Hello, my name is Dr.

George and I'm going to help you work through this lesson called "Plotting magnetic fields." It's part of the unit "Electromagnetism." The outcome for this lesson is I can plot and describe magnetic fields around bar magnets.

Now here are the keywords for the lesson.

I'm not going to read them all out to you now.

If you'd like, you could pause and read this, but I'm going to introduce these keywords through the lesson.

But this slide is here in case you want to come back anytime and check the meanings.

This lesson includes a practical investigation and it has two parts, "Magnets and magnetic fields" and "Magnetic field lines." So let's start.

First of all, magnets attract magnetic materials, and magnetic materials include the metal elements iron, cobalt, and nickel, and also mixtures of metals called alloys that contain these elements.

An example is steel.

It includes iron, so it's also magnetic.

A magnetic material can be made into a magnet.

So for example, an iron nail.

There is a way of making that into a magnet, but most iron nails are not magnets and will simply be attracted to magnets.

Magnets don't attract objects that are made out of non-metals 'cause they're not magnetic or other metals apart from the ones here.

So these items here, well, if that can is steel and those pins are steel or perhaps iron, they'll be attracted.

The other objects aren't attracted, they're not magnetic materials.

Now a question to check if you've been following along.

Which of the following objects are magnetic? Copper pipe, wooden ruler, iron nail, or steel knitting needle? In these questions through the lesson I'll give you five seconds to think, but if you need longer, just pause the video, and press play when you're ready.

What matters here is what the objects are made of.

Two of them, the iron nail and the steel knitting needle, are made of magnetic materials, so we say they're magnetic.

Copper is a metal but is not a magnetic metal and wood is a non-metal, and none of those are magnetic.

Magnetic force is a non-contact force.

It's one of those forces that can act between objects even when they're not touching.

Gravitational force is another non-contact force.

So the force acts at a distance, but it does get weaker the further away from the magnet you get.

If you haven't noticed, you can pick up paperclips with a magnet, but if they're close enough to the magnet.

And in this region around the magnet, the magnet is having an effect, it's influencing other objects, and we call that region of influence a magnetic field.

And when objects come into the magnetic field, they will be attracted to the magnet if they're made of magnetic material.

Or if magnets come into the field, they may be repelled or attracted depending on which way the magnets are facing.

We can represent this invisible field with lines called magnetic field lines, and we'll look more at those later.

Now a question.

Which of the following can a magnetic field do? Look carefully at the options and pause if you need longer than five seconds.

A magnetic field can pull magnetic objects towards the magnet before the magnet actually touches them, and it can also push other magnets away before they touch.

What it doesn't do is attract tiny pieces of tissue.

You might have seen a charged object like a plastic rod or a balloon that's been charged positive or negative attract little things like bits of tissue.

That's electrostatic force and it doesn't happen with magnets.

As you may know, every magnet has two poles, often one at each end, although you can get other shapes of magnet like a horseshoe shape.

Unlike poles, poles that are different from each other, attract.

And like poles, poles of the same type, repel; like this.

You may already know that the Earth acts like a giant magnet and it has a magnetic field around it and that field influences the needles of compasses.

The needle of a compass is a tiny, freely-swinging magnet, and that's why compasses can tell you which way is north.

Now I'm going to use the full name for the N end of a magnet, the north-seeking pole.

The north-seeking pole of a magnet is attracted to Earth's magnetic North Pole.

Notice the capital letters there for North Pole, because the Earth's North Pole is a place, it's a geographical location.

It's at the top of this picture, it's in the Arctic.

So the N pole of a magnet is attracted to Earth's magnetic North Pole.

And the S pole of a magnet, the south-seeking pole, is attracted to the Earth's South Pole at the bottom of the picture in the Antarctic.

So if this magnet could swing freely, could move freely like the needle of a compass, it would want to turn like that so that its N end is pointing towards the Arctic and the S end is pointing towards the Antarctic.

And now a question for you, true or false.

At the North Pole, Earth has a south-seeking magnetic pole.

So choose your answer and then choose an explanation to justify your answer.

Why is it right? How do you know it's right? Press pause if you need longer than five seconds to think.

And the correct answer is true.

Earth has a south-seeking magnetic pole in the Arctic, in the region that we call the North Pole of the Earth.

Now that means I'm going to be very careful with what I call the poles of magnets in this lesson.

So if I say North Pole, that could lead to confusion because I might mean the N end of a magnet or I might mean the location on the Earth that is in the Arctic.

So when I talk about the north pole of a magnet, I'm gonna use its full name, north-seeking pole.

And for the S pole of a magnet, I'll use the full name, south-seeking pole.

So the reason why you could know that Earth has a south-seeking magnetic pole at the North Pole is if you remember that unlike poles attract and you know that north-seeking poles of magnets such as the north end of a compass are attracted to the Earth's geographic north.

Let's look more closely at magnetic fields.

The magnetic field around a bar magnet is invisible but it can be represented using lines with arrows like this.

And if we take a small compass and put it at different places in the field, it's affected by the field.

Normally, a compass points towards the Earth's North Pole, which is the Earth's south-seeking pole.

But when this compass is close to another magnet, it's affected more strongly by this magnet.

So look at the way the compass is pointing in the different places.

Can you see the relationship between the way the compass points and the way the field line and its arrow has been drawn? Next I'm going to ask you to explore a magnetic field using a compass.

But you've already seen the magnetic field of a bar magnet, so I'm going to ask you to put two bar magnets together with unlike poles facing like this, and use a plotting compass, a small compass, to explore the magnetic field and draw field lines.

So put your magnets on a piece of paper, A4 is a good size for this, and get a sharp pencil.

And it's a good idea to draw around the magnets so that if they accidentally move during this you can see where to put them back to.

So take a compass and put it somewhere so that it's touching a magnet and so that one end of its needle is pointing at the magnet.

There are some places where you could touch a magnet and it won't be pointing towards the magnet.

Find somewhere where it is.

And then take your pencil and draw a dot at each end of the compass needle.

The compass needle is the little moving magnet inside the compass, like this.

And then move the plotting compass so that the end of the needle closest to the magnet now points at the dot that's furthest from the magnet.

Sounds complicated, but just like this, okay? And then draw a dot at the other end of the compass needle, like so.

And keep going like that, and either you'll end up coming back around to some other place on the magnet and then stop, or you'll get to the end of the paper and then stop.

Then join the dots with a curve and add an arrow, and the arrow should point away from the north-seeking pole at the end of these bar magnets or towards the south-seeking pole at the other end if it's close to that end.

And what you've just drawn is a magnetic field line.

So to get a sense of the whole pattern, repeat those steps to draw perhaps six to eight field lines, and try to get some that are above some that are below.

Get field lines going to the left and the right of these magnets.

Before you start, a question to check that you understood how to place the compass.

So which of these plotting encompasses is lined up with the dots and the magnet correctly? Pause if you need longer than five seconds.

The correct answer is A.

It's the only one in which the compass is right next to the dot and the needle is pointing to the dot.

So make sure that's how you place your compass when you're drawing these field lines.

And now here's a single page showing all of the instructions.

And I'd like you to do this investigation and you can refer to this page if you need to remind yourself of what to do.

So press pause, take as long as you need, and then press play when you're ready.

And here's the shape of the magnetic field around two bar magnets.

You may not have exactly the same number of field lines, they might not be exactly the same shape, but I hope you can see that your magnetic field is consistent with what's shown on this page.

Now let's go on to the next part of the lesson, "Magnetic field lines." So I hope what you noticed is that the magnetic field around two bar magnets, if they're placed end to end like this, is the same as the field around one bar magnet.

Well, it's the same shape, it's just stretched.

The poles that are touching between these two magnets, they cancel each other out.

We don't see the field of a bar magnet repeated twice, we just see the field of one long bar magnet.

The two bar magnets together is actually just behaving like one long magnet with one north-seeking pole and one south-seeking pole at the ends.

And in fact, even a single magnet, we actually know it's made of lots of tiny magnets placed end to end roughly lining up with each other.

We could represent it as something like this.

If we have a solid magnet that is made up of these tiny magnets, we call them domains.

We don't actually build a magnet by sticking domains together.

There are other ways of turning a piece of magnetic material into a magnet, but what it will have inside it is something like this.

There are actually many, many more domains in a real magnet than this.

They're much, much smaller.

So inside the magnet, all of these little N-S pairs that are next to each other cancel out, and so we only have poles at the ends.

So now how many poles is it possible for a magnet to have? Press pause if you need more than five seconds.

A magnet can have two poles, and that's all.

You've seen that if you stick two magnets together, you end up with one long magnet that still has two poles.

And it also turns out if you break a magnet in half between the N and the S ends, what you don't get is one magnet with just an north-seeking pole and one with just a south-seeking pole.

You end up with two smaller magnets that each have an N and S pole.

And you can understand that by thinking of those domains.

If we just take half of a magnet, it still has all those little N-S domains that cancel out between the two ends and leave us with a north-seeking end and a south-seeking end.

If a magnet had zero poles, well, it just wouldn't be a magnet.

And the magnetic field lines around a magnet show the direction of force acting on a north-seeking pole of a second magnet.

So this is what the arrows mean.

If we bring in this second magnet into the field, its north pole, north-seeking pole, is going to experience a force in the direction of the arrows.

And if it was free to move, perhaps it's on ice or it's got little wheels on it, then it's going to be pulled along a magnetic field line.

And I mentioned earlier that a magnet exerts stronger forces on objects that are closer to it, so this smaller magnet will be pulled towards the larger magnet with an increasing force.

And that's shown in the way the magnetic field lines are drawn.

They're closer together where the magnetic field is stronger, where the magnetic field exerts stronger forces.

And also you may have noticed that magnetic field lines never cross over each other.

So which of the following correctly describes the size of the force attracting magnetic objects to a magnet? Is it greater where lines have a free end? Greater where lines are closer to the magnet? Greater where lines are closer together? Or always greater closer to the magnet? Press pause if you need longer than five seconds to think.

The answer, as I said before, it's greater where the lines are closer together.

So near those N and S poles, we have a stronger force attracting a magnetic object.

Now I'm going to ask you about the magnetic field between two magnets that are a bit different from a bar magnet.

These magnets have their poles on their large flat faces.

Look at how these two magnets are arranged, and I'd like you to have a go at drawing the magnetic field lines between them.

I haven't shown you a field like this, so you'll have to use what you've learned about fields and how we draw field lines.

And then when you've done that, describe the magnetic field between the two magnets.

Take as long as you need.

Press pause, and when you're ready, press play.

Let's look at those field lines now, and they look something like this.

Remember, the arrows represent the force on a north-seeking pole, so they point from the north-seeking pole on the left towards the south-seeking pole on the right.

Don't worry if yours doesn't look exactly like this, but I hope your arrows point in the same direction.

Now how could we describe this field? Well, we could say the magnetic field is the same everywhere between the two magnets.

It is a uniform magnetic field.

It's the same everywhere in the sense that there's the same direction and size of force on a north-seeking pole anywhere in the space between these two magnets.

We could also say the north-seeking pole of another magnet would be forced in a straight line from one magnet to the other.

So well done if you got some of those ideas into your answer.

We're at the end of the lesson now, so here's a summary of what we've done.

All magnets have a north-seeking and a south-seeking pole.

Magnets attract magnetic materials that are made out of the elements iron, cobalt, and nickel, or alloys like steel that contain one of them.

Like magnetic poles repel each other, and unlike poles attract.

Magnetic field lines show the direction of force acting on the north-seeking pole of another magnet.

The magnetic field is stronger where magnetic field lines are closer together.

Magnetic field lines do not cross each other.

So well done for working through this lesson and I really hope you now know more about magnetic fields than you did at the start.

I hope to see you again in a future lesson.

Bye for now.