Professor Dave here, I want to tell you about magnets.
We have known about the magnetic properties of
certain types of matter for thousands of years,
and have long utilized this phenomenon
for navigation in the form of a compass,
and also to display good report cards on
the fridge. A few different kinds of
magnets are instantly recognizable, like
the popular horseshoe variety, and most
people are aware that these attract
iron-containing objects. Any bar magnet
will have ends called poles. One will be
the north pole and one will be the south
pole, which were named as such because of
the way a suspended bar magnet will
orient with respect to the cardinal
directions. This is how a compass works,
it's just a magnetic needle that always
points north. Like poles repel and
opposite poles attract, just like
electric charges. The difference is that
opposite charges can be isolated, whereas
no matter how many times you cut a
magnet in half it will always contain
both poles. To fully understand the
properties of matter that allow for
magnetic behavior, we must understand the
electron configurations of the atoms in
the magnet, and we must therefore learn
some chemistry. If you are interested you
can check out my general chemistry
series for more atomic knowledge than
you can shake a stick at, but for our
purposes here it will suffice to
understand that most materials contain
atoms with all of their electrons paired
up in orbitals with one spin up and one
spin down such that they cancel each
other out, but some substances like iron,
cobalt, and nickel have electrons that
aren't cancelled out, and some of these
can be called ferromagnetic. In these
materials the atoms will adopt an
orientation such that all of their net
spins are aligned in parallel fashion
and this phenomenon which we can refer to as a
magnetic domain will generate a magnetic
field. A magnetic field can be depicted
using field lines just like an electric
field, and though they will appear to
begin at the north pole of the magnet
and end at the south pole, they have no
true ends. They are closed loops that
continue through the magnet itself.
In this way earth itself is a giant bar
magnet with the magnetic north pole
about 1,500 kilometers from the
geographic North Pole, and the magnetic
south pole about the same distance from
the geographic South Pole. The geographic
poles mark Earth's axis of rotation but
the magnetic poles exist because of the
way atoms are distributed in Earth's
iron core, which align their spins just
like the atoms in a bar magnet.
The magnetic poles actually change their
position slightly over thousands of
years as material in the core changes
position. There is so much magnetized
material in the Earth's core that the
magnetic field generated is absolutely
immense, stretching far out into space.
This field interacts with high-energy
charged particles that race towards us
from the Sun and deflects them towards
the magnetic poles to produce the aurora
borealis when these particles collide
with molecules in our atmosphere. These
become excited and release colorful
light upon relaxing back to the ground
state. Without the Earth's magnetic field
we would be at tremendous risk from this
kind of radiation and life on Earth
probably wouldn't be possible. Electric
current is able to deflect a compass
needle, and magnetic fields affect the
paths of particles with electric charge,
so it wasn't long until we realized that
electricity and magnetism were two sides
of the same coin, which was then dubbed
the electromagnetic force. We now realize
this is true because it is the motion
and orientation of electrons that
produce magnetic fields. We can use the
right-hand rule to show that if you
grasp a current carrying
wire in your right hand with your thumb
pointing in the direction of the current,
your other fingers will wrap in the
direction of the magnetic field that is
generated by the current. We later came
to understand how electric and magnetic
fields fluctuate together to generate
electromagnetic waves. This was the first
time two seemingly separate forces were
unified, but it wasn't the last. We have
been able to unify most of the
fundamental forces through our study of
modern physics, and one of the primary
objectives of physics today is unifying
all of them into one grand unified field
theory, a so-called theory of everything.
Everything we have discussed in this
classical physics course was discovered
between the 17th and 19th centuries, and
at the turn of the 20th century, things
got a lot weirder thanks to Einstein and
some of his pals. We learned that things
can be both particles and waves, that
time flows at different rates for
different observers, and that space
itself is curved around massive objects.
So if you feel like you've sufficiently
mastered the concepts presented in this
series, I'll see you in the modern physics course.
Thanks for watching, guys. Subscribe to my channel for
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