelson reasoned that, if the speed of light were constant with respect to the proposed ether
through which Earth was moving, that motion could be detected by comparing the speed of light in
the direction of Earth’s motion and the speed of light at right angles to Earth’s motion. No
difference was found. This null result seriously discredited the ether theories and ultimately led
to the proposal by Albert Einstein in 1905 that the speed of light is a universal constant.
After the important contributions of Hendrik Lorentz and Henri Poincaré, in 1905, Albert
Einstein solved the problem with the introduction of special relativity, which replaced classical
kinematics with a new theory of kinematics compatible with classical electromagnetism.
(For more information, see History of special relativity.)
In addition, relativity theory implies that in moving frames of reference,
a magnetic field transforms to a field with a nonzero electric component
and conversely, a moving electric field transforms to a nonzero magnetic component,
thus firmly showing that the phenomena are two sides of the same coin.
Hence the term "electromagnetism". (For more information, see Classical electromagnetism
and special relativity and Covariant formulation of classical electromagnetism.)
Before ending the video, we want to remind you that, nowadays,
electromagnetism-derived technologies are all around us! Here are some examples:
Musical Equipment Musical equipment that
is being used in events is all operated or works with electromagnets. Big loudspeakers,
electric bells, and other electric recorders all operate by the action of electromagnets.
Generators This is another important use of
electromagnet because we do use the generators almost every day. The generator has a magnet
and a copper coil in which when given electricity would cause the coil to rotate against the magnet
and make the generator operate superbly. MRI scanning device
The magnetic resonance imaging device is a scanner that operates with electromagnetic force.
It is used in hospitals to scan inside the body of humans and detect most fractures and other issues.
It has an inbuilt magnet and operates with the flow of electricity.
Mobile and telephones The electromagnetic force
present in mobile and telephones is what gives us the chance to make long-distance
calls with quality sound without being getting interrupted unless by a network.
Elevators An elevator is
another important application of electromagnets. Without electromagnets, the elevators would not
be able to function properly. With the transfer of electric current to the magnetic elevator,
it would be able to move and operate properly. If there is no transfer of electric current,
the elevator would remain stagnant.
This video ends here! As always, thanks for watching!
See you next time on the channel!