Hello, I'm Dr.

George and I'm going to work with you today on this lesson called the magnetic field of a bar magnet, which is part of the unit magnets and electromagnets.

I hope you'll enjoy it.

The outcome for this lesson is I can plot a magnetic field around a bar magnet.

You might not know what that means yet, but you will by the end of the lesson.

Here are the key words for the lesson.

I'm not going to go through them now because I'll introduce them as they come up during the lesson.

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

The lesson has two parts, plotting a magnetic field and magnetic field lines.

Let's start.

In the photo, you can see the bar magnet must be exerting a force on the paperclip because it's making the paperclip hang in the air.

So that force must be able to go through the space between the magnet and the paperclip.

And we say there is a magnetic field in the space around the magnet.

It's a sort of force field.

That might sound a bit science fiction, but it's science fact.

Now, you can use iron filings to show where this invisible field is.

Iron filings are just thin little pieces of iron.

And the iron filings get magnetised by the magnet, and then they align, they line up with the magnetic field.

So in this picture, the iron filings are showing you something that you can't normally see around the magnet, and it's the field.

What do you think a magnetic field around a magnet does? I'll give you five seconds, but if you need longer, press pause and press play when you're ready.

And answer A is correct.

The magnetic field affects objects.

Even when they're not touching the magnet, it pulls on magnetic objects, like that paperclip, before the magnet even touches them.

Magnetic field also can push other magnets away before the magnets touch.

So a magnetic field affects other things that are in the space around the magnet, even when they're not touching it.

But it doesn't attract tiny pieces of tissue.

You might have seen a balloon with an electric charge attract little bits of tissue, but that's an electrostatic force, and magnets don't do that.

Well done if you picked out both correct answers.

You're going to explore the magnetic field of a magnet.

You're going to make this invisible field visible.

And a way that you can do that is using a plotting compass, which is a small compass.

Like any compass, it's north-seeking pole is attracted to the magnetic north pole of the Earth, as you can see here.

So the red end, the darker end of the compass needle is pointing north.

And that's because Earth has a magnetic field.

It behaves like a giant magnet, and the compass needle lines up along the field.

The south-seeking pole of a plotting compass needle will be attracted to the magnetic South Pole of the Earth, and so the lighter end, the white end of the needle is pointing south.

Here's a question about that.

Which of these images shows the correct alignment of the compass needles? In which image are the needles pointing the right way? And remember that the earth isn't really a flat circle, it's a sphere, it's a ball shape.

So I'll give you five seconds, but if you need longer, pause the video and press play when you're ready.

And the correct answer is A.

Those compasses have their north-seeking pole pointing to the Earth's North Pole.

B looks almost right, but if you remember, the earth is a 3D shape, it's a ball, the right hand and left hand compasses just aren't really pointing the right way.

So a magnet and a plotting compass both have north-seeking poles, because a plotting compass needle is a magnet itself.

Normally the north-seeking pole of the compass, the red end, points to the North Pole of the Earth, and the south-seeking pole of the compass swings around to point to the South Pole of the Earth.

But if you put the compass close to the magnet, it's in the magnetic field of this magnet and it changes the way it points.

So the like poles of the magnet and the compass repel, and the compass points away from the N-pole of the magnet.

If I say N-pole, it's just a short way of saying north-seeking pole, and S-pole for south-seeking pole.

So this little plotting compass is no longer showing you which way is north on the planet.

It's pointing away from the North Pole of this bar magnet.

And if we get some more plotting compasses and put them around the bar magnet, they all point away from the magnet's N-pole.

And so they're revealing something, they're showing us something about the bar magnet's magnetic field.

And if we put a plotting compass near the S-pole of the magnet, it points towards it, and we can put some more here, and they all point towards the S-pole.

So that, again, is showing us something about the invisible field around that end of the magnet.

So, what part of a bar magnet does the needle of a plotting compass point towards? If you need more than five seconds, pause the video and press play when you're ready.

And the answer is the S-pole of a bar magnet.

So the north-seeking end of the needle of a compass points towards the south-seeking end of a bar magnet.

So we can use a plotting compass to trace the field to reveal the field around a bar magnet.

So I'll tell you what we do, and then you'll be doing this yourself soon.

We place the magnet in the middle of a piece of paper, A4 is a good size for this, and draw around the magnet with a sharp pencil.

Draw a dot at the corner like this, and then place a plotting compass so that one end is as close to the dot as possible.

Then draw another dot at the north-seeking end of the plotting compass needle, and then move the compass so the end of the needle is over the new dot.

Draw another dot at the north-seeking end.

Move the compass again, so the end of the needle is over the new dot.

Draw another dot, and you keep going like this.

And when the dots reach the edge of the paper, stop making dots and draw a smooth curve through them.

What you've drawn is called a field line.

Add an arrow to show which way the plotting compass needle was pointing.

And you can draw more field lines by starting at different points next to the magnet, so you can show more about the magnetic field around it.

Before you do this, quick question, which of the following images shows the plotting compass pointing in the correct direction? And I'll give you five seconds, but if you need longer, press pause and press play when you're ready.

The answer is B.

B shows the north-seeking end of the compass needle, the red end pointing away from the north-seeking end of the bar magnet.

Now I'd like you to do this.

So you're going to plot the field around a bar magnet using a plotting compass, and the dots around the magnet show you the starting points for the compass each time you want to draw a field line.

Take care not to move the bar magnet, but if you do, you can put it back where you've drawn around it.

Take as long as you need.

Press pause and press play when you're finished.

Let's take a look at the field lines around a magnet.

This one here is drawn without the dots.

You might not have the exact same number of field lines.

They might be slightly different shape, but your magnetic field should look similar to this one.

Now let's look more at what's going on with these magnetic field lines.

The animation shows a plotting compass moving near a magnet, and we see how its compass needle aligns with the field.

And here are a whole lot of compass needles and the way they would align at different positions.

Let's watch that again.

So here's one compass moving, and wherever we put it, it's lining up with the invisible field of the bar magnet.

And if we had many tiny compasses, they would look like this.

Now, we've said that Earth has a magnetic field.

Well, we've said the Earth is a magnet, so it must have a magnetic field around it, and it turns out to be similar to the field of a bar magnet.

If you could trace it out, its field lines would have the same kind of shape.

So if you put a plotting compass in the Earth's magnetic field, it's affected by that field and it points in the direction of the field lines.

So that's what's going on when a compass points north.

Another way to see field lines is to sprinkle iron filings around a magnet and they get lined up along the field lines.

If you look carefully, you can see that they're making shapes that look a bit like the field lines that you drew using the plotting compass.

And here they are.

So it's another way of seeing this invisible field around a magnet.

By the way, one thing to keep in mind is that in reality, the magnetic field is all around the magnet in 3D.

What we've drawn here is a cut through the magnetic field in two dimensions.

But let's think about how we can describe this field that we've drawn on the paper.

First of all, notice that the arrows all point out of the north-seeking pole.

It looks like the magnetic field lines are coming out of that pole, and going in to the south-seeking pole.

So we know that the north seeking end of the compass needle is pushed and pulled along the field lines from the north-seeking pole of the bar magnet to the south-seeking pole, a bit like this.

And we can also say that the field lines of an isolated magnet, that's a magnet that's not near any other magnets are continuous.

What I mean is where the field lines end, that's because you ran out of paper.

If we could continue the field lines beyond the edge of the paper, get some more paper, we'd find that they follow round to make continuous loops.

So these two field lines are actually part of one long continuous field line, like this.

And these two are also part of a single loop, and that's the same for all of these field lines.

The field lines also don't cross each other.

They never cross over each other, and there are no gaps in the lines.

And the pattern of the field lines is symmetrical.

The arrows aren't, but the lines themselves have symmetry as if there's a mirror line down the middle of this magnet.

Notice also that the field lines are more concentrated.

They're closer together near the poles of this magnet.

And that actually represents a stronger field, and the bar magnet is strongest at its poles.

And now a question, what do field lines and their arrows show about the size of the force of a magnet on magnetic objects or on other magnets? Think about that.

Pause the video if you need longer than five seconds.

And the answer is that field lines show that the force of a magnet is greater where the lines are closer together.

That's what we can see about the size of the force by looking at the field lines.

Well done if you got that.

Let's have a summary of rules about magnetic field lines and what they're like.

They start at the north-seeking pole and go to the south-seeking pole, if we follow the arrow around.

They don't cross each other.

They don't have any gaps.

And there are more field lines concentrated in an area around the poles, and this is where the field of the magnet is strongest.

Now, what do the lines and arrows show about the direction of the force of the magnet on magnetic objects or other magnets? So what do the lines show about force direction? Five seconds.

Pause if you need longer.

And the correct answer is, the lines or the arrows on the field lines show direction the north-seeking pole of another magnet is pushed or pulled.

Try to remember that, that the arrows show the direction a north-seeking pole wants to point, not a south-seeking pole.

And now I'd like you to use what you've learned during this lesson and have a go at writing a description of what magnetic field lines are, and how they can explain how compasses work.

You might like to plan ahead a little bit about how you're going to organise your answer, and what points you'd like to include.

Pause the video and press play when you've done, and I'll show you an example answer.

So here's an example of an answer to this question.

"Magnetic field lines go from the north-seeking pole to the south-seeking pole without crossing and without gaps.

They're symmetrical around a bar magnet, and the greater the concentration of field lines, the greater the strength of the field.

Magnetic compass needles will be pushed and pulled so that the north-seeking end will align in the direction of the arrows on the field lines, and will point to the south-seeking pole of the magnet that's causing the field." So compare your answer with this one.

It doesn't have to be exactly the same.

You don't have to make the points in the same order, but check which points you made and whether they match the points in this answer.

We've reached the end of this lesson, so here's a summary.

The space around a magnet contains an invisible magnetic field.

A compass needle is magnetic, and it will point towards the south-seeking pole of a magnet.

A plotting compass can be used to plot a magnetic field.

Field lines are continuous and go from the north-seeking pole to the south-seeking pole without crossing.

They form a pattern that is symmetrical.

So well done for learning about magnets with me.

I hope you enjoyed the investigation, and I hope to see you again in a future lesson.

Bye for now.