Hello, my name's Dr.

George, and this lesson is called Strength of an Electromagnet.

It's part of the unit Electromagnetism.

This lesson includes a practical investigation, and the outcome for the lesson is I can accurately measure the strength of an electromagnet to analyse how different factors affect it.

Here are the key words for the lesson.

I'm not going to go through them all now.

I'll introduce them as we go along.

But you can come back to this slide anytime if you want to check their meanings.

The lesson has two parts, measuring the strength of an electromagnet, and current and the strength of an electromagnet.

Let's get started with a reminder about what electromagnets are.

To make an electromagnet, you simply take a piece of wire, make it into a coil, the shape of a spring, and put a current through the wire.

And there'll be a magnetic field around it.

It behaves like a magnet.

To make the magnetic field much stronger, you can wrap the wire around a soft iron core.

Iron is a magnetic material, so when the coil becomes magnetised, the iron becomes magnetised too, and that strengthens the magnetic field.

Soft here means it's very pure iron that easily becomes magnetised but also easily loses its magnetism.

So when you switch off the current, the coil stops being magnetised, and so does the core.

So what you have here is a switch on, switch off magnet, an electromagnet.

And the magnetic field around an electromagnet is very similar to the field around a bar magnet.

It has a south-seeking end and north-seeking end.

And we can represent the field using these field lines as shown here.

And they're very much in the same shape as they would be for a bar magnet.

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

So that's how we represent a stronger field when we draw magnetic field lines.

So we have a particularly strong field close to each of the poles, and the field gets weaker as you move further away from a pole.

And so the force of a magnet becomes weaker when things are further away.

And remember, a magnet acts on magnetic objects, objects that are made of magnetic materials.

So in this picture, where is the magnetic field around the bar magnet strongest? Choose one of the letters, A to D.

And when I ask these short questions, I'll wait five seconds.

But if you need longer, just press pause, and press play when you're ready.

And the correct answer is B.

Out of these four places, that's where the field lines are closest together, showing us that's where the field is strongest.

Now, we have a coil with a current in it, and each turn of the coil, each little loop of the coil has a magnetic field around it because it's the current that causes the magnetic field.

And the magnetic fields of each of these turns combine to make the field quite strong around the electromagnet.

So we get a stronger field than if we just had a straight piece of wire.

If we spread the turns further apart, we get a weaker field overall.

If you think about it, if you were next to one of the turns in the right hand magnet, you're further away from the neighbouring turns.

So you're not in a very strong part of their field.

Overall, you don't get a very strong field.

On the left hand side, if you're next to one of those turns, you're close to the neighbouring turns as well.

And so you get a stronger field overall.

Now I have a question for you.

There are two electromagnets here, and I want you to decide which has the stronger magnetic field.

We have one with more tightly wound coils in the upper picture, and that has a current of two amps in the wire.

And then we have one with more loosely wound coils in the lower picture.

And that has a current of four amps.

Which of them has the stronger magnetic field, or is it impossible to say from the information I've given you.

Press pause if you need longer than five seconds to think.

And the answer is it's actually impossible to say.

If you compare B with A, B has more loosely wound coils, which makes the field weaker, but it has a larger current, which should make the field stronger.

And we don't know which of these two is going to have the bigger effect.

So in the end, we don't know if B is going to have a stronger or weaker field than A.

well done if you realised that.

Now I'm going to show you a way of measuring the strength of an electromagnet.

We're actually going to do that by measuring the force it exerts on a bar magnet.

And you can do that quite accurately using this apparatus.

So we have some sort of power supply that's gonna supply an electric current to the coil.

There's the coil.

And we've done something a bit unusual here.

We've wrapped the coil around a boiling tube.

Now, boiling tube is something you might think of as a piece of chemistry equipment, but it's just a really convenient item for holding the coil in a fixed shape and also being able to place the coil very close to one end of a bar magnet, in fact so close that they're just about overlapping here.

And then we have the bar magnet sitting on top of an electronic balance.

When a current flows through the coil, the coil becomes an electromagnet.

And it's going to exert a force on the bar magnet.

It could push down or pull up on it, depending on whether like or unlike poles are next to each other.

But you might be thinking, wait a minute, how are we going to measure force with a balance? A balance measures mass, not force.

Well, it normally does, but the way it does that is not by knowing how much matter is in the object sitting on it, but by experiencing the force of the object pushing down on it because objects have weight.

So we can use that to measure force.

What we have to do first is press the tear button on the balance when there's no current in the electromagnet, and that sets the balance to show zero grammes.

And then any change now in that value is going to be because of the bar magnet being pushed or pulled.

Let's set it up so that the electromagnet and bar magnet are repelling each other, and so the bar magnet will be pushed downwards.

And what the balance does when it experiences a force is it converts that to the mass whose weight would cause that size of force.

The conversion on our planet is that 100 grammes of mass has a weight of one newton.

And so when the balance experiences a pushing force of one newton, it displays 100 grammes on the screen.

So we can convert that back.

Dividing by size by 100 here, if we see the balance saying one gramme, we know that there's a 0.

01 newton force pushing down on it.

And we can do that with any mass that we see.

We divide it by 100 to get the force in newtons.

And this is a really convenient way to measure small forces.

You probably wouldn't have a kind of newton metre in your school lab that could measure forces as small as the ones in this experiment.

When you do measure small forces, instead of writing them all as 0.

0-something, it can be easier to use millinewtons.

Now, milli doesn't mean million or millionths.

It means thousandths.

So one millinewton is a thousandth of a newton, and one newton is 1,000 millinewtons.

Think of millimetres, one millimetre is a thousandth of a metre.

So we can convert a newton into millinewtons by multiplying by 1,000 like this.

So if you read one gramme on the scales, you can convert that to newtons, as I showed you on the previous slide, and then convert that to millinewtons.

But to do that, you are dividing by 100 and then multiplying by 1,000.

And overall, that's the same as multiplying by 10.

So you can convert from grammes to millinewtons in this experiment by multiplying by 10.

So what's the force pushing down on a balance if the balance reads 40.

34 grammes? Press pause if you need longer than five seconds.

And the answer is 403.

4 millinewtons.

As I said before, you can multiply by 10 to convert between grammes and millinewtons.

And now for the investigation.

So you're going to investigate how the size of the current measured with an ammeter, affects the strength of the electromagnet.

You'll need to set up a table like this so that you can put in the current, the mass and then convert to the force.

Make sure you have the units in the table headings.

And in this experiment, you don't have to do repeat readings, you don't have to measure the force several times for each current because you can just take measurements for a lot of different currents, and then you'll have enough points on your graph that you should be able to get a reasonably accurate best fit line.

And remember, you don't have to get round values of current like 0.

20 amps, 0.

30 amps, et cetera.

It's fine to have other values, as long as they're just reasonably spread out within the range of currents that you use.

So press pause, take as long as you need to do the investigation and gather your results.

And when you've done that, press play.

I'll show you a set of example results.

It won't be exactly the same as yours.

Now, in case you're wondering why did this experimenter jump from zero current all the way up to one amp in one go, what they were doing was checking what range of currents were possible with their apparatus so they could then make sure that their measurements were roughly evenly spaced throughout that range.

It's often a good idea at the start of an experiment to work out what range of values of the independent variable are possible, and then spread out your chosen values within that range.

Now for the second part of this lesson, current and the strength of an electromagnet.

So here's another similar set of results.

And it's quite hard to see just by looking at a load of numbers in a table exactly what the relationship is.

You can probably spot that as the current increases, the force increases, but is this going to be a straight line relationship or does it increase more and more rapidly or increasingly slowly? So we're going to plot a graph.

We put the independent variable, the thing we deliberately changed, current, on the x axis, and the dependent variable, the thing we measured on the y axis.

Include the units in your labels.

And then we choose sensible scales so the graph won't be very small and that the scales will be easy to use.

Now, most ammeters in school measure to the nearest 0.

02 amps, although it does vary depending on what kind of ammeter you were using.

Whatever you're measuring to the nearest, you can draw error bars on the graph to show this.

I'll show you an example.

Here's the result for 0.

081 amps.

So the centre of this cross is at 0.

081 for the current, but the horizontal bar goes 0.

02 to the right of it and 0.

02 to the left of it.

And that's representing that we're confident that the true value of that current was within the range shown by that bar.

Now, there's a vertical bar as well for the force.

In reality, the force was probably being measured to the nearest 0.

1 or 0.

2 millinewtons, which is too small to show with an error bar on this graph.

But that little vertical line there is just there to show us where the point is supposed to be.

It's where the two lines cross.

So here are the rest of the points with their error bars.

And now we consider what line to draw here.

And we're looking at the points, and if we don't already know the relationship between force and current, we're seeing do these points seem to fit a straight line or do they look more like they follow a curve? And in this case, they all, nearly all, lie very close to a straight line that we can draw with a ruler.

Any points where the best fit line doesn't go through the error bars, this one here, are likely to be anomalous.

So what we mean here is we don't actually think that the true relationship between force and current is a straight line except for a wiggle at around 0.

7 amps.

What's much more likely is that the true relationship is a straight line, but something went wrong with that particular measurement, somehow that wasn't the current we thought it was or it wasn't the force that we thought it was, or maybe we even just wrote down the number wrong in the table.

So we don't trust that result, and we ignore it when we decide where to put the best bit line.

Now, which of the results on this graph are anomalous? Pause if you need longer than five seconds.

Well, there are two answers here, A and B.

The best fit line doesn't pass through their error bars, so they're likely to be anomalous.

Something probably went wrong when those measurements were taken.

Now let's look at this relationship between force and current.

We can say that the force is directly proportional to the current, or we can say the strength of the electromagnet is directly proportional to the current.

And what does that mean? Well, it's to do with the line being straight and passing through the origin.

The origin is the 0.

00 here.

And this relationship is similar to what you'd see if you looked at a currency conversion graph, for example.

If you had a graph showing you how to convert pounds to euros, it would be a straight line, and it would also pass through the origin because if you double your number of pounds, that's worth twice as many euros.

If you triple your number of pounds, it's worth three times as many euros, and so on.

And the force-current relationship here is similar.

If you double the current, you double the force.

And in fact, if you multiply the current by any number, the force gets multiplied by the same number.

That's a directly proportional relationship.

Now, which of these graphs show a relationship in which the variables are directly proportional to each other? And it's possible to tell, even though there's no labels on these axes.

Pause if you need longer than five seconds.

And the correct answer is all of them.

They're all straight lines that pass through the origin.

And it doesn't matter that they have different gradients, different slopes, they're all direct proportion relationships.

Now you're going to plot a graph of your results for the investigation that you did in task A.

Or if for some reason you couldn't do that investigation or it went wrong, there's a set of sample results here that you could plot instead.

And for each point, draw an error bar for the current, showing that it's accurate to the nearest 0.

02 amps.

That means that whatever current you measured, you're confident that the real current was no more than 0.

02 amps more or less than that.

And then draw a line of best fit and describe the relationship between the strength of an electromagnet and the current through it, so you're drawing a conclusion there.

Pause the video for as long as you need and press play when you're finished.

So here's an example graph.

Here's what the sample results look like on a graph passing through the origin.

So a sensible conclusion is that the strength of an electromagnet is directly proportional to the current through it.

And this is a useful piece of knowledge.

Electromagnets are used in all sorts of ways, and to know how we can affect their strength when we change the current is very useful.

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

The strength of an electromagnet can be increased by using a soft iron core, keeping the turns close together, increasing number of turns on the coil or increasing the current through the coil.

The strength of an electromagnet is directly proportional to the current through it.

I hope you've enjoyed this lesson, and I hope the investigation went well.

And I look forward to seeing you in a future lesson.

So bye for now.