Magnetism (Old Version)

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So, magnetism, sort of like electric charge,
produces a- an invisible force. And these kinds of invisible forces are popular for
special effects, especially in science-fiction. Now, with regards to magnets, a few basic
observations. Most metals cannot be magnetized. So, things like aluminum and copper and such,
you can’t make a permanent magnet out of these metals. However, iron and a few other metals
and some alloys, are said to be ferromagnetic, which means they can be magnetized and remain
magnetized. Now, magnets attract ferromagnetic metals,
so not every piece of iron is magnetized but a magnet will attract a piece of iron whether
it’s magnetized or not. Two magnets can either attract or repel each other depending on the
poles of the magnet. And magnets have a north pole and a south pole. And if we have a south
pole on one magnet and a north pole on another magnet, those two will attract. So, these
opposites attract much like electric charge. And if we have a south pole next to another
south pole, those two repel. Similarly, with a north pole repels another north pole. And
as I mentioned, not every metal is ferromagnetic. So, here’s a stack of coins. The ones on the
left are not ferromagnetic and on the right are. Most of the ones that are ferromagnetic
there simply have some iron or steel. Now, what happens is with a piece of iron
becoming magnetized is first, if it’s unmagnetized then there are in the solid iron magnetic
domains which are pointing in all random directions. So, there’s no synchronization or alignment
of these magnetic domains. Each magnetic domain is like a- a tiny internal magnet. Well, if
we bring a strong magnet close to that piece of iron that was unmagnetized, some of the
internal domains start to become aligned with that strong magnet nearby and then that piece
of iron becomes weakly magnetized. Now, if we bring the strong magnet even closer and
leave it there for a long time, then more and more magnetic domains become aligned and
eventually we have now a new piece of iron that has been magnetized.
So, we can see this in this demonstration. So, I have a strong horseshoe magnet and I’ve
put a nail on it. Now, that nail is magnetized. And you can see that it magnetizes the nail
below it and the one below that. But now if I [Metal Clinking] take it away, it returns
to being unmagnetized. So, as I said, the magnetic domains in the iron nails are aligned
by proximity to the strong magnets, so that each nail becomes a magnet temporarily. And
when I remove the top nail from the strong horseshoe magnet, the domains returned to
being unaligned and the nails lose most of their magnetization. Now, if I left those
nails attached to the strong magnet say overnight, then they would actually retain a permanent
alignment and magnetization. Now, it’s also possible to scramble those
magnetic domains. And the easiest way to do that is by raising the temperature, which
produces a lot of random molecular motion, and that will demagnetize a piece of iron.
So, here you see a nail which is hanging and it is attracted to the horseshoe magnet. Now,
I’m going to heat up the [Torch]- the nail. And the nail is going to get red hot. And
if I get it hot enough- see, once I get it hot enough I have scrambled those magnetic
domains inside the nail and it can’t retain alignment. And so it’s no longer attracted
to the horseshoe magnet. A simpler way of creating a magnet is to make
an electromagnet. So, when we have a current passing through a wire, there’s a magnetic
field that’s produced and we can augment that magnetic field by wrapping the wire into
a coil. And then this current, when it passes through this coil of wire produces a fairly
strong magnet. We can make it even stronger by putting a piece of iron or steel inside
of the coil. Here’s an example of that. I have an electric coil; a coil- a coil is just
wire wrapped many times around in a circle. So now, I’m turning on the electromagnet.
Now, I’m turning it off. [Metal Clink] So, electromagnets are also popular for special
effects like- it’s a plot element in this episode of Breaking Bad where they install
a large electromagnet in a truck in order to destroy some computers.
Now, we know that the earth has a magnetic field. We have a small magnet on a pivot-
that’s a compass- and the magnet aligns with the earth’s magnetic field, being attracted
to it. And since there is iron inside of the earth’s core, you might think that it was
a permanent magnet like the horseshoe magnet. But actually it’s an electromagnet, and there’s
electric currents within the core of the earth. We know it’s not a permanent magnet because,
as we saw, when iron gets hot it can’t retain the alignment and it’s extremely hot in the
core of the earth. Now, one last example of electromagnets is
an electric motor. So, we can have the force of an electromagnet serve to create a rotation
by passing a current through some electromagnets with the appropriate- appropriate orientations.
So, I’m going to turn on this simple motor and, I’ll explain the operation here in a
moment, let’s just watch it. Notice that there’s a switch at the top of the motor,
and as I’m increasing the current, the motor is spinning faster and faster. So, the way
this simple motor works is at one point the central electromagnets are set up so that
the north pole is next to the north pole on one side and there’s a south pole next to
south pole on the other side. And so, of course, they repel each other and the central shaft
turns. Now, when this orange electromagnet, which starts out on the left, when its stands
and it’s over on the right, the switch at the top reverses the direction of the current
and that reverses the polarity of the electromagnet and now this becomes a south. And so once
again it is repelled, and so we continue the cycle back and forth as it spins around.
So, in summary…. and again we know that it’s not a permanent magnet because it’s so
hot. And finally, one example of a electromagnet is an electric motor, which uses electromagnets
to create a force and that force serves to produce the torque that turns the motor. So,
that’s basics of magnetism. And you see from this last part, there’s a strong connection
between magnets and electric currents, and there’s more to say about that. And we’ll
do that in the next tutorial.

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