Electricity and magnetism
Introduction to Magnetism An introduction to magnetism
Introduction to Magnetism
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- 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.
- 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.
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