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Course: Physics library > Unit 13
Lesson 1: Magnets and Magnetic ForceIntroduction to magnetism
An introduction to magnetism. Created by Sal Khan.
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- When a bar magnet is broken in the middle, what is the effect on its intensity?(263 votes)
- the magnetic intensity would decrease as the distance between the poles decreases.(227 votes)
- atSal says a magnet is created when the electrons line up. how were lodestones' electrons lined up in the first place? 10:14(51 votes)
- Lodestones were igneous rocks, which means that they were originally lava. When the lava came up through a volcano, the rock took a significant time to cool. In this time, the Earth's magnetic field automatically aligned the domains, or atoms, of the lava. Then, when the rock cooled, the atoms were set in place, making the rock magnetic. This is the process that factories use to make bar magnets today.(77 votes)
- when you rub a nail across a magnet many times, the nail becomes magnetized.
why does this work?(58 votes)- This is because of magnetic induction. Nail is made of iron. Iron is magnetized easily. But you should remember that in iron there are atomic magnets which line up with each other in groups called domains. In an un-magnetized piece of iron the magnetic domains are pointing out in all directions and so cancel out each other. So when we rub the nail with a magnet it becomes magnetized. This is because all the tiny N-Poles add up at one end and all the S-Poles add up at the other end.(12 votes)
- Sal talks about with electrostatics we find that a point charge whether it is an electron or proton creates its own electric "vector" field and it is a monopole. But then as he is talking about magnets he says if you were to cut a magnet in half, the two halves would create a dipole again and again. He also says even if you were to keep cutting it till you had only an electron left it would still remain a dipole. How is this possible if he stated earlier an electron has a monopole?(23 votes)
- The electrostatic charge of an electron is a monopole. The magnetic field of a spinning electron is a dipole.(23 votes)
- okay so what your saying is that the earth is a big huge magnet????(16 votes)
- Yes. Electromagnetism is one of the most common sources of power on the earth. The earth is constructed with a North and South Pole and a Inner core made up of metallic composites presumably and largely iron. The Magnetic field around the earth is driven largely by this source of electromagnetism. In general the poles and the core of the earth are a lot like a battery on a massive scale. This magnetic field around the earth suffers interference from solar particles released from the sun with large electrical charges and cause the Northern Lights (Aurora Borealis). I might be wrong... and I got a little off topic.(52 votes)
- Why can there never be a monopole for a magnet?(12 votes)
- The indications we have from some of the String Theories have given a mass for a magnetic monopole so high that we would need is way beyond anything we could produce, it predicted mass is about a Planck mass.
In quantum field theory the difference between there being electric monopoles but not magnetic ones is because of a broken symmetry.(5 votes)
- what is the difference between electric and magnetic field ??(8 votes)
- Electric fields are made by charges or changing magnetic fields and create force on charge.
Magnetic fields are made by current (moving charge) or changing electric fields and create force on moving charges only.
Magnetic and electric fields are two sides of the same coin but viewed from different frames of reference.(10 votes)
- How can I explain simple magnetic force to children ages 2 1/2-4 years of age? For example when they play with trains that have magnets that connect them, when they don't connect how can I say, well it's the magnet type that doesn't allow them to connect or does let them connect.(3 votes)
- at that age, it make be sufficient for them to experience the phenomenon.
If you can find some magnets for them play with external to the toy train then, as they play, you might find some discussion starts to emerge.
If they are sufficiently engaged, I would encourage use of proper terminology (north and south) since positive and negative can be a) just difficult to comprehend and b) confusing later in lfe
Cheers(5 votes)
- AtSal says magnetism always comes in the form of a dipole, but after watching this video I read a wikipedia article that said the moon does not currently have a dipolar magnetic field. Would this be because the moon's magnetic field is so weak in comparison to ours? Or something else? 7:25(3 votes)
- Magnetism always comes in dipoles, but that does not require every planet and moon to have any magnetic field at all. It just says that if they do, they will have two poles, not one.
(Scientists recently created a magnetic monopole in a laboratory, so it's no longer accurate to say magnets are always dipoles - but you will never encounter a monopole)(4 votes)
- why do the charges and magnets act so similar? I mean....the like charges repel each other, and unlike charges attract each other. And similarly, the same poles of two magnets repel, and different poles attract. Is this merely a coincidence?(4 votes)
- Magnetism is an effect of electric fields and relativity. The phenomenons of electric and magnetic fields are just different aspects of electromagnetism.(3 votes)
Video transcript
We've learned a little
bit about gravity. We've learned a little bit
about electrostatic. So, time to learn about
a new fundamental force of the universe. And this one is probably second
most familiar to us, next to gravity. And that's magnetism. Where does the word come from? Well, I think several
civilizations-- I'm no historian-- found these
lodestones, these objects that would attract other objects
like it, other magnets. Or would even attract metallic
objects like iron. Ferrous objects. And they're called lodestones. That's, I guess, the Western
term for it. And the reason why they're
called magnets is because they're named after lodestones
that were found near the Greek province of Magnesia. And I actually think the people
who lived there were called Magnetes. But anyway, you could Wikipedia
that and learn more about it than I know. But anyway let's focus
on what magnetism is. And I think most of us have at
least a working knowledge of what it is; we've all played
with magnets and we've dealt with compasses. But I'll tell you this right
now, what it really is, is pretty deep. And I think it's fairly-- I
don't think anyone has-- we can mathematically understand
it and manipulate it and see how it relates to electricity. We actually will show you the
electrostatic force and the magnetic force are actually the
same thing, just viewed from different frames
of reference. I know that all of
that sounds very complicated and all of that. But in our classical Newtonian
world we treat them as two different forces. But what I'm saying is although
we're kind of used to a magnet just like we're used to
gravity, just like gravity is also fairly mysterious when
you really think about what it is, so is magnetism. So with that said, let's at
least try to get some working knowledge of how we can
deal with magnetism. So we're all familiar
with a magnet. I didn't want it to be yellow. I could make the boundary
yellow. No, I didn't want it to
be like that either. So if this is a magnet,
we know that a magnet always has two poles. It has a north pole
and a south pole. And these were just labeled
by convention. Because when people first
discovered these lodestones, or they took a lodestone and
they magnetized a needle with that lodestone, and then that
needle they put on a cork in a bucket of water, and that needle
would point to the Earth's north pole. They said, oh, well the side of
the needle that is pointing to the Earth's north, let's
call that the north pole. And the point of the needle
that's pointing to the south pole-- sorry, the point of the
needle that's pointing to the Earth's geographic
south, we'll call that the south pole. Or another way to put it,
if we have a magnet, the direction of the magnet or the
side of the magnet that orients itself-- if it's allowed
to orient freely without friction-- towards our
geographic north, we call that the north pole. And the other side is
the south pole. And this is actually a little
bit-- obviously we call the top of the Earth
the north pole. You know, this is
the north pole. And we call this
the south pole. And there's another notion
of magnetic north. And that's where-- I guess, you
could kind of say-- that is where a compass, the
north point of a compass, will point to. And actually, magnetic north
moves around because we have all of this moving fluid
inside of the earth. And a bunch of other
interactions. It's a very complex
interaction. But magnetic north is actually
roughly in northern Canada. So magnetic north
might be here. So that might be
magnetic north. And magnetic south, I don't know
exactly where that is. But it can kind of move
around a little bit. It's not in the same place. So it's a little bit off the
axis of the geographic north pole and the south pole. And this is another slightly
confusing thing. Magnetic north is the geographic
location, where the north pole of a magnet
will point to. But that would actually be the
south pole, if you viewed the Earth as a magnet. So if the Earth was a big
magnet, you would actually view that as a south
pole of the magnet. And the geographic
south pole is the north pole of the magnet. You could read more about that
on Wikipedia, I know it's a little bit confusing. But in general, when most
people refer to magnetic north, or the north pole,
they're talking about the geographic north area. And the south pole is the
geographic south area. But the reason why I make this
distinction is because we know when we deal with magnets,
just like electricity, or electrostatics-- but I'll show
a key difference very shortly-- is that opposite
poles attract. So if this side of my magnet is
attracted to Earth's north pole then Earth's north pole--
or Earth's magnetic north-- actually must be the south
pole of that magnet. And vice versa. The south pole of my magnet here
is going to be attracted to Earth's magnetic south. Which is actually the
north pole of the magnet we call Earth. Anyway, I'll take Earth out of
the equation because it gets a little bit confusing. And we'll just stick to bars
because that tends to be a little bit more consistent. Let me erase this. There you go. I'll erase my Magnesia. I wonder if the element
magnesium was first discovered in Magnesia, as well. Probably. And I actually looked
up Milk of Magnesia, which is a laxative. And it was not discovered
in Magnesia, but it has magnesium in it. So I guess its roots could be
in Magnesia if magnesium was discovered in Magnesia. Anyway, enough about Magnesia. Back to the magnets. So if this is a magnet, and let
me draw another magnet. Actually, let me erase
all of this. All right. So let me draw two
more magnets. We know from experimentation
when we were all kids, this is the north pole, this
is the south pole. That the north pole is going to
be attracted to the south pole of another magnet. And that if I were to flip this
magnet around, it would actually repel north-- two north
facing magnets would repel each other. And so we have this notion,
just like we had in electrostatics, that a magnet
generates a field. It generates these vectors
around it, that if you put something in that field that can
be affected by it, it'll be some net force
acting on it. So actually, before I go into
magnetic field, I actually want to make one huge
distinction between magnetism and electrostatics. Magnetism always comes in
the form of a dipole. What does a dipole mean? It means that we
have two poles. A north and a south. In electrostatics, you
do have two charges. You have a positive charge
and a negative charge. So you do have two charges. But they could be
by themselves. You could just have a proton. You don't have to have
an electron there right next to it. You could just have a proton and
it would create a positive electrostatic field. And our field lines are
what a positive point charge would do. And it would be repelled. So you don't always have to have
a negative charge there. Similarly you could just
have an electron. And you don't have to
have a proton there. So you could have monopoles. These are called monopoles, when
you just have one charge when you're talking about
electrostatics. But with magnetism you
always have a dipole. If I were to take this magnet,
this one right here, and if I were to cut it in half, somehow
miraculously each of those halves of that
magnet will turn into two more magnets. Where this will be the south,
this'll be the north, this'll be the south, this will
be the north. And actually, theoretically,
I've read-- my own abilities don't go this far-- there could
be such a thing as a magnetic monopole, although
it has not been observed yet in nature. So everything we've seen in
nature has been a dipole. So you could just keep cutting
this up, all the way down to if it's just one
electron left. And it actually turns out that
even one electron is still a magnetic dipole. It still is generating, it still
has a north pole and a south pole. And actually it turns out, all
magnets, the magnetic field is actually generated by the
electrons within it. By the spin of electrons and
that-- you know, when we talk about electron spin we
imagine some little ball of charge spinning. But electrons are-- you
know, it's hard to-- they do have mass. But it starts to get
fuzzy whether they are energy or mass. And then how does a ball
of energy spin? Et cetera, et cetera. So it gets very almost
metaphysical. So I don't want to go
too far into it. And frankly, I don't think you
really can get an intuition. It is almost-- it is a
realm that we don't normally operate in. But even these large magnets
you deal with, the magnetic field is generated by the
electron spins inside of it and by the actual magnetic
fields generated by the electron motion around
the protons. Well, I hope I'm not
overwhelming you. And you might say, well, how
come sometimes a metal bar can be magnetized and sometimes
it won't be? Well, when all of the electrons
are doing random different things in a metal bar,
then it's not magnetized. Because the magnetic spins, or
the magnetism created by the electrons are all canceling
each other out, because it's random. But if you align the spins of
the electrons, and if you align their rotations, then you
will have a magnetically charged bar. But anyway, I'm past the
ten-minute mark, but hopefully that gives you a little bit
of a working knowledge of what a magnet is. And in the next video, I will
show what the effect is. Well, one, I'll explain how we
think about a magnetic field. And then what the effect
of a magnetic field is on an electron. Or not an electron, on
a moving charge. See you in the next video.