In the late 1800s, Scottish physicist James
Clerk Maxwell published a set of 4 equations
which set the foundation for classical electromagnetism.
Though daunting even in their simplest form,
they describe the reason you can hear my voice
right now.
His last two laws, individually known as Faraday’s
law and Ampere’s law state the following:
a change in magnetic flux over time can induce
an electric field and therefore an electric
current and an electric current can generate
a magnetic field.
What does that mean in plain English?
Move a coil of wire over a magnet and you’ll
generate electricity, or run a current through
a wire and you’ll create a temporary magnet.
This reciprocal relationship is vital to the
mechanism of microphones and speakers.
As sound hits a microphone’s diaphragm,
it causes the diaphragm istelf and a metal
coil attached to it to vibrate back and forth
(not unlike your eardrum) over a permanent
magnet.
As the coil vibrates, it experiences in its
reference frame a changing magnetic field
which induces in it a small alternating electric
current that is carried away and amplified.
Speakers are set up in a similar way with
a slight variation.
A metal coil is attached to a diaphragm but
is placed in front of a magnet.
As the amplified current from the microphone
flows through the coil, it becomes an electromagnet.
And just like an ordinary magnet, the coil
is then either attracted to or repelled by
the permanent magnet.
As the alternating current flips back and
forth, so does this attraction or repulsion,
causing the attached diaphragm to vibrate
and produce sound.
So it’s these 150 year old equations which
are mathematically so elegant and so important
to the history of physics that we take advantage
of every time we record our voice, watch a
video, or make a phone call.
If you’re bored, take a pair of earbuds
and touch them against each other and you’ll
feel just the slightest tug from the magnets
in them as you try to pull them apart.
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