Introduction to magnetism | Physics | Khan Academy

Summary

The script delves into the fundamental force of magnetism, its historical origins from lodestones in Magnesia, and the concept of magnetic poles. It clarifies the distinction between magnetic north and geographic north, emphasizing the dipole nature of magnets where opposite poles attract. The script also touches on the generation of magnetic fields by electron spin and motion, explaining how alignment of these can magnetize a metal bar. The video promises to explore the magnetic field's effect on moving charges in the next installment.

Takeaways

• 🧲 Magnetism is a fundamental force of the universe, second most familiar to us after gravity.
• 📚 The word 'magnetism' originates from 'lodestones' found near the Greek province of Magnesia.
• 🔍 Magnets have two poles, a north and a south, which were labeled by convention based on their interaction with Earth's magnetic poles.
• 🌐 Magnetic north and south are different from geographic north and south, and they can move due to the Earth's internal dynamics.
• 🔗 Opposite magnetic poles attract each other, similar to the principle in electrostatics.
• 📉 Magnetism is always found in the form of a dipole, meaning it always has two poles, unlike electrostatics which can have isolated charges (monopoles).
• ✂️ If a magnet is cut in half, each piece becomes a new magnet with its own north and south poles, indicating that magnetism is intrinsic to the material at all scales.
• 🌀 The magnetic field is generated by the spin and motion of electrons within a material.
• 🔄 A material becomes magnetized when the spins and motions of its electrons are aligned, creating a net magnetic field.
• 🌐 The Earth itself acts as a giant magnet with its own magnetic north and south poles, which do not align perfectly with the geographic poles.
• 📚 The script suggests further exploration of magnetism's deeper concepts, such as its relationship with electricity and the potential existence of magnetic monopoles.

Q & A

What is the origin of the word 'magnetism'?

The word 'magnetism' comes from the Greek province of Magnesia, where lodestones, natural magnets, were found. These stones attracted other magnetic or ferrous objects, and the people living in the region were called Magnetes.

What is a lodestone?

A lodestone is a naturally occurring magnet, which attracts other magnets or metallic objects like iron. They are the basis for the study of magnetism.

How are the poles of a magnet labeled?

The poles of a magnet are labeled by convention. The north pole of a magnet is the side that points towards the Earth's geographic north when the magnet is allowed to orient freely.

What is the difference between geographic north and magnetic north?

Geographic north is the direction towards the Earth's northernmost point, while magnetic north is the location where the Earth's magnetic field points towards, which can be slightly offset from the geographic north.

Why does the Earth's magnetic north move?

The Earth's magnetic north moves due to the complex interactions of the fluid motion inside the Earth and other factors, making it a dynamic and shifting point.

What is the fundamental difference between electrostatics and magnetism in terms of charge representation?

In electrostatics, charges can exist as monopoles (single positive or negative charges), whereas magnetism always comes in the form of a dipole, with a north and a south pole.

What happens when you cut a magnet in half?

If you cut a magnet in half, each half becomes a new magnet with its own north and south poles. This is because magnetism always exists as a dipole.

What generates the magnetic field of a magnet?

The magnetic field of a magnet is generated by the motion and spin of electrons within the material. Even a single electron is considered a magnetic dipole.

How can a metal bar become magnetized?

A metal bar becomes magnetized when the spins of its electrons are aligned. When the electrons' spins and movements are random, the bar is not magnetized as the magnetic fields cancel each other out.

What is the relationship between a magnet's field and the force it exerts on other objects?

A magnet generates a field with vectors around it. When another object that can be affected by the magnetic field is placed within it, a net force acts on the object, attracting or repelling it based on the orientation of the poles.

Why is magnetism considered mysterious and deep?

Magnetism is considered mysterious and deep because, like gravity, it is not fully understood at a fundamental level. The relationship between magnetism and electricity is complex, and they are treated as different forces in classical physics, despite being related through different frames of reference.

Outlines

00:00

🧲 Introduction to Magnetism

The script introduces the concept of magnetism as a fundamental force in the universe, second only to gravity. It discusses the historical origins of the term 'magnetism,' relating to lodestones found near Magnesia in Greece. The video aims to provide a deeper understanding of magnetism beyond common knowledge, such as the relationship between magnetism and electricity, and the fact that magnetism is often mysterious and complex. The script emphasizes that magnets have two poles, north and south, which were named based on their interaction with the Earth's magnetic field. It also touches on the distinction between magnetic north and geographic north, highlighting the complexity of Earth's magnetic field.

05:00

🔗 The Nature of Magnetic Forces

This paragraph delves into the properties of magnetic forces, explaining that magnets always come in the form of dipoles with a north and a south pole. It contrasts this with electrostatics, where charges can exist as monopoles. The script clarifies that opposite magnetic poles attract each other, while like poles repel. It also discusses the concept of magnetic fields, which are generated by the motion and spin of electrons within a magnet. The video promises to explore the effects of magnetic fields on moving charges in the next installment. Additionally, it touches on the historical and potential discovery of the element magnesium and its relation to the region of Magnesia.

10:01

🪢 Magnetization and Electron Alignment

The final paragraph explains how a metal bar can become magnetized when the spins of its electrons are aligned, creating a net magnetic field. Conversely, when the electron spins are random, the bar is not magnetized as the individual magnetic fields cancel each other out. The script suggests that the next video will further elaborate on how to conceptualize a magnetic field and its effects on moving charges, providing a deeper insight into the workings of magnetism.

Keywords

💡Magnetism

Magnetism is a fundamental force in the universe that causes certain materials to attract or repel other materials. It is central to the video's theme, as it discusses the nature and properties of magnetic forces. The script mentions that magnetism is second most familiar to us next to gravity and explores its origins and effects, such as the attraction of lodestones to iron.

💡Lodestone

A lodestone is a naturally occurring magnet that can attract iron and other ferrous materials. The term is used in the script to illustrate the historical discovery of magnetism, with these stones being found near the Greek province of Magnesia, which is also the origin of the word 'magnet.'

💡Magnetic Pole

The magnetic poles are the regions of a magnet where the magnetic force is strongest, typically labeled as 'north' and 'south.' The script explains that these poles are defined by convention based on the way a magnetized needle interacts with the Earth's magnetic field, and that opposite poles attract each other.

💡Magnetic Field

A magnetic field is the region around a magnet where magnetic forces can be observed. The script discusses the concept of a magnetic field as being generated by a magnet, with vectors representing the force that would act on a moving charge placed within the field.

💡Electrostatic Force

The electrostatic force is the force of attraction or repulsion between electrically charged particles. The script mentions that the magnetic force and the electrostatic force are fundamentally the same, differing only in the frame of reference from which they are observed.

💡Dipole

A dipole refers to a pair of equal and opposite charges or magnetic poles. In the context of the script, magnetism always comes in the form of a dipole, meaning every magnet has a north and a south pole, unlike electrostatics where isolated charges (monopoles) can exist.

💡Electron Spin

Electron spin is a fundamental property of electrons that contributes to their magnetic moment. The script explains that the magnetic field of a magnet is generated by the spin of electrons and their motion around protons, which is a key concept in understanding magnetism at the quantum level.

💡Magnetic Monopole

A magnetic monopole is a hypothetical elementary particle that has a single magnetic charge, either north or south. The script mentions that while everything observed in nature has been a magnetic dipole, the theoretical possibility of a magnetic monopole is an area of scientific interest.

💡Magnetization

Magnetization is the process by which certain materials become magnetized, acquiring a magnetic field. The script discusses how a metal bar becomes magnetized when the spins and rotations of its electrons are aligned, resulting in a net magnetic field.

💡Magnetic North

Magnetic north is the geographic location on Earth where the north pole of a magnet will naturally point to. The script clarifies that magnetic north is distinct from the geographic North Pole and is influenced by the Earth's internal magnetic field dynamics.

💡Compass

A compass is a navigational instrument that uses a magnetized needle to show the direction of magnetic north. The script uses the compass as an example to explain how the concept of magnetic poles is applied in real-world scenarios.

Highlights

Introduction to magnetism as a fundamental force of the universe, second most familiar to us after gravity.

Origin of the word 'magnetism' from lodestones found near the Greek province of Magnesia.

Magnetism's deep nature and its mathematical understanding in relation to electricity.

Magnetism and electrostatic force are fundamentally the same, viewed from different frames of reference.

Classical Newtonian view treats magnetism and electrostatic force as two separate forces.

Explanation of a magnet's north and south poles and their labeling by convention.

Difference between Earth's geographic poles and magnetic poles, with magnetic north currently in northern Canada.

Magnetic north and south are not aligned with Earth's geographic poles due to the Earth's complex internal interactions.

Concept of magnetic field lines generated by a magnet and their effect on objects within the field.

Distinction between magnetism and electrostatics, with magnetism always manifesting as a dipole.

Explanation of the impossibility of having a magnetic monopole in nature as observed so far.

Magnetic fields are generated by electron spins and motion within a material.

Magnetization of a metal bar occurs when electron spins and rotations are aligned.

Magnetic fields and their effects will be further explained in the next video.

Historical and etymological insights into the term 'magnetism' and its connection to the region of Magnesia.

The complex relationship between a magnet's poles and Earth's magnetic field, including the concept of magnetic declination.

The theoretical possibility of magnetic monopoles, despite not having been observed in nature.

The role of electron spin in the generation of a magnetic field, even down to the level of a single electron.

The alignment of electron spins as the key to magnetizing a previously non-magnetized metal bar.

Transcripts

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.