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Magnets and Magnetic Fields

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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