[PIANO MUSIC - "IT'S A GIFT TO
 BE SIMPLE"]
 OK, students, WE are now
going to talk about magnetism.
Hmm, what is that?
Now, magnets actually were
know since ancient time
in what was called lodestone,
which were very, very
important for the
navigation of ships
because the lodestone would
always point to the north.
It was a magnet.
And over time, as we will
see, the relationship,
very important relationship,
between magnetism
and the electrical
force became known.
In the previous lecture,
we saw that charges
can be considered
to be monopoles,
or point charges that sort of
radiate out in all directions.
A magnet is different
in the sense
that a magnet has two poles.
And we've always traditionally
called them north and south,
the north pole and
the south pole.
And you've probably heard
from some science class
long ago that if you take a bar
magnet and you cut it in half
or saw it in half, the
two pieces will still have
a north pole and a south pole.
And you also know that the
two north poles, or the two
south poles, if you try to push
them together, they will repel.
But the north and the south
pole will attract each other.
Now, this drawing illustrates
some basic similarities,
however.
With charges, we say that if
we had a positive and negative
charge and we bring
them in close together,
there will be attractive field
lines, as illustrated here.
Now, we can't see these field
lines, but they're there.
And they are what
causes the attraction
between the positive
and negative charge.
With a magnet-- and let's
think about a bar magnet--
there are also these
field lines that
radiate between the
north and the south pole,
as shown here, very,
very similar field lines.
Now, while we can easily see
that for electrical forces--
you've probably seen
demonstrations of this.
If you have a bar magnet
and you have some iron
filings that the bar magnet
will actually reorient the iron
filings and create
these field lines,
and you'll be able to
see these field lines
that are created by the magnet.
Now, there was a
very interesting
accidental discovery by a Danish
professor named Hans Christian
Oersted.
This is kind of the way
science is done sometimes,
just serendipitous findings.
Oersted was turning on and
off an electrical circuit.
And he noticed
that a compass, one
of these lodestone permanent
magnets, or a compass--
that the needle of the
compass would jiggle.
Hmm, so an electrical
circuit is causing
the compass, the magnet
of the compass, to move.
That is, there must
be some relationship
between electricity
and magnetism.
Gadzooks, Oersted
said, which, of course,
is Danish for by golly.
This led him to discover that
a magnetic field can be created
by the motion of
electric charges,
because the electric circuit
is a movement of charges.
Hmm, a movement of charges
can influence a magnetic field
and can actually create
a magnetic field.
This is a phenomena called
electromagnetic induction.
A moving charge, a
moving electric charge,
can produce a magnetic field.
And conversely, a moving magnet
can produce an electric field
in a conductor.
That is, it can produce current.
Hmm, sounds like that
might be important.
That is, there is a
fundamental and very important
relationship between
electricity and magnetism.
And one way to illustrate
that relationship
is shown in the bottom drawing,
which shows an electrical wire
and someone attempting to
grab the electrical wire
to illustrate to what is
called the right-hand rule,
that if you grab
an electrical wire,
and if the electricity
flows in the direction
as shown through
this wire, there
will be a magnetic field that
is produced perpendicular
to the wire, in the direction of
your right hand that is curved
in that particular direction.
Magnetic field extends outside
of the wire perpendicular
to the direction of flow
of electric current.
Wow, a moving charge produces
a magnetic field that can even
extend beyond the wire.
And we will see also later
that a moving magnet can
produce electric current.
You've also probably
heard of electromagnets.
That is, if you take
an electric wire,
and you coil it up
as shown in the left,
pass electricity through
this electric coil,
it produces a magnetic
field very similar,
just like that for
a permanent magnet.
I show an electromagnet
on the left,
and a permanent
magnet on the right.
And you see these
magnetic field lines.
It also turns out that if
you take this wire loops
and you wrap it
around an iron core,
it becomes an even stronger
electromagnet, what
we call a solenoid.
So this is what an
electromagnet is
and how it's similar
to a permanent magnet.
Now, this slide--
very wordy, but it
has a lot of information.
Now, there were
several scientists
who contributed
to the development
of our understanding
electromagnetics--
Franklin, Coulomb, Oersted,
Galvani, Faraday, Edison,
Maxwell, probably we
can name some more.
The phenomena of
electromagnetics
can be summarized
in terms of what
is called Maxwell's equations,
of which there are four.
One, like charges repel,
and unlike charges attract.
That can also be
called Coulomb's law.
Two, there are no
magnetic monopoles.
That is, a magnet occurs
as both north and south,
or it occurs as a
dipole, whereas charges
can be monopoles, single-point
charges, either plus or minus.
A magnet will always have
a north and a south pole.
Three, magnetic
phenomena that can be
produced by electrical effects.
That is, a moving
electric charge
can produce a magnetic field.
And four, which is the converse,
an electrical phenomena
can be produced by
magnetic effects.
Now, these latter
two phenomena are
the basis for our
generation of electricity.
We will see later on that
when we rotate a magnet,
it can produce an
electric current.
Also, electric motors
involve the rotation
of an electromagnet
between the north
and south poles of
a permanent magnet.
OK, so we just
covered a great deal
about the theory of
electromagnetism.
And we'll stop now
and have a little quiz
on what we just learned.
[PIANO MUSIC - "IT'S A GIFT TO
BE SIMPLE"]
