For the recent a few centuries, understanding
how to use electricity and how to generate electric power has been considered
as the most important invention for the human history. Nowadays, there is no
way that our human beings can live without electricity: from big machines like
the refrigerators, the air conditioners, the washing machines, to small devices
like lamps and cellphones, almost every second in our daily life we need to
deal with electric appliances and electronic devices.

Although most people know how to use
electricity and understand its importance, many of us may not be familiar with
the twin brother of the electricity – the magnetism. Perhaps the understanding
of the majority of people on magnetism is still on the level of refrigerator
magnets or compasses, I have to say that the interconnection between
electricity and magnetism is inseparable. From the largest scale of facilities
that is related to electricity like the generators in the power plant, to the
much smaller scale of devices like a headphone and a microphone, the magnetism
plays a critical role in all those areas.

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First of all, I would like to
introduce some fundamentals and history about the relationship between
electricity and magnetism. In the ancient time, people studied and considered
electricity and magnetism as two separated subjects that were paralleled to
each other. However, in 1820, a Danish physicist called Hans Christian Oersted
occasionally observed that a compass needle was deflected when got close to an
electric circuit that was switched on and off, and following by further
research, he found and realized that was the first connection between
electricity and magnetism, and it is known for Oersted’s Law. Oersted’s Law
implies that an electric current creates a magnetic field. In 1831, English
scientist Michael Faraday published his study that indicated when part of the
closed circuit moves cutting the magnetic lines near a magnet, an inducting
electric current would be produce in the circuit, which is known as Faraday’s
law of induction. Faradays’ law of induction reveals the fact that using
magnetism we can generate electric power. From that time, people realized that electricity
and magnetism were tightly connecting to each other, and with the deeper and
deeper studying on them, our human beings were stepping into the age of
electricity.

lives much better and more convenience. We can enjoy an ice cream by plugging
the refrigerator into the wall; we can light up the night by plugging the lamps
into the wall; we can use all electric appliances by simply plugging them into
the wall. But have you even thought about where does the electricity on the
wall come from? Okay, the answer is that the electricity on the wall comes from
the generators in the power plant, and the electric generator is built based on
Faraday’s law of induction. A generator in the power plant is a device that
converts mechanical energy from a gas turbine or a steam turbine into electric
power for our daily usage on the principle of electromagnetic induction found
by Faraday. As introduced before, electric current can be induced by moving a
part of the closed circuit like a wire in a magnetic field that cutting its
magnetic lines, and if we increase the intensity of the magnetic field,
increase the length of the wire that cutting the magnetic lines, and increase
the speed of the movement, we can generate an enormous net electric potential.
That is the basic idea how a generator is built and how it works, coils
spinning in a large magnet, and the electric current will flow in the coils
that is distributed to every houses.

I believe most of people have their
own cellphone today, and many of them are fancy smartphone that can do much
more than a phone should do – making a phone call. Although it is hard to
imagine that there are magnets in our cellphone since they usually are not able
to be attracted on the refrigerator, it is true that every cellphone has
magnets in it, and they are the important parts that makes the speaker and the
microphone in the cellphone work. The speaker in the cellphone turns the
electric signal to the sound signal, and it has three major parts to finish
this job: a cone, a coil, and a magnet. The cone, or sometimes call the diaphragm,
is acting like a drum skin that can make sound when vibrating. The front part
of the cone is connecting to the speaking hole in the front of the faceplate of
the cellphone, and the rear part is connecting an iron coil, that just in the
front of a permanent magnet. The speaker employs the principle of Oersted’s
Law: when the speaker is in use, electrical currents will flow through the wires
to the coil, and the electrified coil will produce a magnetic field based on
the direction of the current. As the electric current in the coil that carries the
information changes back and forth, the coil will either attract or repel the permanent
magnet, so that the moving coil will make the cone vibrating, and that is how
the speaker in a cellphone works. On the other hand, the microphone in a
cellphone has exactly the same physical structure as the speaker, but the
principle of operation is reversed: it turns the sound signal to the electric
signal. When we are speaking to the microphone, the sound waves make the cone
vibrating, and the moving cone pushes the coil back and forth. As how Faraday’s
law of induction implies, the moving coil in a magnetic field produced by the
permanent magnet will generate electric current, and that is how the microphone
in a cellphone turns the sound signal to the electric signal.

After finishing my brief
introduction on the historical background and some daily examples showing how
electricity and magnetism interconnects with each other, I believe most of
people will have a better understanding on the importance of magnetism’s
interdependence with electricity. No matter from the small devices like the
speaker or the microphone in our cellphones, or from a large generator that can
supply electricity for the whole city, it is obvious that magnetism exists
wherever electricity exists, and that is why electricity and magnetism are
twins brothers.

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