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For more information, please see full course syllabus of AP Physics C: Electricity & Magnetism
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Lecture Comments (2)

1 answer

Last reply by: Professor Dan Fullerton
Mon Nov 24, 2014 6:08 AM

Post by QuangNguyen VoHuynh on November 24, 2014

I would like to ask the reason why when the charge moves, it creates a magnetic field.


  • Magnetism is caused by moving charges.
  • All magnets have a north and a south pole. There are no magnetic monopoles.
  • Like poles repel, opposite poles attract.
  • Magnetic field lines make closed loops and run from north to south outside of the magnet.
  • Compasses are magnets which are free to align themselves with the net magnetic field.
  • Magnetic permeability is a material property relating to the ratio of the magnetic field strength induced in a material to the magnetic field strength of the inducing field. Highly magnetic materials have high magnetic permeability.
  • The magnetic dipole moment (or magnetic moment) of a magnet refers to the force that a magnet can exert on moving charges. It is analogous to the strength of a magnet.


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
  • Objectives 0:08
  • Magnetism 0:35
    • Force Caused by Moving Charges
    • Dipoles
    • Like Poles Repel, Opposite Poles Attract
    • Magnetic Domains
    • Random Domains
    • Net Magnetic Field
  • Example 1 1:40
  • Magnetic Fields 2:03
    • Magnetic Field Strength
    • Magnets are Polarized
  • Magnetic Field Lines 2:53
    • Show the Direction the North Pole of a Magnet Would Tend to Point if Placed on The Field
    • Direction
    • Magnetic Flux
  • The Compass 4:05
    • Earth is a Giant Magnet
    • Earth's Magnetic North Pole
    • Compass Lines Up with the Net Magnetic Field
  • Magnetic Permeability 5:00
    • Ratio of the magnetic Field Strength Induced in a Material to the Magnetic Field Strength of the Inducing Field
    • Free Space
    • Permeability of Matter
    • Highly Magnetic Materials
  • Magnetic Dipole Moment 5:54
    • The Force That a Magnet Can Exert on Moving Charges
    • Relative Strength of a Magnet
  • Example 2 6:26
  • Example 3 6:52
  • Example 4 7:32
  • Example 5 7:57

Transcription: Magnets

Hello, everyone, and welcome back to

I'm Dan Fullerton and this lesson we are going to introduce magnets.0003

Let us talk about some of our objectives.0007

First off, we are going to explain that magnetism is caused by moving electrical charges.0009

A key point, moving electrical charges create magnetic fields.0014

We are going to describe the magnetic poles and interactions between magnets.0019

We are going to draw magnetic field lines just like we did with electric field lines.0022

And recognize that mechanic permeability and magnetic dipole moment are properties of matter.0026

Let us start by talking a little bit about what magnetism is.0033

It is a force that is caused by moving charges.0036

Magnets are dipoles.0041

By dipole, what it mean is they had a North and a South.0043

You are never going to see a magnet that is just a North or just a South.0047

They always come in pairs, a North and a South.0050

They are dipoles.0052

The light poles repel, the opposite poles attract, very similar to electrical chargers.0054

Magnetic domains are clusters of atoms with electrons spinning in the same direction.0059

If you have random domains, then you have no net magnetic field, like we are showing here in this diagram.0064

The blue lines, the blue arrows is a representation of the magnetic domains.0070

If this is a material, we would have a bunch of different magnetic domains in different portions, all pointing at random directions.0076

When you add them all up, the net effect is 0.0082

If, however, you can get a bunch of those domains to roughly lineup in the same direction, 0086

you will get a net magnetic field which creates a very strong magnet as shown in the bottom picture here.0091

Types of fields, example 1.0100

Which type of field is presently removing electrical charge?0102

If it is an electrical charge, you must have an electric field.0106

If it is a moving charge, you are also going to get a magnetic field.0110

The answer is going to be C, you have an electric field because there is a charge.0113

And because it is moving, you get the magnetic field.0118

Magnetic field strength, if we give the symbol B, it is a vector.0124

It is a vector quantity and the units are tesla.0129

The tesla is a very large quantity of magnetic field.0132

Magnets are polarized, meaning we have two opposite ends, the North and the South.0136

The end of the magnet, the points toward the geographic North Pole of Earth is called the North Pole of the magnet.0142

The end point toward the geographic South Pole of the Earth is called the South Pole of the magnet.0149

Note that there are no magnetic monopoles.0155

Not that there cannot be theoretically be magnetic monopoles but to this point in time, nobody is ever isolated found an isolated magnetic monopole.0158

A North without a South, those do not exist currently.0168

Alright magnetic field lines make closed loops and they run from the North to the South outside the magnet.0173

If you look at the diagram down here, we are showing magnetic field lines.0181

Outside the magnet they run from North to South.0185

Note that inside the magnet, they continue to make a closed loop.0188

Inside the magnet, they run South to North.0192

Do not oftentimes ask about the magnetic field inside the magnet, more often than not, 0195

you are focusing on outside the magnet or they always run North to South.0199

They show the direction, the North Pole of the magnet would tend to point if it was placed in the magnetic field.0205

If we were to go and place compass right there, which lines up with the magnetic field,0210

our compass would point in the same direction as the net magnetic field.0216

The density of the magnetic field is known as the magnetic flux.0221

Just like we had electrical flux, now we have magnetic flux φ.0225

And because it is magnetic flux, magnetic flux oftentimes we are going to write it as φ B.0230

B being the symbol for magnetism, or sometimes you will see it as φ M for magnetism.0237

Those are equivalent.0242

Talking about the compass, the Earth is a giant magnet.0246

The Earth's magnetic North Pole is located near the geographic South Pole and vice versa.0250

The reason that is, we want compasses, the North end of the compass to point to the geographic North.0256

If this is the geographic North Pole up here, we want the compass that is placed somewhere to point the same direction.0262

That means, that this has to be if the Earth is a giant magnet, 0269

this has to be the Magnetic South here at the top of the Earth, according to this picture, which we call the geographic North.0273

And the magnetic North of the Earth is at the geographic South Pole, down the penguins live.0282

A compass lines up with a net magnetic field, encompasses on Earth point for the geographic North.0288

Because the geographic North is the magnetic South Pole of the Earth.0295

Magnetic permeability is a term that refers to the ratio of the magnetic field strength induced to the material 0302

to the magnetic field strength of that inducing field.0310

We are going to talk quite a bit about free space, or oftentimes we will use air as well, 0313

because the permeability of free space vacuum in air is roughly the same.0318

Free space has a constant value of magnetic permeability that appears in physical relationships.0324

It is given this symbol μ 0 or μ₀, the permeability of free space.0329

Its value is 4 π × 10⁻⁷ Tm/ A.0335

The permeability of matter has a different value from that of free space and 0342

very highly magnetic material such as something like iron will have very high values of magnetic permeability.0345

We can also talk about the magnetic dipole moment of a magnet .0355

That refers to the force of the magnet can exert on moving charges.0360

In simplistic terms, you can kind of think of it as the relative strength of a magnet.0363

Compare the magnetic dipole moment of a hydrogen atom to the magnetic dipole moment 0368

of a highly magnetize iron bar, which was going to have the higher dipole moment?0373

Of course, that is going to be the iron bar.0379

It is a much stronger magnetic field.0382

Let us get into a couple more examples here.0385

In the diagram, we have lines of magnetic force between two North magnetic poles.0388

At which point is the magnetic field strength greatest?0393

Just like we saw when we are dealing with electrical fields, 0397

we are going to have the strongest magnetic field where we have the densest, the thickest magnetic field lines.0401

The answer here would be B, because that is where we have the densest field lines showing.0407

In the diagram down here now, we have 1/2 kg bar magnet and a 0.7 kg bar magnet with a distance of 0.2 m between their centers, 0414

which statement best describes the forces between the bar magnets?0423

Because we have a North and North, remember like poles repel.0428

We are going to have a magnetic repulsion but they both have mass, and gravity always attracts.0433

We are going to have gravitational attraction.0438

Gravitational forces attractive that is true.0440

The magnetic force is repulsive, that is also true.0444

C must be our correct answer here.0448

Looking at a compass in a magnetic field, if we are showing magnetic field lines in the diagram below, 0452

and we put a compass over here at point P, which way is the compass going to point?0458

The compass is always going to line up with magnetic field lines so a compass would point in that same direction.0467

One last example here, the diagram below shows a bar magnet, which way will the needle of the compass place to A point?0477

Let us draw our magnetic field lines first realizing that they go from North to South, outside the magnet.0485

If we place a compass at point A, a compass lines up with a net magnetic field.0494

It lines up with the magnetic field lines so the compass would point to the right.0499

Our correct answer would be to the right.0505

That should give you a fundamental understanding of magnets that we are going to expand considerably in our next lessons.0509

Thank you so much for watching

Make it a great day.0517