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Lecture Comments (2)

1 answer

Last reply by: Professor Selhorst-Jones
Tue Oct 21, 2014 9:50 AM

Post by Jamal Tischler on October 21, 2014

Great course ! Can you record more physics lectures ? Something harder with more problems ?


  • Every magnet comes with two poles. Just like electricity, like poles repel each other, while opposite poles attract.
  • Like electricity, we can describe the space around a magnet with a magnetic field and visualize it through the use of magnetic field lines.
  • Unlike electricity, it is not possible to separate these poles from each other. Magnets always come as a dipole: two poles together.
  • Moving charge creates a magnetic field.
  • On an atomic level, all atoms involve moving charge (the electrons). Thus, they have many small magnetic fields. Normally, the random distribution of these fields results in no net effect.
  • However, in some materials (such as iron), it is possible for these magnetic fields to all align and create a temporary or permanent magnet.
  • Since moving charge creates a magnetic field, we can run current through a wire to create a magnetic field in the space around it.
  • Through some clever arrangement, we can run current through some loops of wire, create a magnetic field, and then have it interact with another magnetic field, causing those loops to spin. Spin them with enough force, and you've got an electric motor.
  • The reverse also works: a changing magnetic field induces a current in a conductor. If you place a loop in a magnetic field and make it spin with enough force, you've got an electric generator.


Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Magnet 1:27
    • Magnet Has Two Poles
    • Magnetic Field
  • Always a Dipole, Never a Monopole 2:22
    • Always a Dipole, Never a Monopole
  • Magnetic Fields and Moving Charge 4:01
    • Magnetic Fields and Moving Charge
  • Magnets on an Atomic Level 4:45
    • Magnets on an Atomic Level
    • Evenly Distributed Motions
    • Unevenly Distributed Motions
  • Current and Magnetic Fields 9:42
    • Current Flow and Magnetic Field
    • Electromagnet
  • Electric Motor 13:11
    • Electric Motor
  • Generator 15:38
    • A Changing Magnetic Field Induces a Current
  • Example 1: What Kind of Magnetic Pole must the Earth's Geographic North Pole Be? 19:34
  • Example 2: Magnetic Field and Generator/Electric Motor 20:56
  • Example 3: Destroying the Magnetic Properties of a Permanent Magnet 23:08

Transcription: Magnetism

Hi welcome back to Today we’re going to talk about magnetism.0000

Here’s some things to think about, an electric plant running on coal, a speaker pumping out music, a compass pointing to the north, an electric car driving down the road, and a magnet holding up a report card on a refrigerator.0006

What do all these things have in common? Now you were probably able to guess because of the title to this lesson, magnetism.0018

They’re all based on the existence of magnetism. None of these things would be able to run if it weren’t for magnets.0026

When we work through magnets, when we’re working through this lesson, we don’t have the math yet to be able to quantitatively describe what’s going on.0033

We need calculus to really understand this and also have a slightly better grasp on some things that go on in vectors and so we’re not going to introduce those math concepts for the small dip that we’re going to have in magnetism.0041

We can still understand a lot of things that are going on qualitatively, like we did with entropy in thermodynamics, we didn’t define anything mathematically but we were still able to get a really good feel for the idea’s going on.0052

That’s what we’re going to get here with magnetism which is great because it’s going to give us a bunch of things to think about and it’s going to help us understand how our modern existence is.0063

I mean, is completely shaped by the fact that magnetism is a force. If it weren’t for that we wouldn’t have electricity running through our walls pretty much.0071

Its incredibly important. Magnetism, really interesting stuff. We don’t have the math to talk about it but that’s okay, we’ll still be able to forge on.0079

Every magnet has two poles, a north pole and a south pole. Just like in electricity, like poles repeal each other while opposite poles attracts.0088

If you put two north poles next to each other they push away. If you put to south poles next to each other they push away.0098

A north pole and a south pole will attract one another. They’ll have a force pulling each other together.0102

Just like in electricity we can also describe the area around a magnet as a magnetic field. We can draw on these magnetic field lines.0108

In this case, the magnetic field lines, they go from the south to the north because once again we do things from the North’s point of view.0115

Then the north is going to push it away, suck it back to the south and as we slid through, we’re going to get these circles moving like this.0122

The north pushes it away but then the South Pole pulls it in and it comes back and then shoots through the magnet.0130

We can draw on magnetic field lines on any of these things. This idea of a magnetic field is really important.0136

Another really important thing though is unlike electricity it’s impossible to separate these two poles from each other. With electricity we could put a bunch of charge on one object and carry that object off somewhere else.0143

We’d be able to separate the positive charges from the negative charges as much as we wanted. Magnetism you can’t separate the poles of a magnet. Magnets always come as a dipole, two poles. They always come stuck together.0154

Even if we took a magnet and broke it into two pieces it’s not like we get the north half and the south half. Now we just have the north half also has it’s south half.0168

The south half now also has its north piece. The other part is always there, it’s just a gradation of south to north, and so if you take it, it’s just the difference between north and south.0176

There’s always going wind up being this magnet coming up. We’re always going to wind up having a dipole no matter what we do.0185

That said, we have never found a monopole and just because we haven’t found a monopole, one pole by itself, doesn’t mean it’s absolutely impossible. It might be possible to get things out from a dipole.0192

I have repeatedly said that its impossible and all these sorts of things but that doesn’t mean that there’s no way that it could be….just because I’ve said its impossible doesn’t mean it’s actually impossible.0204

I’m just saying so far we’ve never found it. It’s really unlikely that we’re going to find one, we haven’t found it, but it doesn’t completely bar the possibility.0216

Like all things in science, we have to be prepared for unexpected changes. Things that we are not ready to…our theory isn’t ready for yet.0225

That said, our theory is pretty strong at this point, it seems pretty likely that we’re not going to find a monopole.0234

Here’s the really amazing thing about magnets. This is mind-blowing. A moving charge creates a magnetic field and the space around it. If you take a charge and slide it through the air, you slide it through space; the area around it has a magnetic field as long as it’s moving.0242

As long as it’s changing location a magnetic field is around it. We won’t get into the why, it involves things that are a little beyond us at the moment.0262

It involves moving electric fields and connections to special relatively and things that we haven’t talked about and are not going to get the chance to discuss.0271

This is amazing. This is huge ramifications which we experience daily. Let’s look at why these charges, why moving charge would cause magnets in like a bar magnet, that we can stick to a refrigerator.0280

Even when an atom is still the electrons are moving. They orbit the nucleus, so they’re moving around the nucleus and at the same time they’re spinning like tops.0297

We’ve got some nucleus, some super dense nucleus in the middle and around we’ve got an electron moving. The electrons moving around it in a circle and as it’s moving it’s also spinning itself.0305

Its spinning like a top as its orbiting the nucleus. That combination of its own motion and its spin and actually its more of the spin than its own motion creates a magnetic field.0319

Its moving charge, so even atom and every object is full of moving charge. Since every object has many, many, many atoms inside of it, each one of those atoms has a lot of moving charge and all those moving charges create magnetic fields.0331

In most objects these electric charge motions are evenly distributed on the whole, half the electrons create a magnetic field pointing one way while the other half point the other direction.0346

The spins wind up…one of them is spinning clockwise, the other is spinning counter clockwise effectively.0356

Because of that, that one of them points one with its magnetic field, the other one points the other way and so they cancel out to nothing.0362

We experience the magnetic field as being nothing. Just like with net charge, there is a lot of positive charge, there’s a lot of negative charge, they’re all right next to one another.0368

From our point of view there’s no charge, there’s no net charge. Just like there’s no net magnetic field.0376

However, in some materials like iron, the motions are unevenly distributed. There’s more of one electron spin type than the other. If we’ve got more of one electron spin type than any other then we’ve got a little bit more of magnetic field than the other type of magnetic field.0382

There are these many tiny magnetic domains each operating as its own magnet. This is a very blown up piece of magnet where this…this is actually not a magnet, this is just some chuck of iron.0399

Each piece of this iron has its own little magnetic domain. Each one of these magnetic domains winds up having its own magnetic field direction.0411

If it’s like this and it just a bunch of magnetic fields, they all cancel each other out. On the whole these tiny magnetized sections, there’s so many of them, they’re all pointing randomly. We don’t notice anything, the iron has a lot of stuff happening inside of it but since it’s pointing all directions at once it’s just noise.0424

It just cancels itself out, nothing really comes of it. If we bring up a magnet near the object, all of a sudden each of those sections, each of those magnetized domains…each of those magnetic domains, they’re each going to spin into alignment and we’ve created a temporary magnet.0442

We’re going to get all those electrons lining up; they’re all going to point the same direction because there’s this other magnet near them.0460

We’re going to temporarily magnetize this chunk of iron. We’ve got this permanent magnet that shows up on the scene.0466

Each one of those magnetic domains goes “Oh, those guys are pointing that way. I’m being pushed…I’m now in line with that”.0474

It’s in line with that, that happens to all of its friends, so all of the friends are in line and they’re also now reinforcing their own magnetism.0481

Each one of the guys, if this guy tried to spin on his own, if he tried to spin back to some other direction even without the magnet there.0487

If all the other guys were already pointing in one way, they all say “Nope, you got to go this way”, and they’d all push him back into position.0496

As long as the whole group is moving as one, the whole group is pointed in some direction; any person who randomly starts to drift off will wind up getting pointed back to it.0502

If we go and we remove the magnet, most pieces of iron, they are not so magnetized over the course of being next to that other magnet that they’re just going to wind up their own random thermo motion.0512

The atoms moving around, it’s going to wind up canceling out and they’re going to sort of bounce their own magnet domains until they’re eventually each random again and we’ll have lost the magnetic-ness.0524

This is why when we take a bar magnet, some sort of magnet, and we stick it to a fridge, the fridge has iron inside of it. We stick it to the fridge and the fridge magnetizes to it. It puts a force on it because on the other side, that iron goes “Oh, it’s a magnet.”0532

It all flips in magnetic direction, so now we’ve got south, north, they stick together, they’ve got a force. We pull the magnet away and we stick up another piece of iron in its place, it doesn’t do anything because already demagnetized itself.0548

It’s lost that magnetism in the period of time it took to just put up a nail or something next to the spot that had had magnet.0561

We’re able to magnetize normal pieces of iron by putting them next to a magnet because on an atomic level they have this magnetic domains where they’re ready to take a magnetism, they just need to have one put over the whole group.0569

Back to that idea of moving charge creating magnetic field. If a moving charge creates a magnetic field around it then it would defiantly follows that current flowing, flowing current will create a continuous magnetic field.0584

If we have an amp of current flowing down a line we’re going to have a magnetic field around that amp of current.0598

The magnetic field makes a circle around the wire. If we’ve got a wire going like this, it’s going to circle around it so it’s going to wind up doing something like that.0605

At the higher up, it’s going to circle…it should be about the same size but it’s going to wind up circling around it.0619

If my arm was the wire, it’s going to go like this around it. At each point we’re going to see this magnetic field curving around it. The idea is the right hand rule, if you put your thumb in the orientation that current is moving and then curl your fingers on your right hand.0626

Thumb with the current and then curl your fingers that’s the direction that the magnetic field is going.0641

If we did the same idea and instead of just having a line we’d had loop like this. Well if we look inside of that loop, we’re going to wind up having a…never said to be the best artist.0647

We’re going to wind up having a bunch of different magnetic things. Along that current, if the current is flowing like this as it goes through, we’re going to wind up seeing…actually I might have put those blue on the wrong way, but we get the idea.0666

You could imagine it yourself, you could curl out a single loop in the air using your right hand and you’ll be able to see it. Kind of hard to draw this stuff, sorry I’m just not the best drawer.0686

We can create a loop that’s got a magnetic field going around it. What if we make a bunch of loops? The more loops we have for a given current, the more times that current is going to have the change to create a magnetic field in the same location.0697

If we stack our loops so it comes in and loop, loop, loop, loop, loop, loop, loop, loop, and then goes back out, well that means that all of a sudden we’ve got this massive magnetic field going through.0711

We’ve got this huge magnetic field because we’ve stacked all of those loops and we’re talking, in reality when you stack this in real life and real engineering, we’re talking hundreds maybe even thousands or even more loops.0724

Lots of turns in wire. We can repeated put down many, many loops and by having the current flow through all those loops at the same time, we’ve got a bunch of charged moving in one combined area.0735

We’ve got a really strong magnetic field. Each loop backs up the magnetic field. Since each loop provides one magnetic field worth of strength depending on the strength of the current running through.0747

If we have a thousand loops, we’re going to multiply that by a thousand because we’re layering it one on top of the other. We’ve got a thousand times what it would have been if it was just one.0757

If we’ve got many loops of wire and we run current through it, we’ve created an electromagnet. We’ve created a magnet that operates when electricity runs through it.0767

When we run current through all these loops we create a magnet. It becomes a magnet because we’ve run current through it, because we’re able to put many magnetic fields on top of it, we can make it into a very noticeable, very, very strong magnet.0776

This idea is what gives us the ability to have electric motors. If we run a current through a loop or loops of wire it’s going to create a magnetic field.0792

If we have that loop and in general, in real life we’d wind up having to engineer with a loops, many, many loops of wire but for ease we’ll say it’s just one loop to imagine it.0803

If we have a loop and we run current through it we’ve got a magnetic field. Then if we place this loop or these loops into another magnetic field, those magnetic fields will interact.0813

The magnetic field of our loops compared to the magnetic field of the permanent magnet it’s next to, we’re going to have two magnetic fields. Those two magnetic fields will interact and a force is going to be applied to our loops.0823

Now with some clever engineering we could put that loop on an axel and we could have the current constantly flow through it, this is the clever part of engineering and we’ll be able to get the force to not just push on it once, but we’ll be able to get it to push on it and push on it and start spinning it.0835

We’ve got some loop of wire and then we manage to have that magnet push down this way so it spins it, it torque it.0852

If we’ve got this attached to some axel like this, where the wire is running down on either side of it.0863

Then we’re able to run a current through it, the axel starts to spin so we’ve got this spinning axel, we’ve got current running through it creating this…we’ve got current running through it creating a magnetic field.0873

It’s interacting with the other magnetic fields where it’s providing a force. That force provides a torque because its pushing on one edge, that we design it to make sure it pushes on one edge and so that torque causes the whole thing to start to spin.0884

We get it to spin fast enough by putting enough current through it by letting it run for a while; we’ve got a spinning axel. A spinning axel is the output shaft of a motor.0897

If we get something to spin then we can put that spin into something else and that’s exactly what happens in the electric motor of a car, the electric motor of an RC, the electric motor of…any electric motor is something has managed to spin.0907

We get something to spin up. This is what happens in a fan, in fan blades. The electric motor there is causing something to spin and it’s causing the spinning because it’s got this resistance between magnetism and the magnetism of the loop.0917

We can create a motor by just having electricity. This is really cool. Now if we do the reverse of it, it turns out that the reverse actually works.0935

Moving charge creates a magnetic field. It turns out the reverse is also true. If a change in magnetic field will induce a current. If we have a magnetic field change around a loop, change around a conductor, it’s going to induce current.0943

Its going to put a voltage on it. If we pass a loop of wire through a magnetic field, say we’ve got some magnetic field just hanging out here and then we take a loop of wire and we push it this way through it.0958

As it passes through the magnetic field it’s going to wind up having some current because it’s going to see a change in magnetic field.0975

It’s moving through it so it’s going to see a change. Alternatively we could put it in place and just spin it. As it spins, it’s going to see different ways of looking at that magnetic field, so from the point of view of the spinning loop, it’s constantly seeing new kinds of magnetic fields.0983

It’s seeing a constantly changing magnetic field. If we spin the loop in a stationary magnetic field, that loop will produce current. If we mount that loop on an axel and then we spin that axel and it’s in a magnetic field, that loop is going to generate current.1000

If we have a bunch of loops on that axel and then we spin the axel really quickly through a motor, then we can generate large quantities of current and this is the idea that powers all of the electric factories, pretty much almost everyone, not true for solar cells, but most electric factories are going to have something like this.1018

If we have an electric power plant it’s going to be based on this idea; coal uses that coal to boil water to cause a steam turbine to spin. That turbine is the spinning thing for this loop. It’s in a magnetic field so it spins, we create electricity.1043

In a nuclear power plant it’s the same idea, we’ve got rods of nuclear…we’ve got nuclear elements that are putting out energy, the put out heat energy as they decompose, as they break down.1062

We’re going to wind up using that heat energy to boil water. We boil water; once again we push it through a steam turbine. That steam turbine spins this loop of wire, we get electricity.1078

A hydroelectric dam, a hydroelectric plant does it even more directly. It just has a flowing amount of water, either going a waterfall or in a river and it just immerses a turbine, the flowing water spins that turbine, that turbine spins the axel, spins the loop.1090

Once that loop is spinning in the magnetic field, boom, we’ve got electricity. This is a really, really cool idea. It’s the reason that this connection between magnetism and electricity, it’s the reason why we can all have TVs, why we can have power outlets that we can plug all of this energy into.1107

Is because we can change these sources of energy, we can move our energy around. It’s not that the electricity is creating energy; it’s that we’re taking the kinetic energy of the water.1123

We’re taking the chemical energy of the coal and putting that into kinetic energy of steam and then taking that kinetic energy and we’re converting it into electricity which we then pipe to our house.1134

Any of these things. We’re converting some form of an energy into something that manages to get it put into electricity and that change over moment, when it changes over from whatever it was before, whatever kinetic….it becomes kinetic energy to spin the turbine and through magnetism is manages to transform to electricity.1142

This is really, really cool and this is the reason we have technology. This is the reason I’m currently able to teach you through the internet is because the fact that we’ve these generators working.1159

That there’s this really cool thing about magnetism. This is just awesome. Alright, we’re ready for some examples.1169

If you isolate a magnet from air currents and you support it in some way that it can rotate frictionless, you’ve created a compass.1176

That compass, its North Pole will eventually point to the Earth’s geographic North Pole. Now it’s actually not precisely the Earth’s geographic North Pole but it’s pretty close for the purposes of exploring the wilderness, it does a great job.1183

It points really close to the North Pole so we can treat it as pointing to the North Pole. If it’s the North Pole of the compass, the magnetic North Pole of the compass pointing to the Earth’s North Pole, what kind of magnetic pole is the Earth’s pole?1196

Remember, North to North repulse. North to South attracts. If we know, if we know that the compass is defiantly a North Pole which we’ve been told that, that’s how it works, that’s how it’s defined.1211

If the compass has a North Pole and it’s spins to point to the Earth’s geographic North Pole, then the magnetic pole has to be South because that’s the only one that would attract it.1231

The Earth North geographic is actually a south magnetic, pretty cool. Second example.1242

To run a generator or an electric motor you need some sort of powerful magnetic field. How could you create such a field if you don’t have a supply of extremely powerful permanent magnets?1258

Even if you did have a supply of extremely powerful permanent magnets, they actually…real life generators pretty much don’t run on permanent magnets in them.1268

What they run on is the idea that you talked about earlier; electromagnets. If you’re going to have…if you need to be able to have some power, some control over what you’re going to have be the field that you want it to run through.1276

If you want to have precise control over what kind of current gets put out, you’re going to need some control over that field or you might want some control over that field.1290

We need to be able to choose it ourselves and to choose it ourselves we use an electromagnet. Then we can choose how many loops of wire we use and what kind of current we run through it.1299

Of course you’ll need some way to generate the current through…for this yourself, so you might need either some sort of strong battery to get it started and then you can just leech off of the electricity being generated in the first place.1307

Off of the generator or you could just crank it by hand or have some other generator that setting it up. All sorts of different possibilities but you are going to have to use some sort of electromagnet.1320

Or you could use extremely powerful permanent magnet but if you don’t have access to them, this is the trick; you’re able to use an electromagnet to generate the magnetic field so that you can then generate even more electricity.1332

You’re not getting something for free here by the way. I just want to point out that really quickly. It’s not that you get free electricity because it’s being….it doesn’t spin freely.1343

The magnetic forces are now actually….the loop has…since it now has current flowing through it, not it has an induced magnetic field as well.1351

That magnetic field is actually fighting it out with the other one so it’s trying to reduce its spin. We don’t get…there’s no free lunch in physics.1359

You don’t manage to spin it and then just get this free unlimited supply of electricity, free unlimited supply of energy.1366

Conservation of energy wouldn’t be happy with that. It’s spins and then its own magnetic field resists the magnetic field that’s already there and so it tries to slow itself down.1371

That’s what the work we have to put into it and the work that we put in to overcome it is the energy that we get out of it electrically.1381

Final example. One way to damage or destroy the magnetic properties of a permanent magnet is to heat it or repeatedly strike it.1388

Why? Remember what a magnetic looks like. If you have an iron magnet, you’ve got all the little arrows pointing in the same way. When it was just a normal piece of iron they were all pointing in random directions.1397

When you get a real full on magnet out of iron the same thing happens, it just manages to point the same way and then stay that way.1410

You’ve magnetized it enough that it’s able to stay locked in and if one guys tries to rotate, he’s going to wind up being pushed back into the orientation that he was by all his neighbors who were also currently in their location.1418

What would happen if you heated it? Remember heat means kinetic energy, if you heat it then you cause the whole thing to shake. This guy shakes, this guy shakes, this guy shakes, this guy shakes, they all start to shake.1430

If they all start to vibrate then it means that they’re vibrating, they’re off a little bit. If they’re off a little bit they might start to point in the wrong direction.1444

If they all start to point in the wrong direction simultaneously they no longer have that peer group effect where they’re all holding each other pointing in the right way.1450

They start to all vibrate a little bit, they start to all get off their track and boom it’s lost. We lose the magnetic property.1459

They’re going to start to disappear and then they’ll start once again pointing in random directions. Now if only one or two manage to turn to a random direction it might manages to remagnetize itself because it could be pushed back into alignment by its peers.1466

If they all start to turn because we’ve heated it up enough, if we heat it to red hot they’re all going to defiantly start to change and jostle and they’re going to lose its magnetic properties.1479

Same basic idea behind striking it. If we strike it, we literally jar, we literally shake those atoms. We might shake those atoms so much; shake those connected molecules that they’re going to wind up starting to spin.1490

They spin a bit and once again we manage to set off this domino effect where they all sort of spin the wrong way and we lose the magnetism. We’ve demagnetized the magnet.1503

Alright, I hope that gives you some idea of magnets and what’s really going on there and why it’s so incredibly useful and important for all of humanity that we have…that there is this thing in nature.1511

I suppose that’s true of all the natural forces. The electric force, the gravitational force, you and I wouldn’t be here if the gravitation force weren’t holding us to the Earth and keeping us spin around that nice warm Sun.1523

We’ve got a lot of cool things happening in physics and all of it coming together is the reason we have the things we have. Hope you’ve learned something; hope you’ve had a great time.1533

It’s been a real pleasure teaching this class and best of luck.1540

See you later on Bye.1544