Download Physics 1001 - Introduction to Magnetism VO Magnets are all

Physics 1001 - Introduction to Magnetism
VO
Magnets are all around us, from the simplest child’s toy to the most powerful electromagnet.
Even the earth acts as a giant magnet. Its magnetic field is responsible for the phenomenon known as
the Northern Lights.
And the discovery of the link between electricity and magnetism has led to inventions such as the
generator and the electric motor. A telephone is an example of a device containing both a generator
and a motor, one in the mouthpiece and the other in the earpiece.
During this unit on magnetism and electromagnetism, we will explore magnetic forces, their causes,
effects and their many uses in our lives.
(Read objectives on screen.)
Instructor
Welcome to the mysterious world of “Magneto Man.” But magnetism won’t be so mysterious to you
after we finish our study of magnetism. See all these things that are attracted to me? Well, they aren’t
actually attracted to me, they’re attracted to the magnets I have hidden in my sleeve and pocket. So
let’s start by looking at some properties of magnets.
We’ll start with a simple bar magnet. Now you’ve all seen a bar magnet before, and you know that it
has two poles, one labeled north and the other south. And I’m sure you know some simple facts about
how these poles interact. But let’s go over them again, just to be sure. We’ll start in the lab.
(students on screen)
VO
A bar magnet is placed in a holder that is suspended from a string. This allows the magnet to move
freely. When the north pole of a second magnet is brought close the north pole of this magnet, it turns
away.
(green chalkboard on screen)
Like poles of magnets repel. The north pole of one magnet will repel the north pole of another
magnet. The same is true of two south poles.
(students on screen)
Next, the south pole of the second magnet is brought close the north pole of the suspended magnet.
This time, they attract.
(green chalkboard on screen)
Unlike magnetic poles attract. North attracts south.
Notice that the forces of attraction and repulsion do not require that the objects touch. Magnetic
forces are field forces.
(green chalkboard on screen)
A magnetic field is the space around a magnet, in which a magnetic force of attraction or repulsion is
exerted. You will learn how to draw magnetic fields in the next program.
Instructor
OK. We’ve established the fact that the north pole of one magnet will attract the south pole of
another magnet. But have you ever wondered why we call these poles north and south? Well, it all
started with a rock like this one.
Over two thousand years ago, the early Greeks discovered a magnetic ore in a region of Turkey,
called Magnesia. That’s where the term magnetism came from. And somehow, in the twelfth
century, the Chinese found that when a single stone made of this ore was suspended, one end always
turned toward geographical north. These natural magnets, called lodestones, or leading stones, were
used to navigate ships. So the Chinese invented the first compass. A compass is just a magnet that is
free to move, like this one. Now do you see where the north pole of a magnet got its name? Better
get some notes on all this.
(diagram of magnet on screen)
VO
The north pole of a magnet, abbreviated with a capital N means north-seeking because it seeks
geographic north.
The south pole means south-seeking because it seeks geographic south.
A compass is a magnet that is suspended in some way so it is free to move.
A compass needle moves to align itself with the earth’s magnetic field.
Instructor
Now you physics types out there should be really bothered by a couple of contradictory statements
you’ve just heard and you’ve even put into your notes. Your teacher will pause the tape so that you
can take a minute and look back over your notes to find the two statements that contradict each other.
Come back after you’ve discussed it in class.
(Pause Tape Now graphic)
(diagram of earth on screen)
VO
The two statements that seem to contradict are these:
“Like poles repel.” and “the North pole seeks north.” Seeks means “is attracted to.” So it seems that
north attracts north. Shouldn’t north be repelled by north?
Yes, and here’s the explanation. The earth acts like a giant magnet -- we’ll learn why later and the
magnetic needle of a compass is affected by the earth’s magnetic field. So if the north pole of a
compass magnet aligns itself with geographic north, which pole of the earth’s magnet is located near
geographic north? Tell your teacher.
The south magnetic pole, of course. The south pole of the earth’s magnetic field is located close to the
geographic north pole. So the north pole of a compass magnet is attracted to this south magnetic pole
of the earth. Opposite magnetic poles still attract. The confusion comes in using the words north and
south in both geographical directions and magnetic poles, which are two entirely different things. So
go back to your notes and add the word, geographic, to the north and south seeking, like this.
(fact or fiction on screen)
VO
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Here’s a physics fact or fiction challenge for you.
On a foggy night, Santa Claus can use a compass to find his way home to the north pole.
Instructor
This one is fiction. A compass would lead Santa to the earth’s south magnetic pole, located in
Hudson Bay, Canada, about 1,200 miles south of Santa’s home at the North Geographic pole. On a
foggy night, Santa would be better off using Rudolph to guide his sleigh.
At any location on the surface of the earth, the difference between the magnetic and geographic poles
is called the magnetic declination. For example, along a line through South Carolina and the Great
Lakes, a compass magnet will point to geographic north. But in Los Angeles, it will point 15 degrees
east of geographic north, so its declination is 15 degrees east.
OK. Now that you know how the poles of a magnet got their names and how compasses work, let’s
answer some more magnetic questions. We’ll start with this one: Why are these washers attracted to
a magnet and these aren’t? What kinds of materials are magnetic anyway? Sounds like a good lab
investigation.
(table on screen)
VO
Your teacher will give you this data table from unit ten of the facilitator's guide. When you’re ready
to make your observations, come back and we’ll go to the lab.
(Pause Tape Now graphic)
(student on screen)
VO
Materials are classified as magnetic if they are attracted to a magnet. Our student will test each object
to see if it contains a magnetic material. On your data table, place a check by the objects that are
attracted to the magnet.
(no audio while objects are tested)
Instructor
Well, I checked off four objects that were attracted to the magnet. We classify the materials
composing these objects as magnetic. Now you need to decide what the four materials have in
common. Talk it over and come back when you’ve made your conclusion.
(Pause Tape Now graphic)
Instructor
What do the paper clip, nail, razor blade, and straight pin have in common? If you said that they are
all metals, you’re surfing. You didn’t notice that the aluminum foil, quarter, penny, and zinc were not
magnetic, and they’re all metals. No, you have to look a little deeper.
Did you notice that all of the objects have iron in them? That’s the common link. Iron is a magnetic
material.
(green chalkboard on screen)
VO
We call elements strongly attracted to a magnet, ferromagnetic. Save room for more notes on
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ferromagnetic elements. But first, think back to chemistry and see if you can remember where the
term “ferromagnetic” comes from. Tell your teacher.
The symbol for iron is Fe, and it comes from the Latin name for iron, “ferum.”
Instructor
Now, what causes the magnet to attract iron and other ferromagnetic elements?
To answer this, we’ll have to look at the electron structure of the atom. Put your pencils down as we
review a little chemistry. I’ll tell you when to take notes.
This is the first of many times that we’ll talk about the link between electrons in motion and
magnetism. The fact is that moving charged particles, like electrons, produce magnetic fields. And
it’s the spinning movement of electrons within the atom that is responsible for the atom’s magnetic
properties.
(diagram of spinning electron on screen)
VO
An electron spins on its axis, producing a magnetic field. If it spins in one direction, the north pole
will be on top and the south pole on bottom, like this.
And if it spins in the opposite direction, the poles will be reversed, like this.
In atoms containing many electrons, most of the electrons will pair up, like this, with their spins
opposite each other, producing magnetic fields that cancel each other out.
However, in ferromagnetic metals, like iron, many of the electrons remain unpaired, like this,
spinning in the same direction, and giving the atoms magnetic properties.
Now it’s time to pick your pencils up and take some notes.
(green chalkboard on screen)
VO
Let’s add to your notes on ferromagnetic elements. You’ve already written that ferromagnetic
elements are strongly attracted to magnets. Why?
Because these elements have many unpaired electrons, spinning in the same direction, that produce
identical magnetic fields.
Iron, nickel, and cobalt are the most common ferromagnetic elements. They have atomic numbers,
26, 27, and 28, and are adjacent on the periodic table, so they are easy to remember.
Instructor
Many students make the mistake of treating magnetic poles like electric charges, saying things like,
“The ends of a magnet have opposite charges.” Remember that the iron and other elements in this
magnet are made up of neutral atoms. There are no charges, just magnetic poles.
And there’s another big difference between electric charges and magnetic poles. You can have a
negatively-charged object, without a positive one. But there is no such thing as a monopole. Even
the smallest magnetic particle, a spinning electron, has both a north and a south pole. You can’t have
one without the other.
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These ring magnets illustrate my point. Their shape may fool you, but each one has a north and a
south pole. Let me show you what I mean. You can see that these magnets attract. But let me place
them on a stand and drop another magnet on top. This one repels so strongly that it defies gravity. So
the top and bottoms of ring magnets have opposite poles.
Oh, I almost forgot another question for you. If magnets are made of iron and nails are made of iron,
why aren’t all nails magnets? Good question, huh? Let’s go to the lab and do some investigating.
(students on screen)
VO
Let’s see if our students can turn a nail into a magnet. A nail is placed on one pole of a bar magnet. It
attracts.
Next, another nails is put at the end of the first nail. It attracts, too. This is continued until the last
nail won’t hold.
Now, the magnet is removed. Some of the nails are still attracted. So our nails have become magnets.
But watch what happens when the nails are dropped on the table and our student tries to use one to
hold up another. It’s obvious that any magnetic polarization the nails obtained is only temporary.
Instructor
So why aren’t all nails magnets? And how does one become a magnet? It can all became explained
with a new physics term: domains.
(green chalkboard on screen)
VO
Domains are large groups of neighboring ferromagnetic atoms whose magnetic fields are aligned,
making the group act like a microscopic magnet.
In an unmagnetized substance, like a nail, the domains are randomly oriented. So the nail does not act
like a magnet.
But when the substance is brought into an external magnetic field, the domains align, giving the
substance a north and south pole.
This is called magnetic induction. The process is similar to the induction of a static charge, but it can
be accomplished with or without touching, as you will see later.
(diagram of domains on screen)
VO
Let’s look at magnetic induction more closely.
This diagram roughly represents a piece of iron, like a nail. Because the iron is ferromagnetic, it has
domains. These small magnets represent the domains, which are randomly aligned.
Now, when the iron is brought into a magnetic field, such as the one around the north pole of this bar
magnet, the domains begin to respond to the field and are induced into moving around, like this, with
south poles being attracted to the north pole of the magnet.
Finally, the domains will be aligned like this. The nail now has a north and a south pole. It has
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become a temporary magnet with its own magnetic field. It can now attract another nail.
But when the external magnetic field is removed, the domains in the nail will go back to their random
pattern due to the natural random motion of the atoms.
Instructor
Have you ever played with this toy? I’m using a magnet to drag around tiny pieces of iron, called iron
filings.
We can use these iron filings to represent the domains in a ferromagnetic substance. Watch this
demonstration.
(students on screen)
VO
Let’s use a test tube filled with iron filings to represent the domains inside a piece of iron. Shaking the
tube makes the filings fall in a random arrangement. When we bring a compass needle close to this
random arrangement of filings, the needle is not affected. So the tube of filings does not act like a
magnet.
Now we’ll place the tube of filings on top of a bar magnet and gently shake both to give the filings a
chance to react to the magnetic field of the bar magnet. Each small piece of iron can turn around, like
the domains of atoms inside the nail can turn. Notice how the filings line up, especially around the
poles of the magnet.
Next, we’ll remove the magnet, being careful not to disturb the arrangement of the filings. This time,
when we bring the compass needle close, it is attracted and repelled by the different ends of the tube.
We have created a magnet out of iron filings, just as a nail can become a magnet due to the movement
of its domains.
Instructor
Just like the iron filings in this tube are all mixed up now, dropping a nail after it has become
magnetized jostles its domains out of alignment, too. Dropping this permanent magnet weakens it,
but it’s not as easy to make a permanent magnet lose its polarity. Why?
Well, it has to do with the type of material it is made of. Iron is considered to be a soft magnetic
material because its domains can slip into and out of alignment easily. Soft iron does not make a
good permanent magnet for this reason.
But hard magnetic materials, like steel or other alloys, are harder to magnetize because their domains
don’t turn as easily. However, once the domains are aligned, the alignment persists after the external
magnetic field is removed. These hard metals make good permanent magnets.
This razor blade is made of stainless steel, an alloy. So the material is harder than pure iron alone.
The domains inside the blade cannot turn easily in response to a magnetic field. We’ll need to use
another method of magnetic induction than merely bringing it close to a magnet. This time the
process of induction will involve touching the object. Watch.
(student on screen)
VO
First, the south pole of a bar magnet is used to stroke the razor blade, starting in the middle, and
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moving toward the top end.
Next, the north pole of the magnet is used to stroke the same blade, from the middle to the bottom
end.
Let’s see how the razor blade has been affected. We’ll place the blade on the surface of a dish of
water, so that the surface tension of the water will hold it up. No matter which direction we turn the
razor blade, it always returns to the same direction, with the top of the blade pointing toward
geographic north. We’ve turned the blade into a magnet, and by making it free to move, we’ve made
a compass.
Watch what happens when we bring a magnet close. Our razor blade responds the same way any
compass magnet would, reacting to the stronger magnetic field.
How do you think a magnet would be affected by breaking it in two? Tell your teacher.
Let’s try it with our homemade magnet. We’ll snap the razor blade in half and place the halves back
in the water. The halves attract each other and return to the original alignment.
Instructor
Breaking a magnet in half just makes two magnets. Did you predict that?
Look at this diagram of the domains in a permanent magnet.
Breaking the magnet in two does not affect the alignment of its domains.
So dropping or hitting a permanent magnet will weaken or destroy it, but breaking it into pieces will
not affect it. What do you think about heating a magnet? Will that weaken it? Tell your teacher. I’ll
wait.
Heating will weaken a magnet. Heating increases the thermal random motion of the atoms, making
the domains lose some of their alignment. It’s like shaking the tube of iron filings, which made it lose
its polarity.
Well, you’ve learned a lot about magnets today, so it’s time to …SHOW WHAT YOU KNOW!!
Jot down your choice for each question. Your local teacher will go over the correct answers with you.
(Read Show What You Know question on screen)
Instructor
Magneto Man has a challenge for you. This is called a “cow magnet.” Does it have anything to do
with a cow? And if so, what? See if you can find out before our next program.
Whoa! I said cow magnet; not a cow-sized magnet. Well, guess I’ll just hang out here until next time.
See you.
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