Lesson video

Hello, my ame is Dr.

George, and this lesson is called an electromagnet.

It's part of the unit, electromagnetism.

The outcome for the lesson is, I can describe how to test the strength of an electromagnet.

So first, I'll be telling you what an electromagnet is and later you'll be planning an investigation to do with the strength of an electromagnet and you'll be analysing some sample results.

Here are the key words related to this lesson.

I'm not going to read you all these definitions now because I'll explain them as we go along.

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

The lesson has three parts, and they're called field of an electromagnet, strength of an electromagnet and more turns on an electromagnet.

Let's get started.

To help you understand what an electromagnet is, I need to start by showing you what happens to a compass if we put it next to a wire when there's a current flowing.

Every time a current flows in this wire, that compass needle is deflected, it changes direction.

And when the current is turned off, the needle goes back to its original position, which is pointing north.

What could make a compass needle do that? Well, a compass needle is a magnet that's able to swing around freely.

And what can affect a magnet is a magnetic field.

So it seems as if when there's a current flowing, there's a magnetic field around the wire.

So moving charge, which is what a current is, is associated with a magnetic field.

Now, is a question for you.

Which of the following would cause a compass needle to deflect? Magnetic material, a magnet, Earth's magnetic field, or an object with an electrostatic charge? There may be more than one correct answer here.

With these short questions, I'll wait five seconds, but if you need longer, press pause and then press play when you're ready.

A magnetic material, a magnet or Earth magnetic field would all cause a compass needle to deflect.

They're all things that are affected by magnets and would affect magnets.

But an object with an electrostatic charge, that's a positively charged or negatively charged object.

If you put that next to a compass, it won't deflect the compass needle because electrostatic forces are different from magnetic forces.

Electric current causes a magnetic field around the wire that carries the current.

It looks something like this if we represent it using field lines.

The field lines show the direction that a compass needle would point if you put it at that position.

If you have several wires carrying current, let's say they all carry the same current in the same direction, you get a stronger magnetic field.

And that's represented here by field lines being closer together than they were before.

So here's the weaker magnetic field and here's a stronger magnetic field.

And now question: which wire or group of wires has the strongest magnetic field? And you'll need to read the labels as well as looking at the pictures.

Pause the video if you need more than five seconds to think.

The correct answer is B.

In B, we have three wires each with.

2 amps of current for a total of.

6 amps.

In A, we only have.

5 amps.

And in C we have.

1 plus.

1 plus.

3 amps, which again is only.

5 amps.

And the largest current, the largest total current will have the strongest magnetic field around it.

Now, we can strengthen the magnetic field around a wire by coiling it like this.

If you do that, each turn, each time the wire goes around has a magnetic field around it, and you now have lots of turns next to each other and the magnetic field combined to make a stronger magnetic field.

And the lines here represent field lines.

They're showing the shape of the field around a coil.

And what you now have is an electromagnet.

It's a kind of magnet.

It has a magnetic field around it and it's magnetic when there's electricity, when there's a current in a wire.

And the magnetic field around an electromagnet is really similar, really the same shape as a magnetic field around a bar magnet, a permanent magnet.

You can strengthen the electromagnet even more by putting an iron rod through the middle of the coil.

Iron is a magnetic material, and so if you put iron in a magnetic field, it becomes magnetised itself, becomes a magnet as well.

And it adds its own magnetic field to the magnetic field of the coil.

And what you get overall is a stronger magnetic field as shown here by the closer together field lines.

Now question: which of the electromagnets shown has the strongest magnetic field if the current through each coil is the same? Pause if you need longer than five seconds.

The correct answer is A.

The only difference here is what the core is made of.

And in A it's iron, a magnetic material, so the iron becomes magnetised when there's a current and that strengthens the field.

Wood and copper are not magnetic materials, so they don't become magnetised and they don't strengthen the field.

Remember, not all metals are magnetic.

In fact, only three metal elements are magnetic, and copper isn't one of them.

And now a longer written task, I'd like you to compare the magnetic fields of an electromagnet and a bar magnet.

In the middle, you're going to write things that they both have in common and at the sides write things that each of them has that the other one doesn't.

You don't have to write in full sentences as long as you make your points clearly.

So pause the video for as long as you need and press play when you're ready and I'll show you the answers.

Okay, so if you're ready, here are some answers.

I'll start with both in the middle.

Both fields have the same shape and they both have a north-seeking pole at one end and a south-seeking pole at the other, as you can see, by the way the arrows are pointing.

Now, an electromagnet is temporary.

This magnetic field is temporary because if you stop the current then the field disappears.

But with a bar magnet, the field is permanent.

You can't switch it off.

Now let's move on to the second part of this lesson: strength of an electromagnet.

You've already seen that when there's a current in a wire, there's a magnetic field around the wire, if you coil up the wire, you get a stronger field.

And if you put an iron core inside the coil, you get an even stronger field for your electromagnet.

When you turn the current off, the electromagnet loses its magnetic field.

It's not behaving like a magnet anymore.

If you use a soft iron core, then the core also loses its magnetism when you switch off.

When I say soft iron, I don't mean soft as in squishy.

Soft in this situation means very pure iron that easily becomes magnetised and also easily becomes demagnetized.

It's easy for it to stop being a magnet.

And so what happens is when we switch on the current, the coil becomes a magnet and that magnetises the iron as well.

But when we switch off the current, the coil loses magnetism and the core immediately is its magnetism too.

If you used a steel rod instead inside the coil, steel is not a soft magnetic material so when the current is turned off, the steel doesn't completely demagnetized, it stays a bit magnetised, which means you can't fully switch off your electromagnet.

Steel's actually an alloy, a mixture of iron with carbon.

It's only about 1/200th carbon, but that's enough to change the properties.

The carbon, actually the steel from losing all of its magnetism.

That makes steel good for making permanent magnets out of but not good as the core of an electromagnet if you want to be able to fully switch the electromagnet on and off.

And our question.

In this picture we have an electromagnet with a soft iron core and a coil connected to a power supply and it's picking up some steel paperclips.

Why will a few of the paperclips not fall off this electromagnet even when it's turned off? Think carefully, and press pause if you need longer than five seconds.

The correct answer is that because these paperclips are made of steel, they'll stay a little bit magnetised after the electromagnet switched off.

So they're touching or close to the electromagnet, so they'll become magnetised, but then they won't completely demagnetize when it switches off.

Well done if you realise that.

Now we're going to think about planning an investigation.

Jacob here wants to plan an investigation to find out how the number of terms of wire on an electromagnet affects its strength.

A basic method you can use is to count the number of paperclips the electromagnet can pick up, and that gives you an idea of how strong it is.

But the first problem we have is, first of all, not all paper clips are the same.

So you have smaller ones and larger ones and different materials.

But also even if you manage to get some identical paperclips after being used once, some of them will be a little bit magnetised because paperclips are made of steel and after they've been magnetised, they don't completely lose that.

Jacob needs a strategy to make sure every measurement is exactly the same except for the number of turns on the coil.

The only thing he wants to change is the number of turns.

He doesn't want other things to be changing as he goes along.

He doesn't want these paperclips to become more and more magnetised as he takes measurements.

So these things that you don't change are called control variables.

You change one thing, you keep other things the same.

Otherwise you're just confused about what your results show.

If you change two or three things, you don't know which of them affected the thing that you measured, the strength of the electromagnet.

So which of these are control variables in Jacob's investigation? Pause the video if you need more than five seconds.

And the control variables are the current, the type of wire you use, either of those might affect the strength of the electromagnet.

Type of paperclip and whether it's been used already, because either of those might affect how many an electromagnet of a particular strength can pick up.

The tightness of turns on the wire.

Perhaps that affects the strength of the electromagnet, so we better keep it the same.

And whether or not you have a soft iron core.

The number of turns on the wire, that's not a control variable.

That's the variable we're deliberately changing.

That's the independent variable.

And the number of paperclips the magnet picks up, that's the one we are looking at how it changes.

That's the dependent variable.

So those two things will change.

We don't want to change any of the others.

Well done if you've got most or all of those.

So the basic method here that Jacob wants to use is not very accurate because it can only measure the strength of the electromagnet in whole paperclips.

Let's say the electromagnet picks up seven paperclips, but it can't pick up eight.

Well, it could be that it can just pick up exactly seven and no more.

Or it could be that it could have picked up seven and a half, but it wasn't possible for us to test that.

Maybe it could have even picked up 7.

9 paperclips, but all we know is that it picked up seven.

So which size of paperclips will give the most accurate measurements? Press pause if you need longer than five seconds.

The correct answer is small paperclips, because with small paperclips you can detect quite a small change in strength of the electromagnet.

If you use giant paperclips then the electromagnet really needs to become quite a lot stronger just to pick up one more.

And you can't measure small differences in strength.

So now I'm going to ask you to write a plan to investigate how the number of terms of wire on an electromagnet affects its strength.

I'd like you to use that basic method of picking up paperclips and along the way explain how to obtain the most accurate results with this method.

So write a step by step method.

Take as long as you need.

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

I'll show you now some of the points you could have made.

And yours doesn't have to be exactly the same as this.

Wrap five turns of intonated wire around the end of an iron nail.

Push them to one end and twist the loose ends of wire together to hold them in place.

That's quite detailed, but the main thing is you're going to make a coil with a small number of turns on an iron core.

Connect to a 3 volt power supply or just a suitable power supply, a low voltage power supply.

Pick up as many small new paperclips as you can.

Remember, making them small and new helps with accuracy.

Move the electromagnet away from the paperclips and turn it off.

Count the number of paperclips and record the amount in a table.

Repeat steps two to four to check for anomalous results, results that don't fit the pattern.

Add five more turns to the coil using the same piece of wire.

Measure and record the number of new identical paperclips it picks up now.

Repeat step six five more times, adding five extra turns each time.

We take measurements for any results you are not sure about.

So this here is quite a detailed method.

Yours might not be as long, but it's helpful because it tells the experimenter exactly what to do.

And it's an important skill to be able to plan an investigation so that it's going to work out smoothly.

Now let's go on to the third part of the lesson.

More turns on an electromagnet.

Here's a table of sample results for the investigation we've been talking about, looking into how the number of turns of wire affects the strength of an electromagnet.

And we can see that for each number of turns, there's a first attempt and a second attempt, and then in some cases there's a third attempt.

Can you see why those particular number of turns had three attempts? It's because the first two attempts were a little bit more different.

There were two or three paperclips difference between the two so the experiment has decided to do a third reading to check.

And what you would do is find the mean for each number of turns of wire, the mean number of paperclips picked up.

And the mean results should be written to the same number of significant figures as the measurements that you calculated it from.

So here, if you calculate the mean of 2 and 3, you'll actually get 2.

5, but we're going to round that to one significant figure, 3.

And here are the rest of the mean results.

Now, I have a question for you.

What's the mean of these three measurements? You can use a calculator if you like.

Press pause if you need more than five seconds.

To calculate a mean, you add up the numbers and divide by how many numbers there are.

So you add these three and then divide by three.

And if you do that, you get this: 10.

866666, et cetera.

Now, if you round that to four significant figures, you do get the answer in B, but you should round it to three significant figures because there were three significant figures in the numbers used to calculate this.

So if you do that, the number rounds to 10.

9 centimetres.

Now, we're going to think about graph plotting, another very important experimental skill.

If we look at these results, it's not so easy to see what this bunch of numbers is telling us, but if we can make a picture of it in the form of a graph, we'll be better able to see if there's any kind of pattern here.

We'll put the independent variable, the number of turns on the x-axis with a suitable scale that's easy to use, and the dependent variable number of paperclips on the y-axis.

Now we're ready to plot the points.

Here's the first one.

And you might be thinking that's a bit of a strange looking cross.

What we've done here is put error bars onto the point.

The actual value is at the centre of this cross.

And the error bars show how accurate the measurements are.

I'll tell you a bit more about that in a minute.

Now, here are the rest of the points also with error bars.

And we need to draw a best fit line.

If the points strongly suggested a curve, we might draw a smooth curve, but these points look as though they all lie close to a straight line.

So we'll draw that using a ruler.

Draw your points and your line using a sharp pencil.

Let's take a closer look at these error bars.

Look at the vertical ones.

They go up one paperclip and down one paperclip from the mean.

And that's showing that we are confident that the true number of paperclips that the electromagnet can pick up is within plus or minus one of the mean that we've calculated.

There's a little bit of doubt there.

We know we can only measure to the nearest whole paperclip.

And also we've seen from the repeated trials that we don't always get exactly the same result each time.

We've also put little horizontal error bars on here.

That's for the number of turns.

You might say, well, surely you know the exact number of turns that you used, but at the ends of the coil we might not quite have exactly whole turns.

We might have a bit more or a bit less.

So we've put plus or minus one turn as the error bars on these points.

And the error bars help you to draw the best fit line.

You try to draw the best fit line passing through all of them, if that's possible.

And the best fit line here is a straight line that passes through the origin, the.

00.

And that shows there's a particular kind of relationship between these two variables.

The number of paperclips picked up is directly proportional to the number of turns of wire.

That's what you have if you have a straight line passing through the origin.

If you haven't learned yet about direct proportion in maths, you probably will.

And as the line passes through all of the error bars here, then we can say there are no anomalous results.

There are no results that really stand out as looking wrong.

Now's a question for you.

It's a different graph from a similar experiment.

Can you identify any anomalous results on this graph? There are five label points.

Are any of these anomalous? Pause if you need longer than five seconds.

And points B and E are anomalous.

And we know that because the best fit line doesn't go through their error bars.

So this is a good way of spotting anomalous points on a graph.

Now I'd like you to plot a graph yourself.

And here we have a table of the weight an electromagnet can lift against the number of turns of wire, so slightly different experiment.

So somebody has managed to measure the weight.

Perhaps they picked up something really small like pins and found the weight that could be picked up.

They've been able to measure it to the nearest.

1 of a Newton.

And I'd like you to include error bars.

So think carefully about how wide they should be.

And also identify any anomalous results.

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

Here's the finished graph.

And let's look at the error bars.

The horizontal ones are the same as before, plus or minus one turn, and the vertical ones are going plus or minus.

1 Newtons.

Look at where we have repeated attempts.

The variation in those values is roughly plus or minus.

1 Newtons either side of the mean.

Occasionally it can be a little bigger than that, but we don't have to be that picky about the exact size of every error bar.

And we've drawn a best fit line and we can't make it go through that point error bars.

It's an anomalous point.

So we haven't drawn the best fit line as a straight line except for a wiggle going through that point.

The best fit line is trying to show what we think the real relationship might be.

And it's very unlikely that the true relationship is directly proportional except for a wiggle just at 25 turns.

It's much more likely that when those measurements were taken some sort of mistake was made.

And now we're at the end of the lesson.

I'll give you a summary.

Electric current causes a magnetic field around the wire that carries it.

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

A soft iron core becomes magnetic and adds to the strength of an electromagnet when the electromagnet is turned on.

The strength of an electromagnet is directly proportional to the number of turns of wire in the coil.

So well done for working through this lesson.

I hope you enjoyed it, and I hope you've learned some things about electromagnets.

I hope to see you again in the future lesson.

Bye for now.