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Science and History behind Electromagnetic Field
In the 1950s, when the United States of America was deemed as the only Super Power in the
world and as a beacon of freedom for other countries, the Russia government established the Iron
Curtain to block all TV programs, such as British Broadcasting Corporation, and radio stations
that contradict the country’s ideals. Nonetheless, applying the science behind electromagnetic
field, people found ways to listen and watch the banned programs. The electromagnetic field is
an invisible, force field that has both electric and magnetic fields, resulting from the charges and
their movements. Before learning about the discovery and applications of electromagnetic field,
it is essential to understand the concept of electric field and magnetic field.
Electric Field
An electric field is an electric force caused by a charge. The direction of the electric field
depends solely on the polarity of the charge causing the field. For a positive charge, the electric
field is pushed away from the charge, and for a negative charge, the electric field is attracted
towards it. A single static1 charge rarely occurs and is considered to be an ideal case. It is more
realistic to look at the interaction of two charges on a same plane. In Figure 1 (a), a positive and
a negative charges are present. Therefore, the electric field lines are directed towards the
negative charge. In Figure 1 (b), the charges are equal and the electric field lines are repelling
one another. This occurrence can be compared to the interaction of two magnets. The magnets
with same polarity resist each other when brought close but magnets with opposite polarity
attract each other.
The strength of the electric field does not depend on the polarity but rather on the quantity of the
charges. The strength decreases as the distance from the charge increases – it is proven by
Coulomb’s Law2. The equation shows that electric field is equipotential everywhere equidistant
from the charge.
Figure 1. Electric Field Lines of Static Charges
http://www.mysearch.org.uk/website1/html/479.Fieldlines.html
1
Motionless
2
𝐸=𝑞=
𝐹
𝑘𝑄𝑠𝑜𝑢𝑟𝑐𝑒
𝑟2
Figure 2. Electric Field of Point Charge
http://hyperphysics.phyastr.gsu.edu/hbase/electric/elefie.html
Magnetic Field
A magnetic field is a force field caused by the movement of a charge, defined as a current. The
magnitude of magnetic field decreases as the distance from the power source increases. The
direction of the magnetic field, unlike that of electric field, is determined using the Right Hand
Rule, also known has RHR. The first picture on Figure 3 shows an example of the right hand
rule. The thumb is pointed in the direction of the current and the other four fingers are curled in
the direction of the magnetic field. The RHR can also be applied in the opposite manner – thumb
in the direction of the magnetic field and the current in the direction of the four fingers.
Figure 3. Magnetic Field Lines of a Bar Magnet
Image from http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfie.html
History of Electromagnetic Field
It was in 1864 when the interrelationship between electric field and magnetic field was presented
by James Clerk Maxwell to the Royal Society of London. Before his publication, electric field
and magnetic field were considered as two distinctive entities. It was Maxwell himself who
combined Ampere’s Law, Faraday’s Law, and Gauss’s Law, and described the electromagnetic
field as a form of wave.
Ampere’s Law states that the magnetic field induced3 by the current is directly proportional to
the magnitude of the current and the length of the current-carrying object or conductor.
Faraday’s Law, which is essential for inductors, demonstrates the relationship between the
voltage and magnetic field. Any changes to the magnetic field in a closed loop or surface will
cause a voltage drop or increase.
Gauss’s Law is the last two equations of Maxwell’s equations. The first equation is for electric
field and it proves that the electric flux4 within a closed surface is equal to the total charge
enclose. The second equation is for magnetic field and it states that there is no magnetic flux
outside of the closed surface or loop.
Maxwell first came to his theory when he recognized the relation between the speed of light and
the speed of electric and magnetic fields in different dielectrics5. Using the relation and the
3
Generated
Product of a field and the area the field encounters
5
Poor conductors of electricity
4
equations from Figure 4 as the basis, Maxwell continued his research and came to the conclusion
that the electric field never exists without the magnetic field and the fields exist in wave form. In
addition, he declared that the change in the electric field causes a change in the magnetic field
and vice versa. While Maxwell’s equations were theories at first, it was later proven correct by
Heinrich Hertz, who demonstrated Maxwell’s theories by showing the existence of radio waves
and their speed.
Figure 4. Maxwell’s Equations
Image from http://www.thunderbolts.info/eg_draft/eg_appendix_3.htm
Applications
We constantly encounter technology that use electromagnetic fields. Some of the instruments
include:
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Cell phone
Microwave
Clothes washer
Hair dryer
LCD/plasma TV
Lightbulb
A cell phone is one of the most widely used devices that depend on electromagnetic fields. It is
composed of circuits and charged using electricity. Using the definition of magnetic field, it
becomes apparent that a cell phone produces electromagnetic field, using the electricity from the
battery. In order to have access to 3G and wifi, the cell phone, using an antenna, receives
electromagnetic fields in wave form from outside sources.
A microwave is an underappreciated instrument that utilizes the amplification of electromagnetic
waves to heat, thaw, and cook food. Inside a microwave, electromagnetic waves are produced
from all sides and is applied to the food placed on the rotating plate. Most microwaves are
designed in such a way that all the electromagnetic fields are concentrated at the center, causing
the central part of the food to be warmer than the rest.